71ie Universi'/y of Chicago Science Series

Protoplasmic Action and

Nervous Action ■

Marine Biological Laboratory

Received.

Oct. 7, 19

Accession No. ^7356

Given By

Place,

Dr. Os ^r Richards

"/American ^^p laical Go
/ Buffalo, IJ. Y.

THE UNIVERSITY OF CHICAGO
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PROTOPLASMIC ACTION
AND NERVOUS ACTION

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PROTOPLASMIC ACTION
AND NERVOUS ACTION

By

Ralph S. Lillie

Biologist, Nela Research Laboratories, Cleveland; formerly
Professor of Biology, Clark University

What am I, Life ? a thing of watery salt.
Held in cohesion by unresting cells . . . . ?

— Masefield

THE UNIVERSITY OF CHICAGO PRESS
CHICAGO ILLINOIS

Copyright 1923 By
The University of Chicago

All Rights Reserved

Published October 1923

Composed and Printed By

The University of Chicago Press

Chicajo, Illinois, U.S.A.

/•

V

TO

HELEN MAKEPEACE LILLIE

PREFACE

The present volume is based in part on lectures
delivered in Clark University and the Marine Biological
Laboratory, on the physico-chemical basis of the more
general or fundamental properties of living matter.
Common to all forms of living matter are certain proper-
ties or modes of action which are absent or imperfectly
developed in non-living matter. The chief of these are
(i) the property of specific growth, and (2) a unification
or integration of activities, of such a kind as to secure the
continued existence of the living system in its environ-
ment. The question of how the living system must be
constituted (in the physico-chemical sense) in order to
exhibit such properties is the fundamental one for
physiology.

Of late years the analytical investigation of the
living organism and its products has made great advances;
on the synthetic side, however, progress has been
relatively shght. The precise manner in which certain
special physico-chemical materials and processes are
combined so as to produce life still remains largely
obscure. It may be expected that properly directed
experiment will throw light on this problem, as it has
on many others apparently equally difficult, but at
present we are at a stage where exact or scientific knowl-
edge is only in its beginning.

In this book I have made no attempt to consider in
detail the many special problems of pure physics and
chemistry which are presented by the organism. It is

ix

X PREFACE

assumed that these problems are of the same kind, and
to be approached by the same methods, as other problems
of physics and chemistry. This point of view seems the
only one possible for the scientific investigator. The
organism exhibits a regularity which, although of a
special kind, is obviously based upon and presupposes
the regularity of its component physico-chemical proc-
esses. Investigation of the latter requires the use of
the exact methods developed by modern analysis; and
these have been shown to yield the same constant and
reproducible results in organisms as in non-living
systems. In fact, one of the most striking features of
organic processes is their exactitude, which is frequently
safeguarded by regulatory devices of the utmost delicacy.
The investigation of many such processes is purely
physico-chemical in its method and results.

It must be remembered, however, that in living
organisms we are dealing with synthetic products of a
higher order. When the materials and energies of the
surrounding world unite to constitute the organism,
new qualities and modes of activity inevitably come into
existence; these special properties of living beings form
the subject-matter of the biological sciences, as dis-
tinguished from the physical sciences. For this reason
the physical and chemical characterization of the con-
stituents, reactions, and processes whose combination
or synthesis produces life is not in itself sufficient;
the biological interest centers in the conditions and
special mode of this combination, and in the nature
of the resulting unity. The problem of the nature
of vital organization remains the fundamental one for
biology.

PREFACE xi

We may safely assume that all qualitative phenomena,
including those of the living organism, are subject to
quantitative laws; but the determination of these laws,
while an essential object of scientific investigation,
cannot be regarded as its only object. The biologist is
primarily interested in the phenomena which are peculiar
to life and in the conditions under which these originate
and manifest themselves. As already indicated, growth,
development, and an integrative correlation of activities
are the chief distinguishing characters of organisms.
Underlying and determining these properties are the
fundamental or universal properties of protoplasm.
The essential problem in the physiology of growth (and
ultimately of development and heredity) is the problem
of the conditions of specific chemical synthesis in proto-
plasm. And the problem of integration resolves itself
largely into the problem of the conditions under which
protoplasmic processes, although spatially separated,
mutually influence one another; i.e., the problem of
transmission. For the solution of these problems we
require first of all a knowledge of the special conditions
under which the chemical reactions in protoplasm
proceed and influence one another.

The general physical conditions under which chemical
reactions are initiated, accelerated or retarded, and
influence other reactions at a distance are undoubtedly
the same in living as in non-living matter; but the special
features of composition and arrangement in the proto-
plasmic system often render detafled analysis difi&cult.
Under these circumstances the study of "models" —
simple artificial systems in which the action of single
factors may be isolated and observed — may be of great

xii PREFACE

service, and I have made use of this method in a number
of instances. For example, the transmission of the
effects of stimulation in nerve and other irritable forms
of protoplasm resembles closely certain types of chemical
transmission or distance-action in metal-electrolyte
combinations; many biocatalytic reactions are identical
with those induced by colloidal platinum or charcoal;
there are also instructive analogies between organic
growth and certain types of inorganic growth. Many
fundamental physical processes which play an important
part in protoplasm are independent of the special
chemical composition of the material; thus the influence
of radiation and electricity on living matter is a special
case of the general influence which these agents exercise
under appropriate conditions upon all chemical reactions.
The detailed nature of the conditions in protoplasm can
be determined only by special investigation.

The more special sections of this book have reference
to the two fundamental problems above defined. The
structural and physico-chemical organization of living
matter, the modifiability of its rate of reaction under
varying conditions (irritability), and its transmissive
property (so highly developed in nervous tissues) are
considered in some detail; and their probable relation
to the polyphasic and film-partitioned character of the
protoplasmic system is indicated.

CONTENTS

CHAPTER PAGE

I. Introduction — General Characteristics of Liv-
ing Matter i

II. The Cellular Organization of Living Matter . 14

III. General Characters of Living Organisms . . 25

IV. General Peculiarities of Protoplasm as a Physi-
cal System 48

V. Physical Nature of Protoplasmic Structure: Im-
portance of Surface Conditions 66

VI. Protoplasmic Structure {Continued): Permea-
bility and Other Properties of Protoplasmic
Membranes 98

VII. General Conditions Determining the Properties

OF Protoplasmic Membranes 132

VIII. Relation of the Inorganic Salts of the Medium

to the Physiological Processes in Protoplasm 151

IX. General Physiological Action of Lipoid-Alter-

ant and Surface- Active Substances . . . . 187

X. Catalysis in Relation to the Chemical Pro-
cesses in Living Matter 217

XL Electrical and Other Factors in the Catalytic

Action of Protoplasm 235

XII. Stimulation and Transmissioisi of Excitation in

Protoplasm 259

XIII. Bioelectric Phenomena 299

XIV. Membrane Changes during Stimulation . . .337

XV. The PHYSico-CnEivncAL Basis of TRANsmssiON in

Nerve and Other Protoplasmic Systems . . . 379

Index f j f /7» ^^J

xiii

/rf'^C

/'

1.^^

CHAPTER I

INTRODUCTION— GENERAL CHARACTERISTICS
OF LIVING MATTER

It is a peculiarity of living matter, as distinguished
from non-living matter, that it is never found in a
diffuse, unorganized, or formless state, but always
composing definite individualized systems or organisms,
of which there are many kinds or species, each with
definite and, on the whole, highly constant physico-
chemical, structural, and active characters. These
organisms form a class of natural systems which, con-
sidered quantitatively, is a very small one in comparison
with physical nature as a whole. This fact in itself
implies that living systems are highly special develop-
ments; they represent a higher order of synthesis, and
it is to be expected that they should exhibit properties
and activities which are absent in non-living systems.
Hence the existence of a sharp contrast between the
living and the non-living — i.e., between organism and
environment — is not in itself surprising. We know,
however, that continuous transitions from the one to
the other have existed and stilL exist; life has evolved
from non-living matter in the past; and in the present
every living organism is the seat of a continual trans-
formation of non-living into living matter. The chief
problem of general physiology is to trace the steps of
this transition; i.e., to determine the nature of the
synthesis by which the living matter, protoplasm, is

2 PROTOPLASMIC ACTION AND NERVOUS ACTION

built up from the non-living material which it incorpo-
rates from the surroundings.

Physiology regards the living organism solely in its
objective aspect as a physical object in external nature;
many aspects and manifestations of living beings do not
form directly a part of its subject-matter, and the general
philosophical question of the essential significance of life
in the cosmos — the question of vitalism or anti- vitalism
— is not one which it makes any pretensions to answer.
It observes simply that certain systems, living organisms,
exist in the external world, presenting a remarkable
combination of properties not found in other natural
systems; and its task is the analysis of these systems in
the terms and by the methods of physical science.

These special or distinguishing peculiarities of living
organisms may be grouped under several general heads,
as follows: (i) metabolism, (2) growth, automatic self-
maintenance, reproduction and heredity, (3) irritability,
(4) regulation and adaptation, (5) spontaneous activity,
having reference to future as well as present conditions.
The essential character and implications of these various
properties will first be briefly considered.

I. METABOLISM

The essential peculiarity which places organisms in
a class apart from most non-living objects is that their
properties and manifestations depend on their continued
chemical activity; in other words, they are metabolizing
systems, formed, maintained, and perpetuated by
processes of chemical transformation. The production
of new chemical compounds by transformation of other
compounds taken from the surroundings, and the

CHARACTERISTICS OF LIVING MATTER 3

organization of these compounds in new structural and
chemical relationships, constitute the fundamental activi-
ties of all living matter. Hence a consideration of
the general features of metabolic processes must come
first in any discussion of the nature of protoplasmic
action.

Under the term metabolism are included primarily
the nutritive and energy-yielding chemical processes in
protoplasm, and secondarily the other chemical processes
subserving or underlying these. The application of the
term is usually clear; but metabolic processes comprise
chemical reactions of all kinds, many of which are in no
sense peculiar to organisms, while others are not met
with elsewhere in nature. The traditional distinction
between constructive and destructive metabolism remains
an essential one; what the organism is at any time is a
resultant of the effects of these two large and, in general,
oppositely directed groups of chemical reactions.
Broadly speaking, the constructive reactions represent
the nutritive processes, and the destructive reactions
the energy-yielding processes. Constructive metabolism
includes the synthetic (anabolic) reactions underlying
growth, self-maintenance, and reproduction. In any
species the end-products of the constructive sequence of
reactions consist largely of certain colloidal compounds,
highly individualized and specific in their chemical
constitution, the proteins; the other synthetic products
(carbohydrates, fats, lipoids, etc.) are chemically non-
specific; i.e., not confined to the species in question;
these form, together with the specific compounds, water
and various dissolved substances, a complex and highly
organized system, or organic individual, which is specific

4 PROTOPLASMIC ACTION AND NERVOUS ACTION

(i.e., definitely characterized and unique) in its structural
and active characters. This building-up, by means of
metabolic construction, of a complex system, specific in
chemical composition, structure, and activity, out of
relatively simple non-specific materials taken from the
surroundings (food, water, salts) is the fundamental
general peculiarity which distinguishes living organisms
from non-living systems. Constantly associated with
the constructive group of reactions is the destructive or
catabolic group by which substances contained in the
protoplasm are broken down, usually oxidized, to yield
the energy freed in vital activity. A great diversity of
compounds are thus utilized by protoplasm as sources of
energy; the catabolic process is non-specific; i.e., sugars,
fats, and proteins are metabolized to yield the same end-
products (CO2, water, urea, etc.) and energy in organisms
of all kinds.

2. GROWTH, MAINTENANCE, REPRODUCTION,

AND HEREDITY

It is essential at the beginning of any study of
fundamental vital properties to recognize the dependence
of the various phenomena designated by the four terms
above upon the fundamental process of specific construc-
tive metabolism. In every organic individual normal
self-maintenance, by which the material lost as a result
of metabolic destruction is replaced by new construction,
involves the same specific synthetic reactions as those
concerned in growth. And growth is obviously a
highly specific process; this becomes evident whenever
a seed or an egg ''grows into" the specifically organized
adult. Organic growth thus involves or implies ''hered-

CHARACTERISTICS OF LIVING MATTER 5

ity"; and since growth is the foundational life-process —
that by which all living matter is brought into existence
— we see at once that the specificity of the underlying
metabolic syntheses is the essential condition underlying
organic specificity. When constructive metabolism
ceases, not only does growth cease but life itself, since
the continual formation of specific material is a pre-
requisite for normal maintenance.

Each of the terms above, however, designates a
feature or aspect of vital phenomena which is as a
rule perfectly definite and distinguishable from the
others. Organic growth is perhaps best defined as
increase in the quantity of the specifically organized
living material.^ Reproduction is the formation of new
individuals by growth from the parent organism, or a
detached portion of the latter (germ, gamete); in
metazoa the replacement of outworn or senescent
individuals is thus accomplished. Reproduction has
been defined as ''discontinuous growth"; thus the
growth of a plant-cutting is a reproduction, and many
cases of asexual reproduction in animals illustrate the
same phenomenon (reproduction by fission, regeneration).
In the lowest organisms, e.g., bacteria, it becomes no
longer a matter of practical interest to distinguish
between growth and reproduction. Heredity, the re-
semblance of offspring or outgrowth to parent stock, is
illustrated in all of these cases; the special problem of
heredity, therefore, is reducible ultimately to the funda-
mental problem of the conditions determining the
property of specific construction possessed by all forms

^ Cf. the discussion by Child, Senescence and Rejuvenescence, Chicago
(1915), chap. ii.

6 PROTOPLASMIC ACTION AND NERVOUS ACTION

of protoplasm/ This consideration is overlooked in
many '' theories of heredity/' which apparently take for
granted the existence of the property which they are
called upon to explain. Ids, pangenes, chromosomes,
and the other representative particles of these theories
are self-multiplying units; i.e., they possess ex hypothesi
this automatic power of synthesizing material and
structure of their own kind. It is well, therefore, to
realize clearly the fundamental identity of the physi-
ological conditions underlying all of the phenomena
grouped under the foregoing head.

To prevent any possible misunderstanding, a few
words may be added here concerning the nature of the
physiological problems raised by the chromosome
theory of heredity, which now seems to be established
on a secure basis through the correlation of genetic and
cytological investigation.^

All the evidence indicates that the chromosomes,
the carriers of genetic factors or "genes," are the elements
or units in a sorting and distributing mechanism, by
means of which special formative metabolic processes
are localized in definite regions of the growing and

^ Haldane's remarks in his British Association address of 1908
{Nature, LXXVIII, 555), "nutrition itself is only a constant process of
reproduction" and "heredity is for biology an axiom and not a prob-
lem," do not dispose of the problem of heredity, but apparently assign
it to a border-line position, somewhere between chemistry and biology.
The property of automatic specific synthesis is the one to be explained.
The original natural systems which exhibited this property were pre-
sumably the ones from which living organisms, as we find them, have
evolved.

2 Cf . T. H. Morgan, The Physical Basis of Heredity, Philadelphia
(1919); also "The Mechanism of Heredity," Nature, CIX (1922),
241, 275, 312.

CHARACTERISTICS OF LIVING MATTER 7

developing organism. The property of self-multiplica-
tion possessed by these units, on which the possibility
of their special action depends, is, however, not peculiar
to them, as already pointed out, but is a property of
protoplasm and of protoplasmic structures in general.^
Once the chromosomes have been produced by this
autosynthetic process, they are free to exercise their
special influence and function. In this respect they
are like other structures which are definite factors in
the formative processes; they must first be synthesized
by the fundamental growth processes before they can
function. There are obvious analogies between the
action of the chromosomes and the action of special
form-determining chemical substances (or hormones)
produced by various organs. Development at certain
stages is demonstrably a consequence, as regards certain
special features, of the previous development of the
thyroid or the pituitary gland or the gonads. An even
more general analogy may be pointed out here, since
it illustrates the nature of many biological sequences.
A prerequisite to the normal activity of the adult is the
development of the normal adult structure; for example,
the formation of hands must precede the construction of
a house, but we do not explain the whole constructive
process by reference to the hands, the tools for sorting
and distributing the materials. How the chromosomes
influence formative metabolism is the essential problem
for physiology; this problem is at present unsolved, but
there can be no doubt as to the existence of this
influence.

^H. J. Muller has discussed the properties of the genes from this
point of view in a recent paper in American Naturalist, LVI (1922), 32.

8 PROTOPLASMIC ACTION AND NERVOUS ACTION

3. IRRITABILITY

It is characteristic of all organisms that they respond
to changes in their environment (stimuli) by changes in
their own activity (response). And since metabolic
reactions underlie all vital activity, this fact implies
that the chemical reactions constituting metabolism
are subject to the influence of external agencies acting
upon the protoplasm. Both constructive and destruc-
tive metabolism may be thus influenced.

In general, what we mean by irritability is this
susceptibility to external influence; irritability, howxver,
cannot be considered as a special property independent
of the continual automatic, chemical, and other activity
of the living system; its existence merely shows that
the chemical reactions of protoplasm are subject to
modification — e.g., acceleration or the reverse — under the
influence of relatively slight changes of state, caused usually
by the action of external agencies upon the protoplasm.

A peculiarity of most intact organisms is that the
changes of activity thus induced are normally of such
a character as to favor the continued existence of the
individual or of the species in the environment; this
general fact may be expressed by saying that the normal
responses to stimulation, however varied in detail, have
a regulative or adaptive character. Adaptiveness, how-
ever, is a peculiarity of the organism as a whole, not an
inherent property of protoplasm in general; this is
shown by the fact that isolated parts may show irrita-
bility quite independently of any adaptive reference;
e.g., nerve or muscle. In this respect irritabihty may be
compared with the chemical instability of explosives,
which may also be applied adaptively.

CHARACTERISTICS OF LIVING MATTER 9

4. REGULATION AND ADAPTATION, INTEGRATION

These characters, while based on irritabihty, have a
more distinctively organic or vital quality — are manifes-
tations of a higher plane of organization — than the simple
property of responsiveness to stimuli. Fundamentally
they are related to the characteristic self-conserving
property of the organic individual or species; this prop-
erty is exhibited by all naturally occurring organisms;
i.e., the structure and activity of the latter are of such
a kind as to favor a permanent or stable existence in the
environment. Under the terms regulation and adapta-
tion, we include, in their broadest appHcation, all of
those features of adjustment — structural, chemical, and
active — which are especially characteristic of living as
distinguished from non-living systems. The organism
is "fitted" to its environment; the reciprocal relations
between the two are so balanced or correlated that the
species persists. In other words, the properties or
activities which have special "survival value" are those
which we designate as adaptive. Adaptations may be
(i) of a static or morphological kind — non-temporal in
their reference — e.g., when the structure of the organism
shows a correspondence with the unchanging features of
its environment. Perhaps the most general and wide-
spread example of such static adaptation is seen in the
general plan of bodily structure common to most free-
living animals — bilateral symmetry combined with
antero-posterior and dorso-ventral differentiation.^ Or

^I have discussed more fully the general conditions that render
this type of structural plan adaptive in a paper on purposive and adap-
tive behavior in the Journal of Philosophy, Psychology and Scientific
Methods, XII (1915), 589.

10 PROTOPLASMIC ACTION AND NERVOUS ACTION

they may be of an active kind; such are classed as
regulations. In this case the activity of the organism
or of its parts changes in such a way as to resist or
compensate departure from the normal; i.e., from the
physiological or other conditions required for continued
life. The automatic regulation of food-intake, gaseous
exchange or temperature, the protective and other
self-conserving reactions or instincts, and the phenomena
of form-regulation are examples. Since in all such cases
the persistence of the organism in the environment is
the condition promoted or secured, and since persistence
in external nature implies equilibrium, we may character-
ize regulations as reactions of an equilibrating type;
i.e., regulation corresponds essentially to equilibration.
In a sense it is obvious that the structure and activities
of an organic species must be such as to secure persistence
in the environment, since the alternative is extinction;
nevertheless the universal presence of regulative modes
of activity is a peculiar and highly remarkable feature
of living as distinguished from non-Hving systems, and
requires special consideration. Regulations or automatic
equilibrations are also met with in many non-living
systems (regulators in machines or other artificial
systems), but for the most part these are of a relatively
simple type.

The conception of organic integration is closely
related to that of regulation; the maintenance of a
definite and unified structure and activity in any complex
system consisting of many parts requires the mutual
interaction and control of the different parts in such a
manner that the activity of each is subordinated to that
of the whole. This integration presupposes the trans-

CHARACTERISTICS OF LIVING MATTER ii

mission of chemical and other influence between different
regions, and in higher organisms is effected chiefly
through the nervous system in co-operation with a chemi-
cal control exercised by special substances (hormones
and other metabolic products) transported from place to
place in the circulation/ The possibility of these two
forms of integration rests ultimately on mechanical or
structural factors, shown in the permanence of morpho-
logical form and organization; hence some authors speak
of a mechanical integration (or correlation) in addition
to the other two.^

5. SPONTANEOUS ACTIVITY

The chief vital phenomena classed under this head
are characteristic of the organism in its action as a
whole, rather than of its special parts, although many of
these are spontaneously active; e.g., the heart. They
are especially developed in animals, and include spon-
taneous activity and trains of activity (instincts)
directed toward the external world and having usually
some definite future reference; purposive and conscious
action, in their physiological aspect, also belong here.
All such characters are based upon, or presuppose,
the other more fundamental characters; i.e., they are
not general protoplasmic properties but appear at
a higher level of vital synthesis; hence they do not
form, strictly speaking, a part of our present subject-
matter.

^ Cf. Sherrington's Integrative Action of the Nervous System.

^ Cf . Child, The Origin and Development of the Nervous System from
a Physiological Viewpoint, University of Chicago Press (1921), chap, i,
p. 12.

12 PROTOPLASMIC ACTION AND NERVOUS ACTION
SCOPE OF GENERAL PHYSIOLOGY

The first four groups of characters appear to be
common to all forms of living matter; i.e., they are the
expressions of the general or fundamental properties
and activities of the living substance or protoplasm
wherever found. We class as ''living" all natural
systems exhibiting these properties in combination; and
general physiology has for its object the study of the
essential composition and activities of such systems.

From this point of view the distinction between
animals and plants becomes one of minor importance.
This difference is essentially one of method of nutrition;
in plants the processes of constructive metabolism start
with more elementary and widely diffused materials
than in animals. A brief reference to the main points
of distinction seems relevant here, since it may assist
in defining the essential problem under consideration.

It is evident that all organisms require for their
normal growth and activities the presence of energy-
yielding (chiefly oxidizable) materials in the protoplasm,
as well as materials for building up protoplasmic struc-
ture; the chief representatives of these two classes of
substances are, respectively, the carbohydrates and the
amino-acids. The main differences between plants and
animals relate to the methods by which these materials
are obtained or rendered available. In green plants
they are synthesized from simpler compounds which in
their unaltered state cannot serve as sources of energy —
CO2, salts, water. In animals the chief ''food " materials
are already complex compounds of high chemical
potential which are not synthesized in the organism
but are prepared outside of the latter (ultimately by

CHARACTERISTICS OF LIVING MATTER 13

plants), and are introduced into the organism from
without by its own special activities. In both groups,
however, the active living substance or protoplasm
consists chiefly of compounds of the same general
chemical type, which in both cases undergo similar
transformations. The fundamental physiological pro-
cesses of plant and animal cells are thus closely similar.
Hence, in general physiology, whose aim is the analysis
of the vital process, wherever occurring, organisms of
both groups come equally under consideration.

CHAPTER II
THE CELLULAR ORGANIZATION OF LIVING MATTER

General physiology has been defined by Verworn^
as '^ cellular physiology," in accordance with the general
conception of the cell theory that the ultimate living
units of any organism are the cells. According to this
conception the cells are the simplest units capable of
independent life; hence general physiology, aiming
at the anatysis and characterization of life-processes,
should be equivalent to cell physiology. There appears,
however, to be a certain arbitrariness in this idea.
The cell is already a complex system with a definite
organization, usually containing a nucleus and exhibiting
other special structural differentiations. The question
of the physiological significance of the cellular organiza-
tion constitutes a special problem in itself. While it
is remarkable that all higher organisms show this type
of organization, it seems hardly justifiable to regard all
organisms as consisting of cells and products of cells.
Such a conception regards the simplest living unit as
having a certain definite type of structural organization;
i.e., it is essentially a morphological conception. A
chemical characterization seems to meet the requirements
of the case more completely. Many organisms are
known which do not show the chief structural feature of
the cell, differentiation into nucleus and cytoplasm; e.g.,
bacteria and blue-green algae. Usually bacteria are
regarded as plant cells of a special kind; it is question-

' Allgemeine Physiologic, 5th edition, Jena (1909), chap. i.

14

CELLULAR ORGANIZATION OF LIVING MATTER 15

able, however, if micrococci, and especially the organisms
in filterable viruses, can be considered as cells in the
true sense. The case of the ultra-microscopic organisms
present in the filterable viruses is of special interest.
These organisms can be demonstrated only by the effects
which they produce (infection); they prove themselves
to be living by their power of automatic growth, shown
by multiplication in the body of the host or in culture-
media, and also by exhibiting other properties character-
istic of protoplasm in general, such as thermolability
and susceptibility to toxic agents of the disinfectant
class. They may be described as complex and chemically
active (metabolizing) material in a fine state of sub-
division (like that of colloidal material), possessing in
addition to the other properties of matter in this state
the special vital properties of assimilation, growth, and
multiplication. As already pointed out, this ability to
transform environmental material into its own specifically
organized and active substance is the distinctive criterion
of living as distinguished from non-living matter.

Our conceptions of the nature of living organisms
must be broad enough to include the ultra-microscopic
forms. Cells, as found in higher organisms, are units
of a relatively complex and highly differentiated kind,
representing a comparatively advanced stage of evolu-
tion. They are by no means to be regarded as the only
systems in nature exhibiting the characteristics of life.

THE CELL

In higher organisms, however, we have definite
experimental evidence that the smallest unit capable of
continued independent life is the nucleated cell. The

1 6 PROTOPLASMIC ACTION AND NERVOUS ACTION

protozoa remain as single cells throughout life. The
higher animals and plants are single cells only at the
beginning of their development — in the germ-cell stage;
in later developmental stages and as adults they consist
of large, closely associated aggregates or colonies of
cells, which, together with the intercellular fluid media
serving for transport (blood) and various other products
of cellular activity (skeletal and other structures), form
a complex and highly integrated system, or organic
individual. Each cell in this organism is to be regarded
as living and capable of independent existence under
appropriate conditions.

The statement that single isolated cells are capable of
independent life has been shown experimentally to be
true, not merely of organisms which throughout their
life are unicellular, but also of many of the cells of higher
organisms when isolated under favorable conditions —
leucocytes, ciliated cells, muscle cells, tissue-cells.
Epithelial cells will grow in suitable culture-media;^
embryonic nerve cells, isolated in sterile plasma, send
out axones in a characteristic manner; i.e., retain the
normal power of growth, differentiation, and develop-
ment;^ and many functional adult cells continue to
live and grow when isolated under favorable conditions
of food and oxygen supply.^

On the other hand, experiment shows that for normal
and long-continued vital activity the cell must be

^ Cf. L. Loeb, Arch. Entwickl. Organ., XIII (1902), 487, and earlier
papers there cited.

2 Harrison, Proc. Soc. Exp. Biol, and Med., Ill (1907), 140; Journal
0} Experimental Zoology, IX (1910), 797; XVII (i9i4)> 521-

3 Cf. Carrel and Burrows, Journal 0} Experimental Medicine, XIII
(191 1); W. H. and M. R. Lewis, Anatomical Record, VI (191 2).

CELLULAR ORGANIZATION OF LIVING MATTER 17

complete, at least in the sense that both nucleus and
cytoplasm (or portions of both) are present. This is
shown by experiments on enucleated cells, such as egg
cells; portions containing nuclei survive; the others
die. But if enucleated portions are fertilized and thus
furnished with nuclei, they continue to live.' An other-
wise complete protozoon such as Stentor will die if
deprived of its nucleus, while fragments of less than
one-twentieth the normal size will survive and reform
a complete organism if a portion of nucleus is present.^
Similar results have been obtained in experiments on
other protozoa; e.g., Verworn's with Thalassicolla.^

There is a large body of similar experimental fact
indicating that the continued interaction of nuclear and
cytoplasmic components is an essential feature of normal
cell-metabolism. It is usually supposed that the nucleus
has special relations to synthetic metaboHsm; hence
its special importance in growth and regeneration, but
the whole problem of the relation of nucleus to cytoplasm
is at present in an unsatisfactory state."*

It is clear, nevertheless, that the nucleated cell of the
higher organism is a complete and autonomous living
unit. But in view of what has just been pointed out
regarding the non-cellular or subcellular constitution of
some organisms, we must avoid regarding the dis-
tinctively cellular features of protoplasmic organization

^ Cf. Delage, Arch, de zool. exp. et gen., VII (1899), 383.

2 F. R. Lillie, Journal of Morphology, XII (1896), 239.

sVerworn, Arch. ges. Physiol., LI (1891); cf. also Allgemeine
Physiologic, 5th edition (1909), p. 620.

4 For a recent study cf. V. Lynch, American Journal of Physiology,
XLVIII (1919), 258.

1 8 PROTOPLASMIC ACTION AND NERVOUS ACTION

as the all-essential ones. To do so would be to imply
that in the early or precellular stages of organic evolution
the assimilative or proliferative types of colloidal
material, which presumably were then the only systems
representing organisms, were not living. The formation
of a particular kind of structure is not the essential
criterion of vitality; the properties which underlie the
formative or structure-building activities are the primary
ones. In most animals and plants these activities give
rise to a cellular type of structure, but this is not neces-
sarily true of all.

IMPORTANCE OF CELLULAR ORGANIZATION

There is a sense, therefore, in which we may regard
the cellular type of structural organization as not so
much the cause or necessary condition of the vital
activities as their product or effect. Obviously all
cellular organisms come into existence through the
constructive processes of growth. This was pointed out
by Huxley, in the early years of the cell theory, in a
well-known passage in w^hich he speaks of the cells as
being not the producers but simply the products or
indicators of vital action. Like the shells on the sea
beach the cells ''mark only w^here the vital tides have
been and how they have acted. "^ This comparison is
an apt one in that it emphasizes the primary importance
of the structure-forming vital activity which expresses
itself in the formation of cells; but it tends perhaps to
subordinate the part played by the cellular structure,
once it has been attained. There is no doubt that this

^ Cf. Huxley, "Review of the Cell Theory" in the British and Foreign
Medico-chirurgical Review (1853).

CELLULAR ORGANIZATION OF LIVING MATTER 19

structure determines, in a quite special way, the character
of the protoplasmic processes; i.e., has its own definite
causative and controlling influence. The metabolic and
other cell processes can be shown to be profoundly
influenced by changes in the physical and other state
of cell structures. For example, there is evidence that
in most cells irritability depends primarily upon the
special properties of the external protoplasmic layer
or plasma membrane; the contractile, secretory, and
similar mechanisms are cell structures; the special
relation of the nucleus to constructive metabolism
has already been mentioned. In general we may say
that physiological activity in all higher organisms is
intimately bound up with the special features of
structure, chemical organization, and activity peculiar
to cells.

A universal peculiarity of living matter, considered
simply as a chemical reaction- system, is that its principal
chemical reactions, especially the specific constructive
group, occur under the control of structural conditions.
If protoplasmic structure is destroyed, mechanically or
otherwise, these essential vital reactions at once cease.
New structure as it arises in growth or development
must therefore have a modifying influence on the meta-
bolic processes and the other physiological processes
dependent upon these. The structural characters pecu-
liar to cells cannot fail to influence profoundly the
chemical activity of all living systems having the cellular
type of organization. One of the fundamental problems
of general physiology has reference to the special nature
of the relations existing between cellular structure and
the chemical processes of the cell protoplasm.

20 PROTOPLASMIC ACTION AND NERVOUS ACTION

The question^ Why is living matter so characteristi-
cally cellular in structure, seems to be equivalent to the
question, Why is it partitioned, subdivided into minute,
usually microscopical, portions (cells), or structurally
discontinuous ? Each portion of protoplasm is separated
from its surrounding medium or from adjoining cells
by a thin, structurally distinct boundary layer usually
called the ''plasma membrane"; and the presumption is
that some definite physiological advantage attaches to
this peculiarity. The most evident general answer is
that this layer serves to separate or insulate the hving
protoplasm from the surroundings, and thus to protect
it from the disintegrative or otherwise adverse influence
of the latter. This view regards the plasma membrane
as primarily a protective structure. Through its
presence each separate portion of living substance, or
cell, is enabled to retain its special composition and
individuality. But this answer, while undoubtedly
correct in part, is too vague and general to be satisfactory.
The recent experimental studies on protoplasmic per-
meabiUty have thrown a more definite light on the
problem. They have shown that in typical living cells
the external protoplasmic layer has the properties of a
semi-permeable membrane; i.e., it is impermeable or
difficultly permeable to the water-soluble substances of
low molecular weight present in the protoplasm and
surroundings (and to chemically similar substances),
while freely permeable to w^ater. Free diffusion of
soluble substances either into or out of the cell is thus
prevented; the protoplasm can preserve a chemical
composition different from that of the surrounding
medium without the interference that would result

CELLULAR ORGANIZATION OF LIVING MATTER 21

from unrestricted diffusive interchange/ It is evident
that if a minute portion of protoplasm is to retain its
special chemical organization, it must be protected
against loss of its water-soluble constituents by diffusion,
and also against the unregulated entrance of soluble
substances from without. Chemical analysis shows
in fact that the crystalloidal content of living cells is
typically widely different from that of the surrounding
medium.^ The presence of a diffusion-proof partition
separating each small portion of living protoplasm from
its surroundings is apparently an essential feature of the
cellular organization.

Without such a diffusion-hindering type of structure,
it is difficult to see how a high degree of chemical differ-
entiation could be maintained in such a system as the
living organism, consisting, as it does, in large part of
an aqueous solution of diffusible substances. Differences
in the distribution of soluble substances between proto-
plasm and surroundings would tend to equalize them-
selves by diffusion, and chemical differentiation would
become difficult or impossible. Morphological differ-
entiation has long been recognized as favored by the
subdivision of the developing germ into cells; this
condition permits morphogenetic processes in neighboring
cells and cell groups to proceed in relative independence
of one another.^ In a similar manner an essential

^ Cf. my paper in Biological Bulletin, XVII (1909), 188, for a fuller
discussion.

^ For a summary of work in this field, cf. Hober's Physikalische
Chemie der Zelle und der Gewebe (1914), pp. 370, 491; cf. also Bottazzi's
article in Winterstein's Handbuch der vergl. Physiol., I (191 1), 37.

3 Cf. F. R. Lillie, "Adaptation in Cleavage," Woods Hole Biological
Lectures (1899), p. 43.

22 PROTOPLASMIC ACTION AND NERVOUS ACTION

condition for the isolation of chemical and physiological
processes in adjacent regions of the organism is the
presence of the semi-permeable intercellular partitions.

There is also evidence that the internal protoplasm of
the single cell is frequently pervaded by a system of films
or closed partitions giving a chambered type of structure ;
and the possibility of intracellular chemical differentia-
tion ('' chemical organization") has been referred to
this condition.^ Such a chambered structure corresponds
essentially to that of an emulsion-like or alveolar system.
Apparently any physico-chemical system which is
built up largely of water and substances in aqueous
solution must be a partitioned system if it is to maintain
within a small space a high degree of chemical differ-
entiation together with a corresponding diversity of
chemical activity.

In general, each living cell can be shown to possess
a surface layer (plasma membrane) with properties differ-
ent from those of the internal protoplasm. At the boun-
dary between this surface layer and the adjoining medium
the general phenomena characteristic of phase-boundaries
are exhibited. A highly characteristic feature of the
living cell is that its surface is sharply defined against
the medium, like the surface of an oil drop, very much
as if the surface layer consisted of water-insoluble
material. This water-immiscible property of living
protoplasm and the semi-permeability of its boundary
layer are closely associated properties; together they
constitute one of the most noteworthy physical peculiari-
ties of living protoplasm. Especially significant is the

^ Hofmeister, Die chemische Organisation der Zelle, Braunschweig
(1901).

CELLULAR ORGANIZATION OF LIVING MATTER 23

fact that they are preserved only while the cell remains
living. All cells disintegrate on death; the vital semi-
permeability and water-immiscibility are then lost.
Any living cell, such as a blood corpuscle, suspended
in its normal medium, exhibits general physical properties
similar to those of a suspended insoluble particle; e.g., an
oil drop. These properties are largely an expression of
general physical conditions present at all boundary
surfaces between adjacent phases, and their consideration
becomes of great importance to the physiologist.

In common with other boundary surfaces between
mutually immiscible phases the cell surfaces have
characteristic electrical properties (interfacial potential
differences), exhibit surface tension, and possess the
property of condensing or absorbing dissolved substances
from the surrounding solution (adsorption). The general
role of adsorption in protoplasmic activity is a highly
important one, to be considered later in more detail;
and undoubtedly this process is a chief factor in the
catalytic or quasi-catalytic action of living matter.
In general, the catalytic properties of finely divided
substances, such as charcoal and colloidal metals, are
referable — at least in large part — to adsorption, and the
same is probably true of the catalytic properties of living
cells. Adsorption appears also to be a factor in the
collection of nutrient and other substances from very
dilute solution, also a highly characteristic feature of
protoplasmic activity.

These considerations show that in addition to limiting
diffusion and thus providing for structural and chemical
differentiation in the manner indicated, the cellular or
partitioned structure of living matter is physiologically

24 PROTOPLASMIC ACTION AND NERVOUS ACTION

important because it furnishes the conditions for another
highly characteristic group of properties, those dependent
on surface conditions. The protoplasm is thus enabled
to utilize (so to speak) the special physical properties
exhibited by matter at boundary surfaces. With fine
subdivision the proportion of surface protoplasm to the
total mass of living substance is large, and the role of
surface processes assumes corresponding importance.
This general point of view recalls Herbert Spencer's
explanation of cell-division as essentially a regulative
process, the effect of which is to maintain a certain
minimal surface-volume ratio in the protoplasmic mass.
The living substance enters into relation with its sur-
roundings through the intermediary of a surface layer,
which has special physiological properties, in corre-
spondence with the special nature of the physical condi-
tions resident at boundary surfaces. Evidence will be
presented later indicating that these electrical, adsorp-
tive, and catalytic properties of the protoplasmic
surface layers determine many of the most characteristic
features of protoplasmic activity, especially the automatic
and rhythmical processes, the susceptibility to electrical
influence, and the various manifestations of irritability.

CHAPTER III

GENERAL CHARACTERS OF LIVING ORGANISMS

CHARACTERS OF ORGANISMS IN RELATION
TO ENVIRONMENT

All organisms have the power of self -maintenance ;
i.e., of maintaining their identity and a certain constancy
of structure, chemical composition, and activity in
spite of continual changes in their surroundings and in
their own living substance. The degree of environmental
change to which different organisms are exposed varies
greatly, and many cells of higher animals pass their
whole life in media which are automatically secured
against all but slight variation. On the other hand,
protoplasmic activity, implying chemical change, is
uninterrupted during life; and, as already pointed out,
is largely the expression of chemical reactions, chiefly
oxidative in nature, by which energy is freed. In all
organisms part of the energy thus freed takes such a
form that the organism is enabled to maintain itself in
equilibrium with its surroundings, grow, and eventually
reproduce itself. A curious and highly characteristic
cycle of activity is thus shown; thus the animal uses
its muscular energy, derived from the oxidation of
carbohydrate, to secure more carbohydrate and other
materials which serve as sources of vital energy; and
this cycle, regulated in accordance with the varying
physiological requirements, is repeated continually
throughout life. Such facts illustrate the general depend-
ence of life upon the interchange of material and energy

25

26 PROTOPLASMIC ACTION AND NERVOUS ACTION

with the environment and explain why so large a part
of biological investigation has reference to the inter-
relations between organism and environment.

We may here recall Spencer's characterization of life
as essentially a continual adjustment of internal to
external relations/ Such an abstract definition, how-
ever, applies to many other systems found in nature; e.g.,
to any system in *' dynamic equiHbrium," such as a
candle flame, a whirlpool, or other physical system in
which there is an automatically regulated balance
between the material and energy supplied to the system
and that lost to the environment. Nevertheless, it is
pecuHarly true of organisms that their processes are of
such a kind as to maintain constantly a certain special
complex of structural and active characters in spite of
internal and external changes. The requirements for
such maintenance vary in the different cases, but certain
conditions are universal. The primary condition is
that material must be taken from the outside that will
serve (i) as a source of energy (to replace substances
consumed in. supplying this energy) and (2) as building
material for the structural substratum (protoplasm) in
which the energy-yielding transformations occur; in
this second class are included substances which do not
serve directly as sources of energy — e. g., inorganic salts.
Considered from the most general point of view, there-
fore, the living organism exhibits (i) a continual trans-
formation of material taken from its surroundings into its
own specifically organized substance; and (2) a continual
chemical decomposition of portions of this substance
of such a kind as to furnish free energy which is utilized

^ Principles of Biology.

GENERAL CHARACTERS OF LIVING ORGANISMS 27

by the organism in the characteristic activities (food-
seeking, etc.) required for its individual maintenance
and the perpetuation of its kind.

From this general point of view the simplest cases
are the most instructive; e.g., that of a single yeast
cell or bacterium introduced into a nutrient medium.
The organism grows and divides until eventually in
place of the single cell there are thousands. Evidently
the material of these additional cells comes from the
surrounding medium, certain constituents of which are
transformed into the living material or protoplasm.
The total quantity of material in the whole system,
organism plus culture-medium, is unaltered; but its
condition has undergone a profound change. A typical
nutrient solution for yeast (Pasteur's solution) contains
sugar and various salts (NaK tartrate, chlorides, phos-
phates, and sulphates of Na and K) together with water
and oxygen. From these relatively simple materials are
built up proteins, lipoids, fats, and other complex bodies;
not only are these characteristic substances synthesized
but they are distributed or arranged (partly in solid
form) in a definite and constant manner so as to give
rise to numerous complex and uniformly constituted
systems, the yeast cells. Each of these, once formed,
becomes the seat of further transformations of the same
kind; and by a repetition of this process the non-living
material of the medium is progressively transformed
into living protoplasm. The transformation is constant
and specific, chemically, structurally, and physiologically ;
''heredity" receives here its simplest manifestation.^

^ Cf. my paper, "Heredity from a Physico-Chemical Point of View,
Biological Bulletin^ XXXIV (19 18), 65.

28 PROTOPLASJMIC ACTION AND NERVOUS ACTION

All organisms and all cells without exception possess
this power, that of transforming certain materials
selectively appropriated from the surroundings into
their own specifically organized and chemically active
living substance. The materials used by different
organisms vary widely in chemical character and accessi-
bility — contrast the case of a yeast cell growing in a
culture-medium with man in his complex social environ-
ment — but in every case the essential process is the
transformation of non-living environmental material
into living substance of a constant and characteristic
organization and activity.

The general as well as the special features ot the
organization of any living being are an index of the nature
and accessibility of the environmental materials required
for its maintenance. This is well illustrated by the
general morphological and physiological contrast between
animals and plants. Since in plants constructive
metabolism begins with simple mobile or diffusible
materials (CO2, water, salts), present everywhere in the
soil and atmosphere, there is no need for locomotion;
and these organisms lead typically a stationary existence,
remaining rooted to one spot where the necessary
materials can reach them by diffusion. The typical
radiating, branching, or dichotomous habit of growth,
reaching out into all directions of space and thus provid-
ing a large area of surface for interchange, is an ''adapta-
tion" to this general environmental condition. Sessile
animals also tend to acquire a radiating plan of structure,
as illustrated in coelenterates and echinoderms. In the
great majority of animals, however, the food supplies
have to be selected from an environment containing

GENERAL CHARACTERS OF LIVING ORGANISMS 29

relatively little utilizable and much non-utilizable
material. In such a case self -maintenance demands, in
addition to the ability to move from place to place, a
selective power of reaction by which food materials
may be picked out and incorporated. Accordingly the
responsiveness to external changes (irritability, motor
activity) reaches its highest development in this group
of organisms. The development of locomotor powers
is especially characteristic of animals; related to this
is their great variety of reactions and instincts. From
such general considerations we may see in a general way
how the distinguishing or prevailing characters of each
group have arisen in evolution in correspondence with
the differences in their methods of nutrition.

In all organisms this selection of assimilable material
from the environment and its transformation into living
protoplasm proceed automatically and are regulated in
correspondence with the physiological requirements, as
these vary with the changes of activity and of external
conditions. Both the automaticity and the regulated
character of these activities are well illustrated by the
changes in the reaction of animals to food materials
during periods of ''hunger." Consumption of the
energy-yielding reserves within the living protoplasm
leads to an increased reactivity of the whole organism
to these substances. Through this means the mainte-
nance of the metabolic equilibrium is assured under the
usual conditions. Regulation of this kind is shown to
a greater or less degree by all organisms, and constitutes
a fundamental condition of self-preservation; typically
if the organism is deprived of any substance or condition
necessary for maintenance, its reactivity and behavior

30 PROTOPLASMIC ACTION AND NERVOUS ACTION

are altered in a manner tending to compensate or remove
the deficiency. Thus hunger is, physiologically speaking,
increased reactivity to food materials; thirst is increased
reactivity to water; the respiratory center of vertebrates
increases its rhythm as CO2 accumulates in the blood;
when the oxygen in the water is decreased, the gill-cilia
of the fresh-water clam beat more vigorously.^ These
and many other instances illustrate the manner in which
a physiological deficiency may itself furnish the means
of setting in motion some physiological mechanism which
remedies the deficiency.^ The end-effect of all such
regulatory responses is to further the persistence of the
organism in its environment. As already mentioned,
the term adaptive is usually applied to those special
peculiarities of structure and activity by which the
organism is automatically conserved in spite of environ-
mental change; hence, from the present generalized
point of view any active adaptation may be regarded
as a special kind of regulation. It is evident that all
such regulations are based upon a highly developed
irritability; this fundamental property of irritability,
therefore, controls all of the active relations between
organism and environment, including the interchange of
material and energy which is the essential feature of such
relations.

SPECIFIC CHARACTERS OF ORGANISMS

The constructive metabolic processes which build up
the living system involve the synthesis of a multiplicity
of new chemical compounds from the food materials and

^ Cf. Babak, Z. allg. Physiol, XV (1913), 184.

^ In Pfliiger's aphorism, in living organisms "the cause of the need
is the cause of the satisfaction of the need."

GENERAL CHARACTERS OF LIVING ORGANISMS 31

other substances (oxygen, salts, water) furnished by the
environment. Of these synthesized compounds the
most individualized and specific are the proteins. These
compounds, characteristically colloidal in their physical
properties, constitute, together with certain other
materials, chiefly lipoid, the relatively stable, solid, or^
permanent (structural) portion of the protoplasmic
complex.

It is significant that the chief structure-forming
compounds should be at the same time those which are
chemically the most specific. Specific form and structure
are the most obvious peculiarities of the living organism ;
hence species are usually distinguished by their structural
characters. It is to be remembered, however, that the
chemical and physiological characters are equally
constant and definite, and must be included in the
complete characterization of any species. The essential
fact, requiring physiological explanation, is that each
individual animal or plant resembles, structurally,
chemically, and physiologically other individuals of the
same species, while differing from those of other species.
As already indicated, the physiological basis of this
specificity is to be sought in the specific nature of the
chemical processes by which the organism is synthesized.
We find in fact that a chemical specificity, corresponding
to the specificity of the organism as a whole, is exhibited
by its constituent proteins, and apparently by these
compounds alone. The other chief biochemical com-
pounds (carbohydrates, lipins) are chemically identical
in widely differing species, while the proteins vary in
their detailed chemical character from species to species.
Apparently each native protein has a special composition

32 PROTOPLASJMIC ACTION AND NERVOUS ACTION

and stereo-chemical configuration, by which it is dis-
tinguished from the corresponding proteins of even
nearly related species. This general fact of an associa-
tion between specific chemical composition and specific
organic structure indicates, together with other evidence,
• that the specific chemical characters of the structural
proteins of any organism determine, in a manner which
cannot be defined in detail at present, its specific pecu-
liarities as an organic species/ Apparently this chemical
specificity determines the more intimate protoplasmic
structure, and hence indirectly the protoplasmic activi-
ties, chemical and other, which are the correlative of
that structure and determine ultimately the physiological
and other peculiarities of the species.

The problem of the conditions of specific form-
determination in organisms has its special physiological
aspects; but on the purely physical side its closest
affihations are with the problem of the relations between
the chemical constitution of compounds and their
crystalline or other molar structure. When similar
molecules unite to form larger molar aggregates, definite
regularities of form and structure usually make their
appearance; this is especially true when substances
separate from solution to form crystals; the axes and
angles of the crystal form are an index of the orientation
which the molecules assume as the aggregate is built
up, and of the linear proportions of the molecules.
Tn most solid compounds this association of structural
specificity with chemical specificity can be readily

^ Cf. Loeb's recent discussion in his Organism as a Whole from a
Physico-Chemical Viewpoint, New York (191 6), chap, iii, "The Chemical
Basis of Genus and Species."

GENERAL CHARACTERS OF LIVING ORGANISMS 33

demonstrated; i.e., each compound has a definite and
characteristic crystalline form, which is similar for
compounds of similar chemical configuration (law of
isomorphism). In colloidal compounds like proteins,
crystals are less easily produced, but under appropriate
conditions many of these compounds can be crystallized,
and it is then found that corresponding or homologous
proteins from different species form crystalline aggregates
which differ characteristically in their specific form-
characters. Specificity of crystalline form has been
demonstrated most clearly in the case of the haemoglo-
bins; i.e., the haemoglobin crystals of the domestic cat
dift'er in a definite and constant manner from those of
other species of the same family, and in different verte-
brates a general correlation between similarity of crystal
form and nearness of relationship can be recognized,^
Such facts indicate that as the molecules unite in the
process of crystallization to form larger aggregates,
structures are built up having definite morphological
characters which are determined by the special configura-
tion of the haemoglobin molecule. The growing crystal
mass takes on definite form characters, like the growing
germ. We may assume that in the living cell, as it grows
and differentiates, similar conditions determine the
physical state assumed by those proteins which are laid
down as microscopic aggregates or deposits to form the

^ Cf. Reichert and Brown, "The Crystallography of Haemoglobms,"
Carnegie Institution Publication No. 116, Washington (1909); also
Reichert's paper, "The Germ Plasm as a Stereochemic System," Science,
XL (1914), 649. NuttaU's work with precipitin reactions demonstrates
a similar correlation between the chemical specificity of proteins and
blood relationship. (Nuttall, Blood Immunity and Blood Relationships,
Cambridge University Press^[i904].)

34 PROTOPLASMIC ACTION AND NERVOUS ACTION

protoplasmic structures; in such a case a specific proto-
plasmic and ultimately a specific cellular structure
would be produced, corresponding to the specific constitu-
tion of the structure-forming compounds.

The general nature of the relation between stereo-
chemical configuration and crystalline form is best
illustrated by Pasteur's classical investigations on the
tartrates. The characteristic spatial arrangement of the
atoms in the d-tartrate molecule is evidently what
determines the production of the specifically formed
asymmetric crystals of this compound. Similarly con-
stituted molecules have a tendency to segregate, hence
the dextro- and laevo-groups in the solution of the
racemic salt unite separately to form separate crystals.
The importance of such conditions in the chemical
processes of protoplasm is illustrated in the characteristic
relations existing between the stereo-configuration of
asymmetric compounds and their assimilability, ferment-
ability and physiological action. The possibilities of a
specificity based on stereo-chemical configuration are at
a maximum in compounds like proteins, built up of
chains of asymmetric amino-acids. As we have seen,
chemical specificity implies structural specificity in the
aggregate formed from such molecules.

As is well known, the chief proofs of the chemical
specificity of closely related proteins are derived from
immunological and related phenomena. Antigenic prop-
erties are apparently confined to proteins, and this
peculiarity is of fundamental importance in relation to
the whole problem of the conditions of specific synthesis
in organisms. When a foreign protein is introduced into
the tissue-media of higher animals, one of its physiological

GENERAL CHARACTERS OF LIVING ORGANISMS 35

effects is to alter constructive metabolism in the cells
of the organism in a definite manner so as to give rise
to other compounds (apparently also protein) of related
or complimentary configuration. These new com-
pounds, anti-bodies, form- specific chemical unions with
the antigens, and hence may serve as a means of identify-
ing the latter or of distinguishing between nearly related
proteins, as in the precipitin and anaphylaxis reactions.
The living protoplasm responds to the presence of the
antigen by synthesizing a compound of similar or
complementary configuration; and this chemical resem-
blance is what determines the intimacy and specificity
of union in the antigen-anti-body reaction. The
anaphylactic guinea-pig is in fact the most sensitive
means at our disposal for distinguishing between proteins
of nearly related composition.'

It is evident that such phenomena have a most
important bearing on the question of the basis of organic
specificity. They indicate not only that the synthesis
of specific compounds by living protoplasm is determined
by the presence of other specific compounds, a fact of
general application in the theory of growth processes,
but also that the specific syntheses characteristic of a
species may be modified under the influence of compounds
having a different configuration from those normally
present. The indications from precipitin and other
tests are that the chemical resemblance between the
corresponding proteins of different species is greatest when
the biological relationship is closest — when the species are
structurally and physiologically most closely similar —
and in general decreases as the organic difference increases.

' Cf. S. Flexner, Science, LII (1920), 615.

36 PROTOPLASMIC ACTION AND NERVOUS ACTION

The general conclusion seems therefore justified that
the specific biological characters of an animal or plant
depend ultimately upon the specific chemical characters
of its proteins. The developing germ, or the growing
and metabolizing organism, builds up proteins of specific
constitution, and these, since they determine the specific
structural characters — with the correlative physiological
activities — of the organism, form the basis of its biological
specificity or special singularity as an organic species.
A fundamental problem, therefore, relates to the condi-
tion determining the synthesis of proteins of its own
specific type by each form of protoplasm. This problem
is as yet unsolved. Apparently the presence of proteins
of a certain composition and configuration promotes or
"catalyzes" the formation of proteins of similar or com-
plementary configuration. A general condition compar-
able with autocatalysis^ thus determines the specific char-
acter of the protoplasmic syntheses, but such a statement
merely defines the problem without solving it. The
problem, however, cannot be solved before it is clearly
defined, and its solution would unquestionably represent
a great advance in biological knowledge, since it would
involve the solution of the fundamental problems of
growth and heredity.

There is some evidence of an identity, or at least
close chemical resemblance, between the specific proteins
of adult tissues or organs and corresponding or repre-

^ For the comparison of organic growth with autocatalysis cf. J
Loeb, Biochem. Zeitschrift, II (1906), 41; T. B. Robertson, Arch. Ent-
wicklimgsmech., XXIV (1908), 581; Wfg. Ostwald, Roux's Vortrdge imd
Aufsdtze, V (1908). Chodat made a similar comparison for plant growth
in 1905 (cf. D'Arcy Thompson's Growth and Form, Cambridge Uni-
versity Press [1917], p. 132).

GENERAL CHARACTERS OF LIVING ORGANISMS 37

sentative proteins in the germ cells. Guyer^ has recently
found that the germ cells of rabbits which have been
injected with anti-lens serum (formed by immunization
in fowls injected with crushed rabbit lenses) are so
modified as to give rise in development to rabbits
having defective lenses and otherwise abnormal eyes.
These defects are transmitted hereditarily by either ova
or spermatozoa through several generations. Since
anti-bodies attach themselves to proteins of correspond-
ing configuration, these observations are evidence of the
presence of the specific lens proteins (or proteins
closely corresponding) in the germ cells. Results of an
analogous kind recently reported by Detlefsen and
Griffith may possibly have a similar significance; rats
which had been subjected to prolonged rotation gave
rise to oft'spring showing characteristic defects in equi-
librium and tendency to circus-movements, and those
abnormalities were also heritable.^

While it is difficult to believe that all, or even more
than a very few, of the proteins in the adult body are
represented by corresponding proteins in the germ, yet
it seems not improbable that there may exist some
correspondence of a general kind between the chemical
organizations of adult and germ, analogous to or parallel-
ing the general morphological correspondence which
Conklin's work^ has demonstrated between the eggs

^ Guyer and Smith, Journal of Experimental Zoology, XXVI (1918),
65, and XXXI (1920), 171. Cf. also American Naturalist, LV (192 1),
97, and LVI (1922), 80.

2 Cf. Griffith, Science, LVI (1922), 676.

3 Cf . Conklin, Heredity and Environment in the Development of
Man, Princeton University Press (1918); also his paper, "The Share
of Egg and Sperm in Heredity," Proceedings of the National Academy of
Science, HI (191 7), loi.

SS PROTOPLASMIC ACTION AND NERVOUS ACTION

and the larval stages in certain animals. That is, certain
proteins with a basic or fundamental relation to the
organization of the, species may be chemically identical
in adult and germ; and they may even be distributed
spatially in a similar way in both; e.g., with reference
to the main axes. In this sense a chemical continuity
between germ and adult may exist, corresponding to the
morphological continuity. At present, however, we
are completely ignorant regarding the details of this
correspondence and can only await the results of further
investigation.

EXPERIMENTAL MODIFICATIONS OF GROWTH
AND HEREDITY

If the metabolic production of proteins of specific
configuration constitutes the essential chemical basis of
growth and development, it must also form the basis of
heredity, since by '^heredity" is meant not a separate
phenomenon but simply the similarity of the constructive
or developmental process in the successive generations of
a particular organic species. We may therefore regard the
factors of growth as identical with the factors of heredity,
and apply the same type of physiological analysis in
both cases.

We find experimentally that while under normal
conditions development follows a highly definite and
constant course in each species, it can be altered in a
definite manner by various procedures; and a large
part of experimental embryology is concerned with
modifying the growth processes in the germ or embryo
and thus controlling the rate and character of develop-
ment. In this manner it has been shown that constancy

GENERAL CHARACTERS OF LIVING ORGANISMS 39

of development in any particular species requires
constancy in the external conditions. For example, the
developing sea-urchin larva forms a skeleton of a charac-
teristic and often complex design in sea water and in
artificially balanced media containing the chief salts
of sea water together with some sodium carbonate;
the formation of this skeleton causes the larva to assume
the triangular and long-armed shape characteristic of
the pluteus stage. But if the carbonate is omitted
from the medium, the skeleton fails to form, and
development does not proceed beyond the gastrula stage.^
The special form of the skeleton is said to be "inherited";
this experiment shows, however, that it is dependent on
the presence of carbonate quite as «iuch as on the
presence of special determinants in the germ.

Such an example shows further that constancy in
the normal sequence of growth processes is the essential
condition for the manifestation of heredity; it also
illustrates the composite nature of the physiological
factors determining the production of any adult form-
character; in all cases the co-operation of definite
"internal" and "external" factors is necessary to yield
the final result. Many cases are also known where
development is altered in a definite manner by the addi-
tion of special growth-modifying substances; a well-
known example of such influence^ exerted by a simple
inorganic substance, is the production of cyclopia in
fishes by increasing the magnesium content of sea
water ;^ other substances and conditions (alcohol

^ J. Loeb, American Journal of Physiology, III (1900), 441.

2 Stockard, Arch. Entwickl. Organ., XXIII (1907), 249; Journal of
Experimental Zoology, VI (1909), 285.

40 PROTOPLASMIC ACTION AND NERVOUS ACTION

anaesthetics, cyanide, cold) have a similar effect/
These substances hinder or suppress the growth of the
anterior region of the forebrain between the optic
vesicles so that the latter tend to approximate and
coalesce, producing a single instead of a double structure.^
The production of exogastralae from sea-urchin blastulae
by adding lithium chloride to the sea water is a similar
instance; in this case the endoderm grows outward
instead of inward.^ The transition is direct from such
simple cases of artificial chemical control of development
to the cases where various developmental processes occur
normally under the control of special chemical substances
produced by the organism itself; the influence of hor-
mones illustrates such^ cases; metamorphosis (in tad-
poles), the growth of the skeleton, and the production
of sexual characters are thus determined.

In general any condition affecting the rate or character
of the formative metabolic reactions has a corresponding
influence on growth and development. Such conditions
include the influence of physical agents like electricity,
light, temperature, contact. It is significant that the
term ''irritability" is applied, especially in plant physi-
ology, to the susceptibility of growth processes to such
modifying influences; in such cases the organism ''re-
sponds" by changing its rate or manner of growth. Any
such response implies a corresponding modification in con-
structive metabolism; hence such facts show that re-

^ Cf. Stockard, American Journal of Anatomy, X (1910), 369;
McClendon, American Journal of Physiology, XXIX (191 2), 289.

' Cf. the discussion in Child's Origin and Development of the Nervous
System, pp. 36 flf.

3 Cf . Herbst, Z. wiss. Zool., IV (1892), 446; Mitteilungen zool.
Sta. Neapel, XI (1893), 136.

GENERAL CHARACTERS OF LIVING ORGANISMS 41

sponses involving metabolic synthesis are called forth
under the same conditions as the other more familiar
types of response, such as muscular contraction in
animals, which depends more directly upon processes of
metabolic breakdown.

The importance of the relations existing between
normal growth and the normal physiological activity of
the organism has been hitherto insufficiently recognized.
Probably the main reason for this is that in the egg and
early embr^^o the development of any organ up to a
certain stage necessarily precedes its functional activity;
often, in fact, development is complete before there is any
possibility of function (generative organs, many muscular
mechanisms). In many other cases, however, normal
physiological activity is a prerequisite for normal
growth and development. Inactivity means lowered
or subnormal metabolism, and this involves subnormal
growth; frequently, when physiological activity is
subnormal, metabolic construction lags behind destruc-
tion, and regression or atrophy ("disuse-atrophy")
results. The need of activity for normal growth is most
evident in the adult stages of higher organisms, and is
especially well shown in intermittently active tissues
like voluntary muscle, where increased activity leads to
increased growth, as shown in the effects of exercise,
while disuse is followed by regression more or less
complete. Other tissues show similar conditions; the
removal of one kidney is followed by increase in the
size of the other, in correlation with the enforced increase
of activity; and valvular insufficiency in the heart leads
to muscular enlargement. Such cases of compensatory
hypertrophy are apparently an example of the above-

42 PROTOPLASMIC ACTION AND NERVOUS ACTION

cited general rule, and indicate clearly that the physico-
chemical conditions determining functional activity are
in close relation to those determining metabolic synthesis
and growth.

Claude Bernard has pointed out that in any living
system a relation of this kind must exist if the system
is to persist and retain its normal properties under
varying conditions of activity.^ All activity involves a
certain breakdown of organized structural material, as
well as of energy-yielding compounds like sugar; hence
a return to the normal or resting condition after stimula-
tion requires that compensatory or constructive processes
should be set in motion by the same condition that calls
forth the destructive or energy-yielding activity.

The general metabolism of any living system repre-
sents an ordered combination of constructive and
destructive processes; the living condition always
involves metabolic construction; as Bernard expresses
it, ''synthesis is life," even during rest. Hence the rate
of metabolic construction is to be recognized as under
the same kind of control as the rate of destruction; i.e., of
energy-production or normal activity. Growth processes
are therefore modified by any condition (cold, poisons,
H-ion concentration, salts, anaesthetics) which alters the
general activity of the living cell. The growth of the
embryo can be temporarily arrested by anaesthetization;
the same is true of seedlings and dividing cells. ^ Such

' Claude Bernard, Leqons stir les phenomenes de la vie, I, 127.

^ Bernard describes the anaesthesia of seedlings and embryos (La
Science Experimentale, Paris [1890], p. 224). For a study of anaesthesia
of cell-division, see my article in Journal of Biological Chemistry, XVII
(1914), 121.

GENERAL CHARACTERS OF LIVING ORGANISMS 43

facts illustrate the unitary character and control of the
metabolic processes underlying the various vital manifes-
tations; they show that growth and development are
controlled by the same conditions as the other forms of
protoplasmic activity. Hence stimulation is a concep-
tion which is applicable to growth processes in the same
sense as to muscular or nervous activity.

Constructive metabolism thus varies with the general
physiological activity of the living system; and this
latter activity is determined largely by the external
agents which act upon or ''stimulate" the protoplasm.
The general property of ''irritability" thus implies not
only the ability of the protoplasmic system to carry
out definite reactions in response to stimuli but also
the ability to vary its constructive metabolism in
correlation with the rate or degree of the energy-yielding
or destructive processes. Restitution, compensatory
growth, recovery from injury, or fatigue and apparently
the normal recovery of the irritable state after stimulation
are different manifestations of this constructive process.

GENERAL FEATURES OF STIMULATION PROCESSES

In general, the term "irritability," as used in physiol-
ogy, designates the universal property of living matter
by which the chemical or other activities of the living
system change, in some specific way, in response to
changes in the surroundings. We say "change in some
specific way", i.e., in a manner distinctive of the living
system, in order to separate true cases of stimulation
from cases where the chemical or other processes occurring
in the protoplasm are changed as a direct consequence
of non- vital factors. For example, within the usual

44 PROTOPLASMIC ACTION AND NERVOUS ACTION

physiological range (5°-40°) a rise of temperature of
io° more than doubles the rate of most chemical reactions
(Qio 2-3); this rule applies to many processes which,
though occurring within the living system, have in them
nothing that is specifically vital; thus the rate of hydrol-
ysis in the digestive tract, the rate of consumption of
oxygen or evolution of CO2 by living cells, and the rate
of autolysis in dead cells are all accelerated to about
the same degree by a given rise of temperature; the
same is true of chemical reactions in non-living systems;
e.g., the hydrolysis of sucrose by acid. Such accelera-
tions are not instances of stimulation in the physiological
sense; true stimulation is illustrated only when the
organism, cell, or other living system makes a response
whose characteristics can be explained only by reference
to the special peculiarities of the system as living.
Thus a muscle can be mechanically subdivided by
scissors, and the purely mechanical action is the same in
the living as in the dead muscle; but, in addition, the
former contracts, i.e., exhibits its characteristically vital
response. Or a living unfertilized starfish egg or frog's
egg mechanically treated in an appropriate way begins
a sequence of cell-divisions; the same result follows
when a starfish egg is kept at 35° for 2 minutes, or treated

with — butyric acid solution for a similar period.
200

The change in the behavior of the living system under

a given stimulating condition is normally a constant one,

but the special nature of this change is determined by

the specific organization or "inherited" character of

the system, as well as by its physiological state at the

time. Hence the same change of conditions acting as a

GENERAL CHARACTERS OF LIVING ORGANISMS 45

stimulus may produce entirely different effects upon
different irritable systems, or upon the same system at
different times. For example, the same intensity of
light will repel one group of animals, and attract another;
mechanical treatment may arouse increased activity in
one motor organ (a muscle) and inhibit it in another
(the swimming plate of a ctenophore). The case just
cited is interesting as illustrating another general feature
in the behavior of irritable systems; the swimming plates
of Mnemiopsis or Eucharis beat rhythmically with
considerable regularity, but instantly cease movement
when mechanically stimulated in the presence of sufficient
Ca salts; e.g., in sea water or artificial media containing
calcium; but in similar media containing no calcium,
mechanical treatment entirely fails .to inhibit the move-
ment, and on the contrary accelerates it.^ This instance
shows that the same external change of condition may
produce different effects in the same tissue according to
its physiological state at the time; under one condition
there is an inhibitory, under another an acceleratory
response. Electrical stimulation of the nerve supplying
a voluntary muscle causes the latter to contract; but
the same stimulus applied to the cardiac branch of the
vagus nerve inhibits contraction.

Such examples illustrate the distinction between the
stimulating effect of an agent or change of condition
upon an irritable living system, and the direct effect
which it produces by its purely physical or chemical
action upon the system. Superposed upon and sequent
to the direct physico-chemical effect is the special or
physiological effect, the nature of which depends on the

^ R. S. Lillie, American Journal of Physiology, XXI (1908), 200.

46 PROTOPLASMIC ACTION AND NERVOUS ACTION

specific vital properties of the system. The given
physico-chemical change calls forth or occasions a
definite change of activity peculiar to the system.
The physiological problem of stimulation has reference to
the physical and chemical nature of the conditions
under which this specific vital reaction is called forth.

Only a special acquaintance with a given living
system or organism can enable us to predict what its
behavior will be under a given stimulating condition.
Irritability as such, however, is a property which is
manifested under comparatively uniform conditions in
all organisms; i.e., the tendency to respond to certain
kinds of physical change is very widely distributed, if
not universal. Such responsiveness is a general character
of living matter and is largely independent of special
features of structure and organization. Thus apparently
all forms of protoplasm are influenced in their activity
by the electric current; in many cases — nerves, certain
receptors, muscles — very weak currents are sufficient
for stimulation; i.e., induce a sudden and profound
change in the activity of the system; in other cases the
sensitivity to the current is less and the response is
more gradual. The electrical sensitivity of living matter
is in fact one of its most characteristic peculiarities.
Evidently there is some feature of protoplasmic structure
or organization, common to all cells and organisms,
that renders all responsive to electricity, although in
varying degrees. The same is true (though perhaps less
universally) of mechanical influences or change of
temperature. Chemical sensitivity is also universal;
since all living matter depends for its existence upon the
incorporation and transformation of the assimilable

GENERAL CHARACTERS OF LIVING ORGANISMS 47

materials present in the surroundings, the existence of a
highly developed responsiveness to the external chemical
conditions is to be expected. It is especially remarkable
that certain groups of compounds — the lipoid-solvent or
anaesthetizing group — have a similar reversible depres-
sant action on protoplasmic activities in all organisms,
from bacteria to higher plants and animals.

We may class therefore as universal properties of
protoplasm: (i) electrical sensitivity, and (2) sensitivity
to the presence of special chemical substances in the
surroundings. In studying the problem of the conditions
of stimulation we are thus brought to consider more
especially the reactions of living matter to electricity
and to chemical substances in the environment. The
fundamental or essential features of protoplasmic
structure and composition must be those which determine
the special responsiveness to influences of these two
kinds.

CHAPTER IV

GENERAL PECULIARITIES OF PROTOPLASM AS
A PHYSICAL SYSTEM

In considering the general peculiarities of living proto-
plasm, it is essential to recognize that its characteristic
properties and activities depend upon features of composi-
tion and structure which are kept in permanent existence
only through a continued process of compensation,
consisting in the metabolic construction of new and
specific compounds to replace those broken down or lost
in vital activity. Without this continual automatic
renewal and repair the system is an unstable one and
cannot persist. Physical diffusion and the normal
chemical processes of oxidation and hydrolysis all act
toward producing a disintegration of the system; these
effects are well seen in experiments on autolysis; the
dead cell digests itself and its soluble constituents diffuse
into the surroundings. During life the structural and
chemical integrity of the system is maintained by means
of its continued synthetic activity; the cessation of this
activity is the essential change in death.

This general conception of living matter, as a system
which holds its own through a balance of constructive
and disintegrative processes, is fundamental in physi-
ology. Other systems exhibiting an analogous type of
equilibrium, i.e., between constitutive and disintegrative
processes, are of frequent occurrence in nature, and have
been classed by Ostwald as ''stationary systems.'"

^ Ostwald, Vorlesungeji iiber Naturphilosophie, Leipzig (1902),
chaps, xii, xv.

48

PROTOPLASM AS A PHYSICAL SYSTEM 49

Whirlpools, candle flames, waterfalls are examples.
Such systems also exhibit a constant configuration, and
are the seat of special activities, in which access of
material and energy from without balances or com-
pensates the tendency to disintegration resulting from
their own activity and the environmental influences.
As with living organisms, their integrity depends upon
continual and balanced interchange with the surround-
ings. A further general resemblance is that they fre-
quently possess permanent features of form and structure
which would be impossible as characters of systems in
static equilibrium. Such types of equilibria — in which
opposed active processes (rather than opposed pressures,
tensions, or potentials) have equal and opposite resultant
effects, so that the system as a whole retains constant
properties — are often called ''dynamic" or ''kinetic"
equilibria. The possibilities of complex structure, and
of correspondingly complex activity, are at a maximum
in systems of this constitution; this is readily seen when
we contrast a fountain with still water, or a candle
flame or fireworks with their components in static
equilibrium. We may say that in such systems the
possibilities of the fourth or time dimension are added
to those of the three spatial dimensions.

Living matter, as a system exhibiting a dynamic
equilibrium of the special kind^ already indicated,
exhibits many characteristic peculiarities, both of
structure and activity, which are derived from^ this
fundamental feature of its constitution. All living
organisms consist largely of structures which would not
be possible, as permanencies, if the structural materials
were not being continually formed and deposited in such

50 PROTOPLASMIC ACTION AND NERVOUS ACTION

a way as to offset the continual breakdown; and these
structures subserve or render actual many activities
which would be impossible in any other kind of system.
In general, the activities which are most characteristic
of living as distinguished from non-living systems
belong in this class. We may thus understand, on the
basis of the general properties of systems in stationary
equilibrium, the possibility of the existence of material
systems of such complex structure and activity as living
organisms. The power of regulation exhibited by
stationary systems, i.e., of returning to the original
state after disturbance, is one of the chief properties
which they exhibit in common with living systems.
So long as the constitutive processes continue in action
such a result is to be expected. The permanence of
such delicate structures as filaments, films, nerve pro-
cesses, and the other finer products of the formative
activity of protoplasm depends on this continual auto-
matic synthesis, which compensates the tendency to
physical breakdown.

When any irritable organism or cell responds to
stimulation, the energy for the response is derived from
the chemical energy of the protoplasmic constituents,
usually from the oxidation of carbohydrates. It is
clear therefore that one of the essential effects of the
stimulus is to alter the rate or character of cell-metab-
oHsm. In many cases this effect may be indirect; e.g.,
in the voluntary muscle cell a large part of the heat-
production following a single stimulus succeeds the
contraction^ (Hill); similarly in the turgor-motors of

^ Cf. A. V. HUl, Journal of Physiology, XLII, XLIV, XLVI, XL VII
(1911-13); also Ergebnisse der 'Physiol., XV (1916), 340; Physiological
Reviews^ II (1922), 310.

PROTOPLASM AS A PHYSICAL SYSTEM 51

plants and possibly in other motile organs. But in all
cases the work performed in the response represents
energy derived from metabolic breakdown, although this
energy may act in the intervals between stimulation by
developing a tension or turgor which is released only at the
moment of stimulation. A fundamental problem thus
arises with regard to the general nature of the conditions in
living matter which render its rate of chemical reaction so
readily alterable by physical changes in the system.

The most significant general fact is that it is only
while the cell is living that its rate of metabolism is
readily and quickly changed by a stimulating condition.
In general, also, it is only during life that the energy-
yielding forms of metabolism have a high rate or intensity;
typically COa-production, heat-production, and con-
sumption of oxygen decrease greatly at death, although
they may not cease entirely. One of the most remarkable
peculiarities of living protoplasm, considered as a
chemical reaction-system, is that its chief energy-
yielding reactions, e.g., oxidation of sugar, proceed
rapidly at low temperatures, and in a medium which is
approximately neutral. To produce a corresponding
speed of reaction in vitro, high temperatures or strong
reagents are required. It is probable that the conditions
which determine the susceptibility to stimulation are
the same as those which are responsible for the high
velocity of the energy-yielding reactions. The nature
of these conditions is imperfectly understood at present;
but apparently they are especially favorable to certain
types of oxidation; e.g., of carbohydrates.

The indications are that structural rather than purely
chemical factors are of chief importance, since the

52 PROTOPLASMIC ACTION AND NERVOUS ACTION

oxidation-furthering enzymes (oxidases) extractable from
the cell have a relatively slight influence on the physi-
ologically important oxidations; e.g., of sugar. We may
thus regard the possession of a certain type of structure,
characteristic of the living state, as chiefly responsible
for the facility with which chemical reactions proceed in
living protoplasm, as well as for their modifiability
under stimulating conditions. The synthetic reactions
appear to be largely dependent upon the oxidations;
this is indicated by the importance of oxygen for growth
processes, as well as by various other facts, although the
precise nature of this interdependence is not understood
at present. The whole problem of the relations between
the structure of protoplasm and its chemical activity
is one of fundamental interest, and some of the more
general facts and considerations bearing on this problem
will now be briefly reviewed.

CHEMICAL REACTIVITY OF LIVING MATTER AS
RELATED TO STRUCTURE

All Hving matter is characterized by the possession
of a certain structural organization or permanent
arrangement of components which is essential to its
normal activity. ' If we destroy protoplasmic structure
by heat, mechanical injury, or chemical treatment, the
specific metabolic activity of the system and its respon-
siveness to stimulation are lost.

In general, the chemical reactions of living matter
may be grouped under two classes according to their
relation to protoplasmic structure: (A) those reactions
which continue in an essentially unaltered manner after
the ^4ife" of the cell has been destroyed; e.g., in cell-

PROTOPLASM AS A PHYSICAL' SYSTEM 53

extracts or in the residue remaining after complete
mechanical or other disintegration of the protoplasm;
and (B) those which continue only while the protoplasm
remains structurally intact and 'living." The former
group (A) includes a large number of hydrolyses and
some oxidations; e.g., those due to oxidases; but, as
already indicated, the physiologically significant oxida-
tions, especially of sugar and other energy-yielding
compounds, cannot be accomplished, at least with
anything like the normal velocity and completeness,
under the influence of enzymes or cell-extracts. Yeast
cells which have been mechanically destroyed, or even
simple watery extracts of yeast, rapidly hydrolyze cane
sugar, just as does the living cell, and autolyzing yeast
cells split proteins rapidly into amino-acids. It has been
found, however, that the alcoholic fermentation of sugar
proceeds much more slowly in the press-juice of yeast
than it does under the influence of the living protoplasm.^
Many other cases are known where biochemical reactions,
although proceeding in dead cells or under the influence
of cell-extracts, do so at a slower rate than in living
protoplasm.

The latter group of reactions (B) include the specific
syntheses, i.e., of protein, together with those syntheses
which require the expenditure of considerable energy,
like the building up of fats from carbohydrate, or of
amino-acids and other compounds of high chemical
potential from compounds of lower potential. The
energy required for these syntheses is apparently de-

^ Cf . Harden, "Alcoholic Fermentation," in Monographs on Bio-
chemistry, edited by Plimmer and Hopkins.

54 PROTOPLASMIC ACTION AND NERVOUS ACTION

rived from the oxidation of other compounds, especially
carbohydrates.^

It is especially signiiicant that in all cases the synthesis
of specific proteins, the reactions essential to growth
and maintenance, requires the intact protoplasmic
structure. These syntheses constitute the chemical
reactions most highly characteristic of the living state.
At one time it was believed that all of the metabolic
syntheses were the result of the special activity of
living protoplasm, and that the chemical reactions of
dead protoplasm or cell-extracts were always of a
catabolic (splitting) kind; it is now known, however,
that various syntheses involving little change of energy,
e.g., the synthesis of esters and disaccharides, readily
occur under the influence of enzymes alone. Yet the
fundamental fact remains that the more important or
specific part of the synthetic activity of protoplasm is
exhibited only during life. A relation of the normal
protoplasmic structure to certain types of chemical
action, especially synthetic action, is thus indicated.
With the alteration of structure occurring at death, as
indicated by loss of semi-permeability, coagulation of
cell-proteins, and other phenomena of disintegration, is
associated a loss of synthetic power. Only the living
yeast cell can build up from a solution of sugar, tartrates,

^ Hence the importance of carbohydrates for growth and assimila-
tion; e.g., in plants, carbohydrate is indispensable for the assimilation
of amino-acids by yeast and molds (cf. the series of papers by F. Ehr-
lich, Biochem. Zeitschrifi, I, VIII, XVIII, XXXVI (1906-11); similarly
in higher plants the synthesis of proteins from amides in germination
requires the presence of carbohydrates (cf. Jost's Physiology of Plants,
p. 175, for a summary of the chief facts). The sequence of metabolic
derangements associated with diabetes shows the fundamental importance
of carbohydrate metabolism in higher animals.

PROTOPLASM AS A PHYSICAL SYSTEM 55

and inorganic salts the various special compounds,
definite and constant in number, proportions, and
distribution, which compose the yeast protoplasm.

Some of the changes in the chemical reactivity of
protoplasm resulting from mechanical or other destruc-
tion of the living cells or tissue are well illustrated by
Fletcher's and Hopkins' work on the formation and
disappearance of lactic acid in muscle;^ also by the work
of Harden and Maclean on oxidation by isolated animal
tissues f and more recently by Warburg's determinations
of the oxygen consumption in living cells (sea-urchin
eggs, blood corpuscles, bacteria, etc.) as compared with
that of the same cells after death or fine mechanical
subdivision.^ In all of these cases chemical activity
is greatly decreased when the protoplasmic structure is
artificially destroyed. Warburg has also shown that
when certain cells, the blood corpuscles of birds, are
mechanically broken down by freezing and thawing,
the oxygen consumption exhibited by the residue is
associated with the more solid part of the complex — that
which can be separated by centrifuging ; similarly, in
liver cells the separable granules have a relatively high
oxygen consumption."* A relation of oxidative activity
to the solid part of the protoplasmic structure is thus
indicated. In some cases it can be shown micro-
chemically that certain oxidations (the indophenol
reaction) occur most actively at the surfaces of solid

^ Fletcher and Hopkins, Journal of Physiology, XXXV (1907), 247.
* Harden and Maclean, Journal of Physiology, XLIII (1911), 34.

3 Warburg, Ergebnisse der Physiol., XIV (1914), 253.

4 Warburg, cf. Biochem. Zeitschrift, CXIX (1921), 134, and references
to earlier papers there given.

56 PROTOPLASMIC ACTION AND NERVOUS ACTION

structures, such as the nuclear and plasma membranes.^
It is interesting to note that a visible alteration or
breakdown of protoplasmic structure seems always to
be associated with the death process, however induced;
even after natural death, coagulative or other alterations
occur in most forms of protoplasm; death rigor, increased
permeability, and loss of tensile strength in muscle cells,
are examples of such effects. The death change involves
a structural disintegration, with which is associated a
loss of normal chemical activity.

A second class of cases, in which certain chemical
reactions may be promoted instead of hindered by the
breakdown of normal cell structure, also throws light
upon the relation of structure to the chemical activity
of protoplasm; an example is the autolytic breakdown of
proteins or of glycogen in dead liver cells or other
autolyzing cells. The rate of such breakdown is
increased when the structure is altered by death, and
still more so (according to Chiari's observations) in
the presence of lipoid-solvent compounds like chloroform.^
Such facts illustrate another form of chemical control
exercised by protoplasmic structure. Apparently they
indicate that a partitioned or alveolar structure exists
during life; enz}Tiie and substrate, for example, may thus
be kept apart while this structure is intact, but on death
the interalveolar partitions are broken down and inter-
action results. It has been suggested by Hofmeister^
that this chambered type of architecture is what renders
it possible for a variety of chemical reactions to occur

* R. S. Lillie, Journal of Biological Chemistry, XV (1913), 237.
^ Chiari, Arch, exper. Path. Pharmakol., LX (1909), 256.
3 Hofmeister, lac. cit.

PROTOPLASM AS A PHYSICAL SYSTEM 57

within the limits of a single cell without mutual inter-
ference; different metabolic processes are thus localized*
a necessary condition for a definite "chemical organiza-
tion" of the cell.

It is well known that when living protoplasm is
acted upon by cytolytic agents or heat (40°) or is altered
mechanically or osmotically (i.e., by hypertonic or
hypotonic media) beyond a certain degree, chemical
changes are induced in it which are absent or inappreci-
able under normal conditions. These changes are as-
sociated with profound structural alteration, as shown
by coagulation of the cell-proteins, changes of permea-
bility and water content, and loss of the normal tensile
and other mechanical properties of the protoplasm.
Thus in muscle, and probably in most other cells, lactic
acid is formed in large quantity; in many cells autolytic
changes are initiated; in oxidase-containing fruits and
tubers (apple, potato) the browning reaction occurs;
and in many cases (muscle) there is a marked temporary
increase in the output of CO2. Of special interest is
the fact that these changes are associated with a loss
of the normal semi-permeability of the plasma mem-
branes, coincidently with a loss of the characteristic
water-immiscibility of the protoplasm as a whole; hence
disintegration by diffusion processes follows rapidly.
These effects are such as might be expected to result
from a breakdown of the normal partitioned structure
of the system. Materials which during life are kept
apart by the interposition of films are thus enabled to
interact; hence (as already cited) autolysis is accelerated
by cytolytic compounds like chloroform. For a similar
reason the minuter structural elements — which normally

58 PROTOPLASMIC ACTION AND NERVOUS ACTION

are prevented by the pervading film-stnicture from fusing
or otherwise losing their identity — undergo alteration or
breakdown; a general coarsening or increase of opacity,
indicating coagulative changes in the cell proteins, is
characteristic of dying protoplasm, and is associated with
the changes in mechanical properties described above/

The basis of these changes is insufficiently understood
at present, but their ready production by lipoid-solvent
compounds seems to indicate that the lipoid constituents
of the protoplasm are specially involved. Apparently
the lipoids have a relation to the protein constituents
resembling that which a "protective colloid" (gelatine)
added to a suspensoid hydrosol (gold) has to the colloidal
particles of the suspensoid. In the presence of the
protective substance the particles remain separate under
conditions, such as the presence of salts or increase of
H-ion concentration, which otherwise lead to fusion or
precipitation;^ this stabilizing influence is apparently
dependent on the formation of thin adsorption films
about the particles. In the case of living protoplasm,
the evidence from cytolysis and similar phenomena
indicates that the normal fine subdivision of the struc-
tural proteins — shown by the characteristic translucency
during life — is dependent on the presence of thin lipoid
films (possibly soap) at the surface of the protein particles,
fibrils, or other structural elements. When these films
are broken down or destroyed, a coalescence of particles
and a coarsening of structure result; these effects involve
a loss of semi-permeability, together with the changes

^ The progress of structural changes of this kind can be followed by
the microscope under dark ground illumination; cf. Aggazzotti, Z. allg.
Physiol., XI (igio), 249.

" Zsigmondy, Colloids and Ultramicroscopy.

PROTOPLASM AS A PHYSICAL SYSTEM 59

in mechanical and chemical properties already described.
On such a view the cytolytic action of lipoid-alterant
compounds may be explained. Such compounds act
by destroying the film-structure; hence, in addition to
destruction of semi-permeability, they break down the
intracellular, partitions and induce chemical reactions of
the above-described kind and cause coagulation of the
cell-proteins. In irritable cells such compounds have
also a strongly stimulating action of an irreversible kind,
as shown in the contraction produced in muscle cells,
and similar effects.

The above-described loss of translucency accompany-
ing cytolytic or mortiferous processes is a phenomenon of
much interest, which has an intimate bearing on the
general problem of protoplasmic structure. This change
is shown with great clearness in all of the more transpar-
ent forms of protoplasm; e.g., the eggs of marine animals
(starfish, etc.), protozoa, and muscle cells. Some years
ago while studying the conditions of activity in the cteno-
phore swimming plate — a beautiful example of a clear
translucent protoplasm, consisting of parallel contractile
fibrils (fused ciha) — I was struck with the constancy and
definiteness of the relations existing between changes of
translucency and changes of contractile activity. In
dying animals the plates become partially clouded and
adopt a rapid unintermittent movement, differing from
the normal movement in being of quicker rhythm and in
no longer showing the mechanical inhibition described
above; this movement continues until finally the plate
becomes white and opaque and all activity ceases.^

'R. S, Lillie, American Journal of Physiology, XVI (1906), 117;
XXI (1908), 200.

6o PROTOPLASMIC ACTION AND NERVOUS ACTION

A similar cycle of alteration is passed through, only
more rapidly, when the normal plates are transferred
from sea water to various unbalanced solutions, such as
pure isotonic NaCl; the plates then exhibit for a brief
period (one or two minutes) an extremely active vibratory
movement, which is associated with a progressive
whitening or coagulation. In general the rate of coagula-
tion is more rapid the more energetic the contractile
activity; and it is especially noteworthy that the coagula-
tive process does not begin until the plate starts vibrat-
ing; the vibration then continues until the whole struc-
ture is opaque. This change of structure is irreversible,
and at the end the plate is so altered in consistency that
it readily falls to pieces when shaken. Evidently the
contractile activity is associated with the removal of some
substance or condition which prevents the coalescence of
the protein particles forming the fibrils. A film-structure
of the kind suggested above seems indicated, which is
broken down by the action of the solution with the
production of both chemical and mechanical effects.
The general relations between such effects and stimula-
tion processes will be considered in more detail below.
Apparently in the swimming plate the essential effect
produced by the unbalanced solution is an acceleration
or intensification of the normal processes of stimulation
and contraction; a dependence of these processes on
the alteration or removal of film material is thus indicated.
The indications are that during the normal rhythm of
contraction in sea water the film-structure is alternately
broken down and reformed in each contractile cycle.
Presumably under the abnormal conditions resulting
from the action of the pure NaCl solution the rate of

PROTOPLASM AS A PHYSICAL SYSTEM 6i

breakdown is increased, and the restoration of film-
structure between successive contractions becomes im-
perfect, with the result that eventually the whole struc-
ture disintegrates. These effects may be compared with
those of excessive fatigue, which also leads to irreparable
structural breakdown.^

STRUCTURE OF PROTOPLASM

It will be evident from the preceding discussion that
structure is only one factor in the chemical activity of
protoplasm; undoubtedly many other factors — those
entering in all chemical reactions, such as concentration,
temperature, special affinities, catalysis — enter in deter-
mining the rate and character of the metabolic reactions.
But the controlling factor — that which is subject to
rapid and reversible alteration under the influence of
stimulating agencies — appears to be the peculiar structure
of the living substance. By the conception of ''struc-
ture" as applied to protoplasm is meant, generally
speaking, the distribution of the physically stabler
components, usually the solid components, of the system.
Evidently, as already pointed out, this structure is
itself a product of metabolism; but having once been
formed, it influences the further course of metabolism — in
the general manner of which Child's comparison of the
living organism to the flowing river^^ gives a good illustra-
tion by analogy. In the living organism there is always
structure of a definite kind; even the simplest ''undiffer-

^ Cf. the instances of structural alterations in the central nervous
system described in Crile's recent book, A Physical Interpretation of
Shock, Exhaustion, and Restoration, London (1921).

^ Cf. Child, The Regulatory Process in Organisms, Journal of Mor-
phology, XXII (1911), 171; also Senescence and Rejuvenescence, chap. i.

62 PROTOPLASMIC ACTION AND NERVOUS ACTION

entiated" protoplasm is not homogeneous, and it is
necessary to reach a clear conception of the essential
nature of this structure in the most generalized forms
of living substance if we are to be in a position to under-
stand the fundamental conditions of physiological activity.

That metabolism is controlled by structure is seen
in many well-known physiological facts already referred
to in part; e.g., the course of development, with the
associated constructive metabolism, may in many eggs
or embryos be profoundly modified by artificially
altering the structure of the system. Developmental
processes are frequently initiated by mechanical means;
cases of regeneration illustrate this, or cases where
injury of the egg-surface (pricking in the case of the
frog's egg,^ or any kind of cytolytic action in echinoderm
eggs)"^ initiates cleavage and development. Mechanical
treatment causes stimulation in innumerable instances;
in others it causes inhibition. In all of these cases the
energy for the developmental or other response comes
directly or indirectly from metabolic processes. This
sensitivity to the action of mechanical agents, which by
their impact, pressure, or other effects locally modify
cell structure, is perhaps the clearest proof of the intimate
relations existing between structure and function in
living protoplasm.

In physical chemistry the importance of structural
conditions as modifying factors in chemical reactions is
illustrated in the so-called heterogeneous catalyses.
In these phenomena the acceleration of reaction is

^ Guyer, Science, XXV (1907), 910; Bataillon, Arch. zool. exper.
et generate, XL VI (19 10), 103.

^ Loeb, Artificial Parthenogenesis and Fertilization, University of
Chicago Press (1913).

PROTOPLASM AS A PHYSICAL SYSTEM 63

dependent chiefly on surface effects, of which two classes
appear to be especially important from the biological
point of view: (i) adsorption effects, leading to increased
concentration at surfaces and hence increased reaction-
velocity; and (2) electrolytic effects, due to the existence
of local potential differences between different regions of
the surface separating the two phases; when both phases
conduct electricity, local circuits may thus arise, furnish-
ing the conditions for electrolysis. This latter effect may
also be regarded as a form of catalysis, and is illustrated
in the spreading of rust spots on iron surfaces, or the
periodic catalysis of H2O2 by mercury. Both kinds of
effects are of fundamental importance in protoplasmic
processes, as will be shown in more detail later. Other
conditions characteristic of surfaces may also enter
(see pp. 217 ff.).

As already pointed out, all forms of protoplasm
exhibit the power of specific synthesis characteristic of
life. The constructive metabolism by which the specific
structural elements are built up and maintained must,
like other forms of metabolism, be under the control of
structure. This synthetic activity, being a universal
property of living matter, is undoubtedly to be correlated
with the most general or fundamental type of structure
exhibited by protoplasm. In correspondence with its
uniformity of essential chemical composition and chemical
behavior, protoplasm must also possess a uniformity in
its essential type of physical structure; underlying the
variety of structural detail must be some characteristic
type of structural composition common to all forms of
protoplasm, and determining the special features of its
chemical activity. The traditional problem of the

64 PROTOPLASMIC ACTION AND NERVOUS ACTION

structure of protoplasm is thus intimately bound up
with the basic problem of general physiology.

The problem relates to the nature of the structure
in livmg protoplasm. All observers agree that cell
structure is profoundly altered by death; disintegration
then follows, accompanied by diffusion of the cell
constituents into the surrounding medium. As already
described, the death of the cell is associated with loss of
its normal osmotic properties or semi-permeability; the
normal electrical polarization also disappears at the same
time; both phenomena are characteristic, and indicate
interruption in the continuity of the protoplasmic
boundary layer. The most obvious general structural
changes occurring in the cell interior at death are of a
coagulative kind ; the protoplasm loses its normal trans-
lucency and becomes more opaque (death rigor or death
coagulation). This effect is seen in the greatest variety
of cells and organisms, especially those with translucent
protoplasm, as cited above. The protoplasm of muscle
cells becomes more opaque and loses its coherency or
tensile strength; dying swimming plates whiten and fall
to pieces on shaking, and other phenomena of a similar
kind are well known to all biologists. Many observations
on the postmortem alterations of structure have been made
since the introduction of the methods of microdissection.
Kite and Chambers describe dying cells as losing their
viscidity and as being easily torn to pieces. Chambers
describes the isolated nerve-ganglion cells of the lobster
as undergoing irreversible structural changes when
mechanically injured; the protoplasm then ''sets into a
coagulated non-viscous mass which may be broken into
non-glutinous pieces." Taylor describes a similar break-

PROTOPLASM AS A PHYSICAL SYSTEM 65

down of protoplasmic structures in Protozoa after injury
with the microdissection needle.^

Facts of this kind show again that the maintenance
of a certain characteristic type of structure is an essential
part of normal protoplasmic activity. The structure of
living protoplasm is not to be conceived as resulting
from a combination of static parts like the structure of a
machine; it is the product or expression of continual
synthetic activity and persists only while metabolism
persists; it expresses the constructive activity of metab-
olism, very much in the same manner as the structure
of a flame or of a fountain expresses the dynamic activity
of such a system. If the activity disappears, so also
does the characteristic structure or configuration which
is maintained by that activity. In this sense, structure
in living protoplasm is to be conceived as continually
in process of formation; i.e., as an index of the underlying
synthetic reactions which, as already seen, are inseparable
from the chemical activity of the system during life.
The apparently static condition represents in reality a
state of balance between construction and disintegration.

Yet a certain permanent or stable structural consti-
tution (at least relatively permanent) has to be assumed,
just as in the case of the fountain or candle flame. This
is necessary if the dependent processes of chemical trans-
formation are to exhibit constant characters. The
physical nature of this permanent or persistent structural
substratum of living protoplasm has first to be considered.

^ Kite and Chambers, ^dmce, XXXVI (191 2), 640. Chambers, Trans.
Royal Soc. Can., XII (1918), Series 3, 43; Taylor, University of Calif ornia
Publications, XIX (1920), 403, cf. pp. 420, 424, 434. Mention has already-
been made of Aggazzotti's observations with dark ground illumination on
the structural changes produced in blood corpuscles by cytolytic agents .
Cf. also Traube and Klein, Biochem. Zeits., CXXX (1922), 477.

CHAPTER V

PHYSICAL NATURE OF PROTOPLASMIC STRUCTURE:
IMPORTANCE OF SURFACE CONDITIONS

It is not possible here to review in detail the numerous
and frequently conflicting conceptions of protoplasmic
structure. The details made visible by microscopical
technique are of so varied a kind that none of the many
attempts at unification have met with universal agree-
ment. A chief difficulty has been that most histological
investigators seem to have conceived of protoplasmic
structure as existing independently of the chemical and
physiological activities of the living system, and not as
both dependent upon and determining these activities.
Some conception of structure is required which will be
general enough to apply to all of the forms of living
matter, and which will at the same time enable us to
understand the dependence of the fundamental vital
properties of specific synthesis and irritability upon
structure. It may be doubted whether we are yet in a
position to form a clear and permanently valid concep-
tion of protoplasmic structure, but with the progress in
our knowledge of the properties of colloidal systems has
come what appears to be an increased insight into the
possibilities. The problem may be defined in its essential
terms, as follows: Can a system, with components of the
kind which we find present in all living matter, be ima-
gined which will exhibit, as a correlative of its structural
composition, the above-described properties of specific
growth, sensitivity to electrical conditions, catalytic

66

PROTOPLASMIC STRUCTURE 67

activity, and automatic regulation of composition and
properties ?

The experimental studies and observations of the last
twenty years have led more and more to the conclusion
that the general or fundamental structure of protoplasm
corresponds more closely to that of an emulsion than to
that of any other simple non-living physical system.
The most general facts of its chemical composition are in
agreement with this conclusion. Water-insoluble con-
stituents (lipoids) occur in association with colloidal
constituents which have water-combining powers (pro-
teins). The whole resulting complex is during life
immiscible with water, and typically is bounded from
the external watery medium forming its immediate
environment by a layer or surface-film having semi-
permeable properties. The semi-permeability and the
water-immiscibility of the surface layer appear to be
interdependent properties; they suggest the existence of
a continuous external layer of water-insoluble material
of fatty or similar nature.^ The unit of organic struc-
ture, the cell, would thus appear to be a system with
an aqueous internal phase limited externally by a thin
water-insoluble phase or boundary layer. The aqueous
internal phase forms one component of a system, the
cell protoplasm, which is structurally and chemically
highly complex, and emulsion-like in its general physical
constitution.

It is evident that the general properties of emulsions
do not in themselves explain the properties of living
matter. What seems highly probable, however, is that
the original structural foundation upon which the proper-

^ Cf. Quincke, Ann. Physik., XXXV (1888), 580; cf. pages 629-30.

68 PROTOPLASMIC ACTION AND NERVOUS ACTION

ties of living matter have arisen — or which has made it
possible for systems with vital properties to evolve —
is that of an emulsion; i.e., a polyphasic system with thin
interfacial films separating two or more component
phases which have fluid or solvent properties. From
general considerations it seems clear that some kind of
pol}^hasic structure must be assumed in order to account
for such a universal property as that of growth; the unit
of living matter, even while it continues to increase in
size, retains a complex and specific composition different
from that of the surroundings; and this peculiarity is in
itself incompatible with structural homogeneity, since
the elementary need of providing against free diffusive
interchange with the surroundings requires a surface
layer with properties different from those of the internal
protoplasm. This must be true even of the simplest
forms of living matter. We cannot compare the proto-
plasm of ultra-microscopic organisms with self-propa-
gating enzyme-like material (supposing such material
possible), as has been done, since the physical con-
ditions necessary for metabolism and growth must
exist in even the simplest living systems; and this
requires at the very least a differentiation between the
more permanent or solid components of the system and
the liquid components which contain in solution simpler
materials (nutrients and oxygen) which are continually
being renewed.

It has long been recognized that colloids form the
basis of protoplasmic structure. Hardy's investigations
showed that many of the characteristic structural
appearances presented by fixed and stained protoplasm
in microscopic preparations were incidental consequences

PROTOPLASMIC STRUCTURE 69

of the colloidal composition of the system and not expres-
sions of any distinctively vital condition or structure.
He and others have demonstrated that similar appear-
ances can be produced by fixation in apparently homo-
geneous colloidal solutions or gels (egg-white, gelatine).^
Hardy also pointed out various parallels between the
processes of gelation in artificial colloidal systems and the
changes of physical state in living protoplasm.^ From
these and related facts it became clear that if we are to
draw conclusions regarding protoplasmic structure from
the appearances seen in microscopic preparations, the
general nature of the changes produced in colloidal
systems by physical and chemical agents must first be
determined. Great impetus was thus given to the
study of the physics and chemistry of colloids, a subject
then in its early stages, and also to the study of the
structure and physical properties of protoplasm in the
living condition.

Various resemblances between living protoplasm and
emulsions were long ago described by Biitschli.^ These
resemblances relate both to structure and to certain
peculiarities of behavior; e.g., amoeboid movement
and modifications of activity by changes in the sur-
roundings. Biitschli reached the conception that a
''foam structure," corresponding essentially to a film-
pervaded or chambered structure, is the t>"pe most
generally exhibited by living protoplasm."^

*W. B. Hardy, Journal of Physiology, XXXV (1899), 158; cf. also
Alfred Fischer, Fixierung,Farbung,2md Ban des Protoplasmas, Jena. (1899).

2 Hardy, Proceedings of the Royal Society, LXVI (1899), no.

3 Biitschli, Microscopic Foams and Protoplasm.

4 Cf . the discussion by E. B. Wilson, Jour. Morph., XV (1899),
Supplement.

70 PROTOPLASMIC ACTION AND NERVOUS ACTION

The subject of the physical chemistry of emulsions
forms a part of the now extensively developed field of
colloid chemistry, and cannot be considered here in
any detail. Some of the more general facts relating to
the structure and properties of emulsions must, however,
be discussed briefly, since a clear conception of the physi-
cal conditions existing in these systems is necessary
before proceeding to the consideration of the more
complex types of structure and behavior which have
evolved in living matter, apparently with emulsion-
systems of a relatively simple kind as a basis.

EMULSIONS'^

Emulsions and foam structures are essentially similar
systems, with the difference (as usually defined) that in a
foam the disperse or discontinuous phase is a gas, in an
emulsion a liquid. Jellies or gels also resemble these
systems in constitution in many cases. It is now known
that various different types of gel structure exist;
many jellies, however, are essentially dense emulsions;
thus stiff* foams of air with a soap solution (or solutions
of albumin, saponin, or other surface-active colloidal
substances) have many of the characters of solids or
semi- solids; i.e., permanence of form, elasticity, high
viscosity; and all transitions between liquid and solid
systems of the emulsion type are known — a fact familiar
to anyone who makes a lather of soap solution. The
same is true of an emulsion of one liquid in another;
thus an emulsion of oil in a soap solution may be made

^For a general review cf. Bancroft's series of articles on '"The
Theory of Emulsification " in Journal of Physical Chemistry, XVI-XIX
(191 2-15); also the recent book of Clayton, The Theory of Emulsions
and Emulsification.

PROTOPLASMIC STRUCTURE 71

so concentrated — with more than 90 per cent of oil as a
disperse phase in some of the emulsions prepared by
Pickering^ — that the whole mass has a jelly-like con-
sistency. The differences of opinion as to whether
living protoplasm belongs to the ''sol" or ''gel" type
are thus seen to be unimportant, since transitions between
these states are continuous, and in fact many forms of pro-
toplasm exhibit a liquid consistency at one stage (or under
certain conditions) and a solid consistency at another.^

In a foam of air and soap solution the individual
bubbles do not coalesce, although the intervening films
may be extremely thin; evidently the structural stability
of the system as a whole is determined by the properties
of the films. If we break down these films, mechanically
or otherwise, the whole foam structure collapses. The
stability of emulsions of oil in aqueous media is similarly
conditioned; in this case the coalescence of the separate
oil droplets is prevented by interfacial films of soap or
other material, and such an emulsion can be also de-
stroyed (de-emulsified) by altering the material compos-
ing the films, e.g., by adding strong acid if soap is the
emulsifying material. Such facts lead to the general
question of the conditions determining the stability of a
foam structure or emulsion.

Two chief conditions for the persistence of an air
foam are that the layer of solution separating the
adjacent bubbles should have (i) a low surface-tension
and (2) a high viscosity. A low surface-tension is
favorable because the tangentially acting forces tending

^ Pickering, Journal of the Chemical Society, XCI (1907), 2001.

* Cf . Bayliss' recent paper in Proceedings of the Royal Society, B,
XCI (1920), 196.

72 PROTOPLASMIC ACTION AND NERVOUS ACTION

to rupture the films or laraellae are then small; i.e.,
the natural tendency of the film material to minimize
its surface, or "draw together," is slight. But this
condition is not alone sufficient, as seen in the fact that
pure liquids of low surface-tension against air, like ether,
benzol, alcohol, etc., do not give permanent foams,
any more than does water. It is well known that
mixtures of alcohol and water foam more readily than
either liquid alone ; and this is especially true of mixtures
of water and a second liquid of great surface-activity
and high viscosity, such as amyl alcohol; hence the
important generalization that pure liquids do not foam —
do not form permanent disperse systems with air. Nor
do mixtures of two pure, mutually immiscible liquids
readily form permanent emulsions. Typically the pres-
ence of a third substance is necessary, and it is important
that this third substance should be of such a kind as to
lower the surface-tension at the boundary between the
phases, and also to impart to the surface layer a relatively
high viscosity or resistance to displacement. Under some
conditions this viscosity may be sufficient to impart to
the interfacial layer the properties of a solid film. In
general, a third substance is effective as an emulsifying
agent in proportion to its power of forming at the
boundary a film having these properties of low surface-
tension and high viscosity. Most substances which
*form stable emulsions of oil in water (soap, proteins,
gums) are of this kind. The interfacial films or lamellae
then resist disruption and the disperse droplets are
prevented from fusing. If the film is considered as a
phase, most emulsions would be classed as three-phase
systems (triphasic).

PROTOPLASMIC STRUCTURE 73

Another condition favoring stability in an emulsion
is a small diameter in the disperse droplets. The
disperse particles in suspensions and emulsions are
electrically charged, and if they are sufficiently minute
the forces due to their mutual electrostatic repulsion
may be sufficient to prevent contact and fusion/ We
must therefore qualify the statement that at least three
components are necessary in a permanent emulsion-
system by the proviso that the subdivision be not
excessively minute. This factor, however, is of minor
importance in most emulsion systems, and probably is
not of great importance from a biological point of view.
It is worthy of note, however, that in some cases mutual
electrostatic repulsion appears to play a part in deter-
mining the distribution of colloidal particles, droplets,
or other minute freely mobile particles in cells; e.g.,
the distribution of the chromatin in the equatorial
plates and spiremes of mitotic figures shows evidence of
this factor.^

An emulsion, being a system of disperse charged
particles, resembles in this respect any colloidal sus-
pension; hence electrolytes influence the stabiHty of
emulsions, because of the influence of the ions on the
interfacial potentials, just as they influence the stabihty
of other colloidal systems.^ Generally speaking, any
mechanical, chemical, or electrical conditions which
alter the surface lamellae affect the stability and other*

' Cf. Lewis, KoUoid-Z., V (1909), 91.

2 R. S. Lillie, American Journal of Physiology, XV (1906), 46.

3 For the action of electrolytes on the stability of emulsions cf ,
Powis, Z. physik. Chem., LXXXIX (1914-15), 186. Cf. Northrop
and De Kruif, /. Gen. Physiol., IV (1921-22), 639, for an account of
the analogous action of electrolytes in the agglutination of bacteria.

74 PROTOPLASMIC ACTION AND NERVOUS ACTION

properties of an emulsion system. Emulsions of oil in
alkaline water or soap solution are destroyed by adding
strong acid (HCl) which breaks down the soap films.
Similarly a foam structure may be destroyed mechani-
cally or by adding a surface-active substance of low vis-
cosity; thus a few drops of ether destroy a beer-foam,
a fact explained by Quincke as due to the displacement
of the material composing the surface lamellae.^ Similarly
a saponin solution to which sufficient alcohol is added
does not form a permanent foam; the addition of iso-
butyric acid to a saponin solution also prevents foaming,
but if alkali is added to neutralize the acid and form the
surface-inactive salt, foaming results.^ Many other
facts of a similar kind are well known. The conditions
of de-emulsification ('' cracking" of emulsions) deserve
careful study by biologists, for changes of this kind are
almost certainly concerned in many forms of protoplasmic
activity; e.g., secretion and the processes of activation,
stimulation, and cytolysis.

In general, therefore, we may define the chief condi-
tion of stability in emulsion systems as the presence of
material, differing from that composing the two chief
phases, in the form of thin continuous layers or films
deposited or adsorbed at the boundary surfaces. The
thickness of these films may be extremely slight; when
a material is surface-active and is free to spread over
the surface separating the phases, conditions of equi-
librium may not be reached until the layer is only one
or two molecules thick. ^ Such a film, however, is

' Quincke, Ann. Physik, XXXV (1888), 580.

" Zawidski, Z. physik. Chem., XXXV (1900), 77.

3 Cf. Langmuir, Journal of the American Chemical Society, XXXIX
(1917), 1848; cf. also Freundlich's Kapillarchemie, p. 278.

PROTOPLASMIC STRUCTURE 75

capable of holding one liquid finely dispersed in another
in a permanent state of emulsion. We may infer that in
at least some forms of protoplasmic emulsion-structure
the interfacial films are of molecular thickness, a con-
sideration of much importance in relation to the proper-
ties of irritability and transmissivity (or propagation of
excitation-states), as will be seen below.

ADSORPTION

It will be clear from the above that in the formation
of emulsions — and hence of living protoplasm as a system
based upon the emulsion type of structure — the con-
ditions determining the formation of interfacial films
are of primary importance. Adsorption, the process
by which material collects or concentrates at boundary
surfaces, is thus a fundamental factor in the formation
and behavior of emulsion systems and of colloidal sys-
tems in general. The physics and chemistry of adsorp-
tion processes have recently been discussed fully in
several excellent textbooks,^ so that it is unnecessary here
to give any detailed account. One general fact, however,
which may be emphasized as especially important from
the physiological point of view, is that adsorbed sub-
stances are typically more subject to chemical change
than substances uniformly distributed in a solution.
Both the increase of concentration and the presence of
surface factors are concerned in the increase of reactivity,
the catalytic action of many finely divided materials
(charcoal, platinum, etc.) is usually referred to the
increased concentration of the chemically altered material

^ Hober, Physikalische Chemie der Zelle und der Gewehe (1914);
Freundlich, Kapillar chemie, Leipzig (1909); Bayliss, Principles of Gen-
eral Physiology; Bancroft, Applied Colloid Chemistry.

76 PROTOPLASMIC ACTION AND NERVOUS ACTION

at the surface of the catalytic agent, but it appears
probable that other factors (electrical) also enter in
many cases of adsorption-catalysis (see below).

In considering the case of protoplasmic systems, we
may regard adsorption as of importance in two chief
respects: (i) as an essential condition in the determina-
tion of structure (through the formation of the
adsorption-films of the protoplasmic emulsion and in
membrane structure in general), and (2) as a main
factor determining the character and velocity of the
chemical reactions; i.e., as influencing or controlling
cell-metabolism.

It is well known that the adsorption of dissolved
substances of low molecular weight is, as a rule, a strictly
reversible process, with the equilibrium conditions
defined by the formula xjm = kcn , where x is the quantity
adsorbed, m the mass of the adsorbent, c the concentra-
tion of the substances in solution, and k and n constants.
On the other hand, in the case of colloidal substances or
other substances of high molecular weight, adsorption
frequently leads to a change of properties, the substances
becoming converted into relatively insoluble or resistant
varieties^ (possibly polymerized). In such cases the
process may be difficultly reversible or irreversible, a
fact of much interest as bearing on the question of the
conditions under which the more permanent portion
of the protoplasmic substratum is formed. In general,
organic growth appears to depend on the deposition of
relatively stable or persistent structural elements or
material in apposition to other elements or material of

^ Cf. Hober, op, cit., p. 220, for instances of anomalous or irre-
versible adsorption.

PROTOPLASMIC STRUCTURE 77

the same kind, a process suggesting an irreversible
type of adsorption. A few examples of irreversible
adsorption may be cited for illustration. Freundlich
and Losev^ found that a solution of the dye, crystal
violet, was completely decolorized by animal charcoal,
and that washing would not give back the dye. This
may mean that the concentration of the dye in solution
at equilibrium is indefinitely small; but more probably
it points to the formation of an insoluble modification
as the result of adsorption. Proteins like egg-albumin
when adsorbed at the surface of drops of chloroform,
form thin highly insoluble and resistant pellicles; i.e.,
the protein undergoes the change usually described as
"denaturation." Various other cases of anomalous
adsorption are probably to be referred to conditions of a
similar kind; the adsorbed material apparently under-
goes some chemical modification.

A further fact of fundamental biological interest is
that the adsorbent action of many materials has a
certain specificity or selective character; i.e., the action
varies from adsorbent to adsorbent independently of
the latter's state of subdivision. This phenomenon has
apparently the same ultimate basis as have the specific
chemical affinities between substances; in fact, the
distinction between adsorption and the formation of
true chemical compounds is now very generally recog-
nized as ill defined.^ The phenomena of cohesion, adhe-
sion, and capillarity are closely related to adsorption;
thus water wets (is adsorbed by) certain solid surfaces,

' Freundlich and Losev, Z. physik. Chem., XLIX (1907), 284.

* Cf. Langmuir, op. ciL, pp. 1900 fif.; also Journal of the American
Chemical Society, XL (191 8), 1361.

78 PROTOPLASMIC ACTION AND NERVOUS ACTION

but not others. It is well known that the interfacial
tension between an adsorbing surface and a solution of
an adsorbable substance is a direct function of the degree
of adsorption of the latter.

When the adsorbed substances are of low molecular
weight — e.g., in homologous series of alcohols, organic
acids, or similar compounds — it is usually found that the
order of relative adsorption is not altered by altering
the adsorbent, although the degree of adsorption may
vary widely with the different adsorbents. With more
complex molecules, however, relations of an apparently
arbitrary or specific kind often enter, and presumably the
relations between the molecular structure or configura-
tion of the adsorbing surface and that of the adsorbed
substance then become important. Specific adsorptions,
like specific chemical combinations (between enzyme and
substrate, or antigen and anti-body) are thus probably
largely dependent on similarities of chemical configura-
tion. Hence a good adsorbent for one substance may
be a poor one for another.^ Freundlich cites various
instances illustrating the differences between the adsorb-
ent powers of different materials for the same substance.^
Thus charcoal adsorbs crystal violet 20 times as effec-
tively as silk, and 156 times as effectively as cotton wool.
He found that different adsorbents usually showed the
same order of relative adsorption for the dyes used;
with four solid adsorbents the general order of adsorbent
action was charcoal>wool> silk > cotton, but the ratios
of the adsorption constants varied with different dyes.

^ Cf. Bayliss, op. cit., p. 60, for instances of specific adsorption; also
Bancroft's Applied Colloid Chemistry, p. 3.

' Freundlich, Kapillarchemie, p. 155.

PROTOPLASMIC STRUCTURE 79

Wohler and Plliddemann^ found that iron oxide adsorbed
10 times as much benzoic acid as acetic acid; chromic
oxide adsorbed both about equally; while platinum
sponge adsorbed more acetic than benzoic, but both
shghtly. According to Freundlich, gelatine adsorbs
sugar only after having been treated with formaldehyde.''
The influence of the specific molecular structure of the
adsorbent on its selective adsorption is well shown in
the investigations of Marc. A crystalline adsorbent,
BaC03 (rhombic) adsorbs KNO3 (rhombic) but not,
or slightly, NaN03 (hexagonal); CaC03 (hexagonal)
adsorbs NaN03 but not KN03.^ These observations
throw an interesting light on the phenomena of crystal-
lization; it is well known that the specific molecular
configuration of a substance determines the form in
which it crystallizes, as originally shown by Pasteur's
observations on the separation of laevo- and dextro-
tartrate in separate crystals in the crystallization of the
optically inactive solution. Apparently the abstraction
of molecules from solution and their deposition to form
the regular solid structure or crystal are determined by
conditions of the same kind as those determining selec-
tive adsorption. Adsorption of molecules at the surface
of the crystal is a preliminary to the growth of the latter;
this growth is evidently dependent on mutual apposition
of molecules similar in configuration ^and dimensions and
with their axes parallel."* In organic growth — another

^Wohler and Pliiddemann, Z. physik. Chem., LXII (1908), 664.

2 Freundlich, op. cit., p. 514.

3 Marc, Z. physik. Chem., LXXXI (1913), 641.

4 Crystal growth, in fact, appears to afford the clearest cases of
specificity in adsorption.

8o PROTOPLASMIC ACTION AND NERVOUS ACTION

type of specific growth and form-determination — similar
processes are almost certainly concerned.

The reversible character of many adsorption processes
is a property of great biological importance and appar-
ently one essential to certain physiological effects, such
as narcosis, which are universal in living protoplasm.
There is no doubt that this reversibility also plays an
essential part in the normal chemical processes of proto-
plasm. The displacement of one adsorbed compound by
another of greater surface-activity presupposes reversible
adsorption, and various biological instances of this effect
are known. Thus an adsorbed enzyme can be removed
from an adsorbing surface by adding a more surface-
active substance; e.g., rennin adsorbed by charcoal and
added to milk will not coagulate the latter, but on the
addition of saponin the rennin is set free and causes
coagulation. A solution of rennin is inactivated by
shaking with air, but not if saponin is present; the latter
protects the enzyme by preventing adsorption at the
air-water interface.'

Similar cases of inactivation by shaking are cited
by Meltzer and Shaklee.^ Apparently only the adsorbed
enzyme is inactivated; it has already been mentioned
that changes of physical state frequently result from ad-
sorption; Ramsden's observation that proteins can be
coagulated by shaking with air is an instance of the same
phenomenon.^ Hence the prevention of adsorption
through the presence of another surface-active com-

*Cf. the observations of Jahnson-Blom (1912), and Schmidt-
Neilson (1910), cited in Bayliss' Principles of General Physiology, p. 70.

* Meltzer and Shaklee, Amer. Jour. Physiology, XXV (1909), 81.

3 Ramsden, Z. physik. Chem., XL VII (1904), 343.

PROTOPLASMIC STRUCTURE 8i

pound may be a factor in preventing physical and
chemical alteration in living protoplasm. Preventive
effects of this kind probably form a chief factor in
the anaesthetic action of surface-active compounds, as
well as in the protective action which they often ex-
hibit (against salt action, haemolysis, or mechanical in-
jury)/

ADSORPTION AND DEPENDENT PHENOMENA IN
COLLOIDAL SYSTEMS

In general, colloids of the suspensoid group are less
readily adsorbed than those of the emulsoid group, in
correspondence with the fact that the latter are usually
surface-active, the former not; For the same reason
the suspensoids do not usually act as emulsifying agents,
while many emulsoids are highly effective in this regard.
The distinction, however, is not absolute, since finely
divided insoluble substances of various kinds may
emulsify oils under certain conditions;^ what is essential
is that the material should collect and form a continuous
layer at the surface between the phases.

As a rule proteins are surface-active; their solutions
have lower surface-tensions than pure water and they
are readily adsorbed. The conditions are, however,
complex; the degree of adsorption may vary with the
same protein according to its state of subdivision (which
varies with the salt content) or according to the H-ion
concentration of the solution; the latter condition deter-
mines the proximity to the isoelectric point and hence

^ See pp. 207 ff.

^Cf. Bancroft, loc. cit.; Journal of Physical Chemistry, XVI
(1912), 475.

82 PROTOPLASMIC ACTION AND NERVOUS ACTION

the electrical properties of the particles, a factor which
influences their adsorption.^

Observations on the relation between the concen-
tration of proteins in solution and the degree of their
adsorption give many abnormalities, probably referable
chiefly to variations in the aggregation state.^ The
particles cohere and form larger aggregates which con-
dense at surfaces, forming films of modified protein.
Such processes are largely irreversible, and chemical
change probably also enters as a factor; the changes
in the properties of enzymes, dyes, and other colloidal
compounds in adsorption are probably to be thus
explained. Many cases of abnormal and irreversible
adsorption belong here; such abnormalities are especially
characteristic of colloids of the emulsoid group. The
surface-activity of proteins, and the readiness with which
they form films, filaments, and other coherent structures,
at the surfaces where they are adsorbed, are undoubtedly
properties of great biological importance; and we may
assume that in the deposition of proteins in a solid or
semi-solid state to form the more permanent structural
elements of protoplasm such processes play a chief part.
As already indicated, it seems probable that specific
adsorption, a process apparently based on the tendency
of molecules of similar configuration to cohere or coalesce
to form larger aggregates, is the fundamental factor
underlying the specificity of growth processes.

As already pointed out, adsorption may furnish the
conditions for many of the chemical reactions in cells;

^ Cf . Christiansen, cited by Pauli, Colloid Chemistry of Proteins,
Philadelphia (1922), p. 89.

2 Cf. Hober, op. cit., pp. 2175.

PROTOPLASMIC STRUCTURE 83

i.e., material concentrated or condensed at the proto-
plasmic surfaces may in this manner first become capable
of chemical interaction. In a recent paper^ Bayliss
gives instances showing that in many reactions in poly-
phasic systems adsorption is the initial process which
forms a necessary preliminary to the true chemical com-
bination following.

We may conclude that the determination and control
of chemical reactions by adsorption are universal in
living protoplasm. The presence of colloidal complexes
or ^'adsorption compounds" — e.g., compounds of lecithin
with proteins, such as lecithin-vitellin (in the ethereal
extract of egg yolk) and jecorin (dextrose and lecithin
plus protein) — is frequent in organisms. Inorganic
salts and ions are probably also largely present in a con-
dition of adsorption;^ and the indications are that the
action of the surface-active pharmacological compounds
(especially the anaesthetics) is largely so determined.
According to Loewe, the chief relation between lipoids
and narcotic compounds is one of adsorption,^ although
the relative solubilities of these compounds in the
different protoplasmic phases (partition-coefficients)
probably also enter as an important factor in narcotic
action."*

The promotion of chemical action by adsorption
is often called ''adsorption-catalysis." Well-known
examples are the formation of H2SO4 from SO2 in the
presence of platinum, the reduction of various compounds

^ Proceedings of the Royal Society, B, LXXXIV (1911), 81.
* Cf . Pauli, loc. cit.

3 Loewe, Biochem. Z,, LVII (1913), 161; cf. pp. 200 flf.
1 See chap, ix for the relation of adsorption to narcosis.

84 PROTOPLASMIC ACTION AND NERVOUS ACTION

by hydrogen in the presence of platinum, and the oxida-
tion of compounds by blood-charcoal (oxalic acid, etc.).
The combination of tannin with leather is a good illus-
tration of the determination of a chemical reaction by
a previous adsorption; the tannin is first adsorbed, then
it combines. The same condition is shown in the union
of dyes with heat-denatured egg-white; the process
is at first readily reversible (by acid), but not later,
indicating that the first stage of the process is a close
contact or adhesion, which is then followed by chemical
combination.^ The toxin-antitoxin reactions and the
opsonin reaction with leucocytes are further biological
instances of a similar kind. According to Morgenroth,
tetanus toxin is taken up or attached by living cells
at 8°, but does not become active until 20°. In the action
of enzymes adsorption processes play an important part,
as already indicated.^

INFLUENCE OF ELECTRICAL STATE OF SURFACE

ON ADSORPTION

The relations of electrostatic attraction or repulsion
between the charged surface of the adsorbent and the
charge on the particles of the dissolved substance consti-
tute a factor of decisive importance in many adsorption
processes. For example, acid dyes (whose colloidal
particles are negatively charged) are as a class more
readily adsorbed by suspended alumina (with positive
particles) than by kaolin, a silicate with negative
particles, and vice versa. Color bases show the reverse
behavior, being adsorbed by substances which form

^ Unpublished observations of my own.

' Cf. the data and discussion in Bayliss' textbook, p. 324.

PROTOPLASMIC STRUCTURE 85

negatively charged surfaces; e.g., silicates, carbon, and
adsorbents of chemically acid character, but not by
hydrates like alumina or other positive adsorbents.^
Suspensoids of heat-denatured albumin show the same
behavior; when the particles are made positive by the
addition of a little acid they become adsorbents for
acid dyes, which also cause precipitation; while in the
negative condition, i.e., on the alkaline side of the iso-
electric point, they are precipitated by (and adsorb)
basic but not acid dyes. The mutual precipitation of
oppositely charged colloidal solutions when mixed in
suitable proportions is an example of the same phe-
nomenon.

The importance of the electrical factor in the general
behavior of colloids is indicated by the remarkable
changes in adsorptive and chemical properties which
a given protein exhibits when the H-ion concentration
of the solution passes from one side of the isoelectric point
to the other. On the acid side precipitation is induced
by the anions of an added electrolyte, on the alkaline
side by the cations, as Hardy first showed in the case of
heat-modified egg-albumin.^ Recently the relation
between the charged condition of the protein aggregates
and their chemical and other behavior has been investi-
gated in much detail by Loeb,^ who has determined many
striking correlations between the physical and the
chemical properties of proteins at varying H-ion con-

^ Cf . Michaelis, Physikalische Chemie iind Medizln, edited by Koranyi
and Richter, Leipzig, II (1908), 341.

^^ W. B. Hardy, Proceedings of the Royal Society, LXVI (1899), no.

3 For a summary of these important investigations cf . Loeb's recent
book Proteins and the Theory of Colloidal Behavior, New York, 1922.

86 PROTOPLASMIC ACTION AND NERVOUS ACTION

centrations. As Pauli^ also has pointed out, many
properties (viscosity, osmotic pressure, refractive index,
precipitability by alcohol) pass through a minimum
which is coincident with the isoelectric point.

In all colloids the electrical factor plays a special
part in changes of aggregation state or dispersion. In
precipitation by electrolytes adsorption processes enter;
one or the other ion is adsorbed predominantly, i.e.,
the ion of opposite sign to the colloidal particle; accord-
ingly this ion is the precipitant. In general the more
readily adsorbed an electrolyte is, the more effective it is
as a precipitating agent. Freundlich has shown this
clearly for the salts of a number of organic bases. ^
The relative precipitating effectiveness of the several
salts, with colloidal arsenious sulphide, is shown in the
following table:

g ,. Precipitating

Concentration

Aniline chloride 4.1

P-chloraniline chloride 2.2

Strychnine nitrate o. 39

Morphine chloride o. 36

Neufuchsin 0.30

The order of precipitating concentrations is the
reverse of the order of relative adsorption.

Theoretically the two ions of an electrolyte should
have different adsorption constants in relation to an
adsorbing surface. The nature and quantity of the
ions adsorbed will influence the electrical conditions at
the interface, and, secondarily, all processes in which
these conditions are a factor, such as colloidal stability,

' Cf. W. Pauli, loc. cit.

' KoUoid-Z., I (1907), 328; Kapillarchemie, p. 351.

PROTOPLASMIC STRUCTURE 87

state of dispersion, cataphoresis and electrical endosmose,
and catalytic action. The action of electrolytes on
colloids shows many indications of adsorption effects;
those ions which other evidence indicates are in general
the most strongly adsorbed (H, OH, polyvalent cations)
have a correspondingly marked influence on the colloidal
state. The influence of adsorption is shown with
especial distinctness in the action of salts on protein
solutions, as Pauli's work has especially shown; the
characteristic curves relating temperatures of heat-
coagulation, melting points of gels, and precipitability
by alcohol to concentration of salt (when the salt is
present in excess of that required to form stoichiometric
compounds) are clearly of the adsorption t>pe.^ The
same is true for the influence of salts on the osmotic
pressure of protein solutions.^

Apparently, in all processes where surface effects are
concerned, the different salts of the same metal differ in
their action according to the nature of their anions;
according to Rontgen and Schneider^ the order of rela-
tive adsorption of anions is S04<Cl<Br and N03<I;
this series corresponds to the characteristic lyotropic
series of Hofmeister, which is shown not only in the
above-cited work on proteins and in many of the physio-
logical effects produced by salts^ but also in various
purely physical phenomena involving surface factors

^ Cf. Pauli's book {loc. cit.) for references.

^ The data in my paper {American Journal of Physiology, XX (1907),
127) show a relatively great depressant action on osmotic pressure for
low concentrations of salts and a curve corresponding to the adsorption
type.

3 Rontgen and Schneider, Ann. Physik., XXIX (1886), 165.

88 PROTOPLASMIC ACTION AND NERVOUS ACTION

(influence of salts on surface tension, solubility, vis-
cosity, catalytic action, etc.)/

ADSORPTION OF IONS

The difference between the adsorbability of the two
ions of an electrolyte implies an influence on the potential
difference at the phase-boundary; the surface receives
the charge of the more adsorbable ion and the adjacent
layer of solution the opposite charge. Potentials
arising in this way have been called ''adsorption poten-
tials'' by Freundlich,^ and they undoubtedly play an
important part in colloidal phenomena, since the altera-
tion of the surface charge is a chief factor in the changes
of aggregation-state and other phenomena characteristic
of colloids. Adsorption potentials may also be of great
importance in influencing the character of the chemical
changes occurring under the catalytic influence of
surfaces.

There are many indications that the ions of water,
OH and H, are among the most readily adsorbed ions.
This property is of special biological importance, since
most forms of protoplasm are highly sensitive to varia-
tions in the concentration of these ions; and in certain
cases this sensitivity has become a regulatory factor of
the utmost delicacy. Free organic acids and bases are

^ For a summary of the physical effects of salts showing the lyotropic
series cf. Hober, op. cit., p. 310.

* According to Freundlich the adsorption potential is not the poten-
tial between the interior of the solid phase and the adjoining solution,
but that between an adhering immobile layer of solution and the mobile
layer adjoining. Cf. Kapillarchemie, p. 243; also Report on the Physics
and Chemistry of Colloids by the Faraday Society and the Physical
Society of London (1921), p. 146.

PROTOPLASMIC STRUCTURE 89

as a rule more readily adsorbed than their salts. Many
organic acids (the lower members of the fatty series)
are highly effective in lowering surface-tension, while
their salts have little influence; this effect, however,
may be in large part attributable to the undissociated
molecules. As a class, acids have a marked effect on
the surface charges of indifferent solid substances,
tending to make these surfaces positive; similarly bases
make them negative. Both effects appear in very low
concentrations (Perrin)^ and are undoubtedly due to
H and OH ions, respectively. The fact that adsorbent
surfaces of the most widely varying chemical composi-
tion (carbon, hydrocarbons, silicates, oxides, metals)
are thus affected indicates that adsorption rather than
chemical combination in stoichiometric proportions lies
at the basis of the effect. A slight change in H and OH
concentration may thus have a very marked effect
upon the potential difference across a surface; this is
well shown in the curves given by Haber and Klemensie-
wicz,^ and in the results of Perrin.^ The great effective-
ness of the H-ion as a precipitant for suspensions of
indifferent substances probably depends on its high
adsorbability as well as on its high velocity and special
chemical properties.

At a certain concentration where OH and H-ions are
adsorbed in certain proportions the surfaces will be
electrically neutral; this condition defines the isoelectric

^ Cf. Perrin, Jour. Chim. Phys., II (1904), 601.

=^ Haber and Klemensiewicz, Z. physik. Chem., LXVII (1909), 385.

3 See especially Perrin's curve for naphthalene, reproduced in
Freundlich's KapUlarchemie, p. 236. Ellis also describes this effect in
the cataphoresis of oil droplets, Z. physik, Chem., CXXVIII (1911), 321,

90 PROTOPLASMIC ACTION AND NERVOUS ACTION

point. The H-ion concentration corresponding to this
neutral position varies according to the special chemical
character of the material and its adsorbent properties.

Various chemical effects apparently depending on
unequal adsorption of ions are described in Freundlich's
book; frequently these appear to be consequences of the
displacement of one ion from an adsorbent surface by
another which is more readily adsorbed. For example,
many basic dyes (crystal violet or basic fuchsin) are
chlorides of organic color-bases; when these dyes are
adsorbed by charcoal, a large part of the combined
chloride goes into solution as inorganic chloride. This
result is explained by Freundlich as due to the high
adsorbability of the color cation (dye-HCl = dye-H+
and Cl~) which displaces from the adsorbent the adsorbed
cation;^ according to this view the chemical splitting is
due to the unequal adsorbability of the two ions. The
phenomenon may, however, be regarded as a consequence
of the high adsorbability of the free base, which is present
in the solution in consequence of partial hydrolysis; this
base is removed, leaving the chloride in solution.^ It is
known that many organic free bases are more highly sur-
face-active (adsorbed) than the salts; thus Traube points
out that the surface-tension of solutions of hydrochlorides
of the alkaloids, cocain, atropin, and quinine is lowered
by adding a little alkali, an effect due to the liberation
of the free base;^ the latter will tend to be adsorbed and
the hydrolysis will be promoted. The separation of

^ Freundlich, op. ciL, p. i68.

* Cf. Michaelis, Arch. ges. Physiol., XCVII (1903), 634.

3 Traube, Kolloidchem. Beihefte, III (191 2), 237; cf. also Hober,
op. cit., p. 215.

PROTOPLASMIC STRUCTURE 91

dyes on adsorbing surfaces in an insoluble form and the
''denaturation" of proteins on surfaces are apparently
phenomena of the same general kind; adsorption will
promote hydrolysis or other chemical alteration in a
compound if any reaction-product is more readily
adsorbed than the original compound. From this
point of view the general kinetics of adsorption-catalysis
appear in a clearer light.

The heavy metal ions and the ions of trivalent
metals appear to be adsorbed with especial readiness in
many cases, and the solutions of their salts have a corre-
spondingly great influence on the surface charge of col-
loidal particles and of porous partitions. In the case of
suspended oil drops, Ellis^ found the following concen-
trations of three chlorides to be equally effective in
removing the charge; i.e., in rendering the particles
electrically neutral.

Mols.perLitre Con^'centrlSon

AICI3 o . 00026 I

CuCla 0.0089 ca. 40

NaCl 0.40 ca. 1600

The relative actions of Na, Cu, and Al are approxi-
mately as I to 40 to 1,600, indicating a rapid increase
of action with increase of valence. The same rule is
found in the precipitation of suspensoid colloids by
electrolytes (rule of Schulze and Hardy), and indicates

'f Ridsdale Ellis, Z. physik. Chem., LXXVIII (1912), 321. Loeb's
recent study of the effects of ions in altering the charge on suspended
collodion particles gives a similar result (/. Gen. Physiol., V [1922J, 109).
In this case the collodion particles undergo precipitation when the P.D.
against the medium falls below 16 millivolts. Bacteria are agglutinated
at 15 rnillivolts according to Northrop and de Kruif (loc. cit.).

92 PROTOPLASMIC ACTION AND NERVOUS ACTION

that a relatively great adsorption of polyvalent ions is a
general rule. The relative effectiveness of polyvalent
cations in certain characteristic physiological effects,
e.g., ion-antagonism, is of a corresponding order, indicat-
ing that in such cases the ions act by adsorption at the
structural surfaces of the living system. Thus Al and
Cr ions greatly prolong the activity of cilia in isotonic
NaCl solution when present in concentrations of less
than M/ 100,000.^

It should be pointed out that even if the adsorption
constants of mono-, di-, and trivalent ions were equal,
the trivalent ions should be effective in less than a third
of the concentration of the monovalent ions, because,
on account of the characteristic form of the adsorption
curve, the ratio between the quantity adsorbed and the
quantity remaining in solution is much higher in dilute
than in concentrated solution; hence sufficient trivalent
ions to neutralize the surface charge may be adsorbed
from extremely dilute solution. This is Freundlich's
explanation of Schulze's rule."^ Whetham's explanation,
based on chances,^ is probably insufficient, although
purely mathematical considerations would indicate
that the statistical conditions to which he calls atten-
tion must play a part in the total effect. The ratio
between the effective concentrations of Al and Na in the
above-cited experiments seems, however, too great to
be accounted for on this ground alone, and a high degree
of adsorption of polyvalent cations must apparently be
assumed.

^ R. S. Lillie, American Journal of Physiology, X (1904), 430.
* Freundlich, Z. physik. Chem., LXXIII (1910), 385.
3 Whetham, Theory of Solution, p. 396.

PROTOPLASMIC STRUCTURE 93

The importance of adsorption-potentials is shown
most clearly in the phenomena of electrical endosmose,
which are of great physiological importance and will be
considered briefly below. The marked influence which
slight quantities of acid and alkali have in altering the
phase-boundary potentials, especially in the neighbor-
hood of the isoelectric point, is a fact of special biological
interest. Near this point the effect of variations in the
H-ion concentration upon the phase-boundary potentials
is at its maximum; farther from the isoelectric point
the difference caused by a given change in the H-ion
concentration is relatively slight.^

The reactions of the tissue fluids in higher animals are
slightly on the alkaline side of neutrality, and the reaction
of the living protoplasm (because of the higher tension
of CO2 within the cell) is presumably somewhat less
alkaline and is probably not far from the isoelectric
point of some of the structural proteins. Hence slight
variations of the H-ion concentration within the cell
should have a correspondingly great effect upon the
boundary-potentials of the corresponding cell-structures.
Variations in the H-ion concentration of the cell-medium
would affect first of all the boundary-potential of the
cell as a whole; i.e., that existing across the plasma
membrane, and this is probably the chief reason why
living tissues are frequently so sensitive to changes in the
external H-ion concentration. As already pointed out,
this sensitivity has in some cases become the controlling
factor in regulatory processes of vital importance to
the whole organism, as in the respiratory nerve, cells of
vertebrates, which show an accelerated rhythm in

^ Cf. Haber and Klemensiewicz, loc. cit.

94 PROTOPLASMIC ACTION AND NERVOUS ACTION

response to an extremely slight rise of acidity due to
increase in the CO2 of the blood.

ELECTRICAL ENDOSMOSE

Certain physical phenomena of great biological
interest, having similar relations to the charged char-
acter of the interphasic surfaces, are those classed as
electrical endosmose. The essential phenomenon is
the transfer of fluids with or against an electric current
traversing a porous partition immersed in an electrolyte
solution. The fundamental conditions of this transport
are the same as those determining the electrical con-
vection of colloidal particles or emulsion droplets,
except that in electrical endosmose the continuous fluid
phase is the mobile one, the solid phase which forms
the substance of the partition being fixed in position.
Colloidal gels interposed in the path of a current exhibit
this phenomenon, and it is well known in protoplasmic
systems.^ There is no question but that effects of the
same kind must occur normally in living matter, since
the latter, during functional activity at least, is continu-
ally being traversed by the currents of the bioelectric
circuits; and it seems probable that electrical endosmose
plays a special role in the processes of secretion and
adsorption, as suggested by various physiologists.''

^ For example, Hermann describes experiments showing the trans-
port of water through tissues (muscle and nerves) in the direction of
the positive stream when a constant current is passed through the
tissue {Arch. ges. Physiol., LXVII [1897], 240).

'Engelmann, Arch. ges. Physiol., VI (1872), 97; Waymouth Reid,
Phil. Trans., Series B, CXCII (1900), 239; Hober, Arch. ges. Physiol.,
CI (1904), 607; cf. also my recent paper in Biological Bulletin, XXXIII
(1917), 135, 170 flf.

PROTOPLASMIC STRUCTURE

95

At present, however, our knowledge is insufficient for
any final estimate of its physiological importance.

Since the effect depends upon the potential difference
between the walls of the pores and the mobile fluid layer
adjoining, all conditions that influence this potential
difference affect the character of the movement. The
rate of the movement or its direction or both may be
thus affected. The effects of salts, acids, and alkalies
are well illustrated in Perrin's investigations published
in 1904.^ The following table gives the results of a
typical experiment. The diaphragm consisted of naph-
thalene, and varying solutions of HCl and KOH were
used:

Solution

H. cone, (n)

OH cone, (n)

Charge of
Diaphragm

Flow

n/so HCl

. 2X10-2

5X10-^3

—

38 to anode

n/ioo HCl . . . .

. 10-2

10-12

+

39 to anode

n/iooo HCl . . .

. 10-3

IO-"

+

28 to anode

n/5000 HCl . . .

. 2X10-4

5X1O-"

•3 to anode

n/5000 KOH . .

. 5X1O-"

2X10-4

—

29 to cathode

n/ 1000 KOH. .

. IO-"

10-3

—

60 to cathode

n/50 KOH . . . .

. 5X 10-13

2X10-2

—

60 to cathode

This experiment and others of a similar kind show that
when a current of given intensity is passed through a
partition, the rate and direction of transport are deter-
mined by the charge of the partition substance, and
that this varies in a definite manner with the nature
and concentration of the ions present.

Perrin also showed that the isoelectric point varied
with the nature of the diaphragm; thus partitions of
iodoform and glass were persistently negative, while
those of BaC03 and CrCl3 were positive. The electro-

^ Perrin, /. Ckim. Pkys., II (1904), 601.

96 PROTOPLASMIC ACTION AND NERVOUS ACTION

positivity of any surface, whatever its chemical composi-
tion, was invariably increased by adding monovalent
acid to the solution, and decreased by adding monovalent
base. Polyvalent ions were very effective, reversing
the sign of the partition-charge in extremely low con-
centrations; e.g., for a partition of chromic chloride
these results were found.

Solution 4'^^^/^ Flow

Water slightly acid -f 59

Same-f .001 n K^Fe (CN)6 ... + 2

Same-|-.02 n K^Fe (CN)6. ... — 20 (reversal)

Freundlich^ calculated from Perrin's data the con-
centration of salts (millimols per litre) required to reduce
by 50 per cent the endosmose through positive and nega-
tive partitions, respectively, and obtained the following
results.

I. CrClj (positive) Cone, (millimols per litre) for

Solution 1/2 velocity

Dil. acid-f KBr 60

Same-hMgS04 i

Same-hK3Fe(CN)6 o. i

II. Carborundum (negative) Cone, for 1/2 velocity
Soluiioji

Dil. alkali-f-NaBr 50

Same-|-Ba(N03)2 2

Same-|-La(N03)3 o. i

The effect thus increases very rapidly, in accord-
ance with Schulze's rule, with increase in the valence
of the active ion, which is always that having the opposite
sign to that of the partition. EUissafoff^ in an investiga-

* Kapillarchemie, p. 238.

» Ellissafoff, Z. physik. Chem., LXXIX (191 2), 385.

PROTOPLASMIC STRUCTURE 97

tion with glass and quartz capillaries also found the
Perrin-Schulze valence rule to hold for alkali and alkali
earth salts, but heavy metals (Hg, Ag) were more
active than corresponded to their valence; organic
cations (morphine, neufuchsin) were also highly active.
Reversals with Th(N03)4 and methyl violet were
observed, but not with acids up to n/iooo; no higher
concentrations were used.^

^ A general review by T. B. Briggs in the Journal of Physical Chem-
istry for 191 7 (XXI, 198, and XXII, 256) gives a full review of the
subject of electrical endosmose. Recently its bearing on physiological
Dhenomena and its relation to cases of anomalous osmosis have been
considered in an important series of papers by J. Loeb (/. Gen. Physiol.,
II [1920], p. 557, and later papers in the same journal). For the action
of ions see further, A. Gyemant, "Elektroendosmose und lonenadsorp-
tion," Kolloid-Z., XXVIII (1921), 103.

CHAPTER VI

PROTOPLASMIC STRVCTVRE— Continued: PERMEA-
BILITY AND OTHER PROPERTIES OF
PROTOPLASMIC MEMBRANES

We have seen that the stability of emulsion systems
and many of their essential properties are determined by
the presence of thin interfacial films. In the formation
of these films, and in the determination of their special
properties, electrical and other factors enter, of a kind
characteristic of boundary surfaces in general. In
general, it may be said that heterogeneous mixtures
containing substances that influence the surface-tension
or the electrical polarization at the phase-boundaries
tend to reach a state of equilibrium in which they have
what may be called a ''structure"; i.e., a more or less
orderly and definite distribution of the components.
The gathering of surface-active compounds at the
phase-boundaries, in conformity with the principle
of Gibbs and J. J. Thomson, is a chief factor determining
the character of this distribution; and the latter second-
arily determines the character of the chemical changes
occurring in the system. Living protoplasm is an
example of a heterogeneous system in which the control
of chemical change by structural conditions has reached
perhaps its highest development.

Not only are the structural elements of protoplasm
(alveoli, nuclei, colloidal particles, fibrils) bounded by
surfaces at which adsorption and chemical change
occur, but the whole mass of protoplasm, the living

98

PROTOPLASMIC STRUCTURE 99

cell, is similarly bounded. Suspensions of living cells —
suspensions of blood corpuscles, eggs, spermatozoa,
bacteria, or yeast — in their normal aqueous media
may be regarded as similar in many respects to emulsions.
Each cell is a small discrete particle with a surface-
tension and an electrical potential-difference against its
medium. The existence of this potential is shown in
convection experiments; just as oil droplets migrate to
the anode in an electrical field, so do suspended living
cells; each cell, although living and highly differentiated
internally, behaves in this respect like a negatively charged
colloidal particle. As in the case of a suspended oil droplet,
the sign of the charge carried by the living particle may be
changed by acids or polyvalent ions; and like colloidal
particles in general the cells may be precipitated from
suspension (or agglutinated) under various conditions.^

Red blood corpuscles travel in neutral media like
isotonic sugar solution to the anode, thus showing the
presence of a negative surface charge; by passing CO3
through the media or adding weak acids, the sign of the
charge may be reversed; the corpuscles then become
positive or cathodic."^ At an intermediate concentration
the charge is abolished (isoelectric point); and it is
interesting to note that at this point the corpuscle tends
to break down or undergo •haemolysis.^ This fact is of

^ For data and references in this field, cf. Hober's Physik. Chemie
d. Zelle, pp. 247, 300, 483, and 599. More recently the cataphoresis of
bacteria and their agglutination by electrolytes and other substances has
been studied by Northrop and De Kruif (/. Gen. Physiol., IV [1922],
629, 639, and 655).

^ Hober, Arch. ges. Physiol., CI (1904), 627; CII, 196.

3 Michaelis and Takahashi, Biochem. Zeitschrift, XXIX (1910), 439.
The isoelectric point is also the optimum for agglutination; cf. C. R.
Coulter, Jour. Gen. Physiol., Ill (1920), 309.

lOO PROTOPLASMIC ACTION AND NERVOUS ACTION

special interest as indicating the importance of electrical
factors in the structural stability of the protoplasmic
surface layer, and suggests a reason why depolarizing
influences have in general a stimulating action on irritable
cells. The relations between structural change and
stimulation will be considered in more detail later.

Hober and Kozawa^ found this isoelectric point to
vary for different species of corpuscles and to be char-
acteristic for a particular species; i.e., certain corpuscles
are more readily made positive than others by H ions
and polyvalent ions; thus the corpuscles of rabbits and
guinea pigs were found to require the least H-ion con-
centration for reversal and those of the ox and pig the
highest; those of the dog, cat, goat, and man were
intermediate.

Animal Isoelectric Point (PH)

Rabbit Between 3.8 and 3.4

Guinea pig ca. 3.4

Man (and cat) ca. 3.13

Dog Between 3.13 and 2.98

Sheep Between 2.98 and 2.77

Pig ca. 2.77

The relative order with La ions was the same. Such
differences are to be referred to the specific peculiarities
of structure, composition, or permeability characteristic
of each kind of corpuscle.

In their general or common features, these phe-
nomena indicate that the behavior of the protoplasmic
free surface with reference to the ions present in the
medium resembles that of the surface of an individual
oil-droplet in an emulsion. It may be assumed that

^ Hober and Kozawa, Biochem. Zeitschrift, LX (1914), 146.

PROTOPLASMIC STRUCTURE loi

the external surface of the cell is not peculiar in this
respect, and that other protoplasmic interfaces (those
within the cell) have similar properties. Film-formation,
variations of electrical polarization, and dependent
phenomena are thus to be regarded as constant features
of the processes at such interfaces; e.g., at the surfaces
of contractile fibrils, neuraxones, vacuoles, nuclei,
alveoH, and other protoplasmic structures. Secondary
effects peculiar to living protoplasm and dependent on
metabolism may be superposed on these simple physical
effects.

An important difference between the structure of
protoplasm and that of a typical artificial emulsion,
like oil in water, is that the two phases separated by a
protoplasmic film are not necessarily aqueous and non-
aqueous, respectively, but may both be aqueous. Any
suspension of living cells, such as blood, illustrates this
condition; both the interior of the corpuscle and the
serum are complex aqueous solutions, and the film
bounding the corpuscle is too thin to be optically detect-
able as a separate structure. Yet it can readily be
shown by osmotic methods that a semi-permeable
membrane is there present. Similarly within the limits
of a single cell various structurally distinct regions,
sometimes optically distinguishable, sometimes not, are
to be regarded as separated by thin films. The details
of this film structure will naturally vary in different
types of cell.

The surface layer of the entire cell, the plasma
membrane, is the modified surface-film which separates
the internal protoplasm from its surrounding medium
and through which the necessary interchange of material

I02 PROTOPLASMIC ACTION AND NERVOUS ACTION

is effected. During life it behaves as if it were semi-
permeable and water-insoluble; yet it is evident that
it allows passage, at least at certain times, to water
and many dissolved substances; e.g., food-substances.
Because of its special character as an intermediary
structure or organ, its properties require special con-
sideration in any study of the properties of protoplasm.
In some respects the properties of the plasma
membrane appear difficult to explain on known physical
grounds or even paradoxical. If a diffusible food-
substance passes through the membrane into the cell it
may there undergo metabolism, but it is difficult to
see how any such substance can enter without other
important substances leaving. If simple diffusion alone
is the chief factor in the entrance and exit of dissolved
materials, we should expect that any increase of per-
meability sufficient to allow the entrance of substances
like sugar would also involve a loss of diffusible cell
constituents to the exterior. It seems necessary to
assume that the conditions determining the normal
transport of substances across the cell boundary are of a
special kind, and that diffusion is only one of a number
of factors. In fact we know that in secretion and
absorption special physiological mechanisms of transport
are concerned, by which water and dissolved substances
are actively conveyed into and out of the cell, fre-
quently against concentration-gradients. This process
requires the performance of work by the cell, just as
does the process of muscular contraction, although
its exact conditions are at present unknown. Many
substances (sugar, salts, amino-acids, urea) may thus
be transported from regions of lower to regions of higher

PROTOPLASMIC STRUCTURE 103

concentration, a process necessarily involving osmotic
work; in this work O2 is consumed and energy set free.
The problem of the nature of this physiological trans-
porting mechanism is often distinguished as the problem
of "physiological permeability" from the simpler prob-
lem of the conditions of simple diffusion across the cell
surface, or '^ physical permeability." The majority of
recent studies on protoplasmic permeability have had
reference to this latter problem.

It will be apparent that the problem of the nature
and conditions of cell-permeability is not a special or
limited one but underlies the whole problem of the
essential nature of protoplasmic structure and activities.
Apparently the most fundamental property of the plasma
membrane is semi-permeability; water can pass, although
with considerable resistance in many cases; but the
normal water-soluble constituents of the protoplasm
and its surroundings do not diffuse across the cell bound-
ary, or only under special conditions. This property
of "semi -permeability" is associated with a high degree
of electrical resistance (signifying impermeability to
crystalloidal ions) and water-insolubility. The physico-
chemical conditions of these properties of the plasma
membrane have been much investigated of recent years,
and since the results of this work have an important
bearing on our general problem, a somewhat detailed
review will be given.

The general importance of membranes in organic
structure and activities has long been recognized, and
much study has been devoted by physiologists to various
artificial types of membrane, such as precipitation
membranes, membranes of parchment and collodion,

I04 PROTOPLASMIC ACTION AND NERVOUS ACTION

and various impregnated types of membrane, in the
hope of throwing light upon the pecuKarities of hving
membranes. The resemblance of these artificial struc-
tures with the protoplasmic membranes seems, however,
in many cases remote. The conditions observable in
living protoplasm indicate that membranes of a different
and somewhat special type, viz., the interfacial films
formed between the separate phases in emulsions and
other polyphasic systems, have a closer affi.nity with
protoplasmic membranes than any other simple type of
physical structure.

The property of forming thin films at structural sur-
faces of all kinds is highly characteristic of living proto-
plasm. The entire cell body is separated from the
surrounding medium by the plasma membrane; within
the cell the nuclear area is sharply delimited by a mem-
brane which usually disappears only during mitosis;
structurally distinct membranes are also formed about
fibrils, vacuoles, alveoli, chromatophores, spheres,
chromosomes, and various cell-inclusions. Apparently
the conditions required for the formation of thin solid
films are present everywhere in living protoplasm; at
any structurally well-defined surface, continuous sheets
of material may be deposited; the formation of mem-
branes at cut surfaces and about extruded portions of
protoplasm is simply one illustration of this general
property.^ The fertilized eggs of marine animals
(sea-urchin, starfish) are especially favorable objects
for showing this property; new films are rapidly formed

^ For a recent account of these phenomena, see the review by Seifriz,
Annals of Botany, LXXXVIII (192 1), 269; cf. also Botanical Gazette,
LXX (1920), 360.

PROTOPLASMIC STRUCTURE 105

at the surfaces of cuts made with microdissection
needles, or around isolated portions of protoplasm.^

Apparently the nearest inorganic analogies to these
protoplasmic film-structures are the thin surface-films
deposited at the boundaries between mutually insoluble
liquids, one or other of which contains surface-active
(especially colloidal) materials in solution; good illus-
trations are the films of soap or other material surround-
ing the droplets in an oil-water or other emulsion, or the
solid films formed about globules of oil, mercury, chloro-
form, or other insoluble liquids immersed in protein-
containing solutions, or the haptogen membranes formed
at the surfaces of warm soap solution, milk, or solutions
of protein, peptone, saponin or other surface-active
colloidal substances. Under certain conditions these
films may acquire a solid consistency and exhibit con-
siderable structural density, and thus limit diffusion
between the two phases; the resulting system may then
be described as triphasic, the three phases being the
internal medium, the external medium, and the inter-
vening thin phase or membrane. The ''artificial cells"
described by Harvey,^ made by breaking up a chloroform
solution of lecithin in dilute egg-albumin, may be cited
as examples of structures formed under these conditions.

In living protoplasm it is to be presumed that
condensation-films of protein and other surface-active
substances are formed at all of the surfaces bounding

^ Chambers, American Journal of Physiology, XLIII (19 17), i; Pro-
ceedings of the Society of Experimental Biology and Medicine, XVII
(1919), 41; Jour. Gen. Physiol., V (1922), 189. For plant cells cf.
Prowazek, Biol. Zentr., XXVII (1907), 737. Drops of protoplasm
from Vaucheria form membranes about their surfaces.

^ E. N. Harvey, Science, XXXVI (191 2), 564.

io6 PROTOPLASMIC ACTION AND NERVOUS ACTION

the different phases; and the physical resemblances
between protoplasm and emulsions are apparently
referable to the presence of these surface-films. Both
systems are examples of what may be called film-
pervaded or film-partitioned systems. By the formation
of these films a certain structure is imparted to the whole
system; this structure is largely the expression of
surface-forces, in which chemical, electrical, and mechan-
ical (surface-tension) factors all enter.

It should be noted that according to this conception
of protoplasmic structure no essential distinction is to
be drawn between the intracellular surface-films or
membranes (alveolar membranes, vacuole membranes,
nuclear membranes) and the surface-films inclosing
entire cells (plasma membranes). In plant cells it can
be shown experimentally that vacuole membranes and
plasma membranes are similar in osmotic properties,
and Hamburger has shown the same for the nuclear
membranes and plasma membranes of animal cells.'

In forming a general conception of the physico-
chemical characteristics of protoplasmic membranes,
the properties of colloidal gels, especially in their relation
to diffusion-processes, may be taken as a starting-point.
In a gel, i.e., a solid mixture of colloidal material (such as
gelatine) and water, diffusion is hindered only slightly if
the concentration of the colloid is low. When the gel
is made denser — as the proportion of water is decreased —
diffusion becomes slower and more restricted. At a
sufficiently high density certain solutes, especially
colloids, can no longer diffuse through the gel, although
water and cr^-stalloids may still pass. If the density

' Hamburger, Osmotischer Driick u. Io7ienlehre, III, 8 fit.

PROTOPLASMIC STRUCTURE 107

be still further increased, crystalloids and finally even
water fail to pass/ This last condition is exemplified
in water-proof organic membranes like the external
skin of most animals and the membranes of certain eggs
(the Fundulus ^gg). From an elementary or purely
physical point of view a membrane may be regarded as
essentially a thin sheet consisting of a gel of the kind
above. Such a gel has a large surface-area in proportion
to its total volume, and by virtue of its diffusion-hindering
property it prevents or retards the transfer of material
(colloidal particles, molecules, ions) between the two
solutions which it separates.

Connected with this diffusion-hindering property,
which conditions the rate of interchange and hence the
rate of chemical activity at the surface between the
solutions separated, are certain electrical properties
C' membrane-potentials"), resulting from the influence
of the membrane on the distribution and transfer of
ions between the two solutions.^ These properties are
apparently of fundamental importance to the bioelectric
processes, and their conditions will be considered more
fully later.

Independently of its structural density, a membrane
of complex chemical composition may exhibit a selective

^ The conditions may be compared with those presented by a series
of "ultra-filters" of graded densities, as described by Bechhold {Colloids
in Biology and Medicine); the permeability decreases as the density
increases.

2 Cf. Lewis, A System of Physical Chemistry, II (1920), 320, for an
account of membrane potentials. The type of equilibrium investigated
by Donnan, in which solutions are separated by a membrane which is
impermeable to some but not all of the ions, plays an important part in
many membrane processes, as shown especially by Loeb in his recent
work. Cf. Proteins and the Theory of Colloidal Behavior, Part 2.

io8 PROTOPLASMIC ACTION AND NERVOUS ACTION

permeability with reference to substances which are
unequally soluble in its different chemical components.
Substances soluble in a given component (lipoid-soluble
substances) may thus pass a protoplasmic membrane,
while chemically similar but lipoid-insoluble substances
may not. Overton's experiments on the differences
between the rates of diffusion of various organic com-
pounds and salts into living cells illustrate selective
permeability of this type.^

It is important to recognize that the plasma mem-
brane is not a dead structure or a purely passive partition,
but represents in reality a portion of the living proto-
plasm, characteristically modified in its structure and
physical properties by surface conditions.^ Hence it
is the seat of metabolic and other activities which influ-
ence its physical properties. Evidently it is that part
of the cell which comes into the most direct relations
with the surroundings. Hence many changes in the
surroundings influence the cell primarily through their
action on the plasma membrane, and there is evidence
that in irritable cells this structure plays the part of a
specially sensitive and reactive intermediary between
the living protoplasm and the external world; it thus
exerts a far-reaching control over the metabolic and other
processes occurring in the cell-interior. The relations of
the plasma membrane to stimulation will be considered
in detail later.

^ Cf, Overton's summary of his work in Nagel's Handhuch der
Physiologie, II (1907), 744.

* Cf. my paper in American Journal of Physiology, XLV (19 18), 406,
for a more complete discussion of this phase of the problem of perme-
ability.

PROTOPLASMIC STRUCTURE 109

The normal semi-permeability of the plasma mem-
brane is a function of the living state of the cell, i.e., is
dependent upon the continuance of the normal construc-
tive metabolism. When metabolism ceases, as at death,
the membrane soon loses its insulating or semi-permeable
properties, and free interchange of diffusible substances
then occurs between the cell and the medium. The loss
of turgor in plant cells after death is the most familiar
example of this type of effect; leaves and other paren-
chymatous parts then wilt because of the diffusion of the
osmotically active substances through the now permeable
plasma membranes. The osmotic tension which during
life keeps the cell walls in their normal stretched and rigid
condition disappears, the tissue becomes soft and flaccid,
and the protoplasm shows other evidences of increased
permeability (increased electrical conductivity, loss of
diffusible materials to the surroundings, ready entrance
of dyes and other substances). Similarly in animal cells
various substances, such as pigments and other com-
pounds, normally confined within the protoplasm,
diffuse rapidly into the surrounding medium on death,
and the plasma membrane admits substances such as
alkalis and salts, to which previously it was impermeable.

It is a remarkable and apparently paradoxical fact
that the living plasma membranes usually show them-
selves highly impermeable to many dissolved substances
which are essential to the cell as foods or otherwise
(sugars, amino-acids, and neutral salts), and some
general explanation of this peculiarity seems required.
If we were to express the matter teleologically we might
say that the advantage to the cell consists in the insula-
tion of the living protoplasm from its environment,

no PROTOPLASMIC ACTION AND NERVOUS ACTION

which has an entirely different composition. The
maintenance of the normal vital properties requires that
the essential diffusible constituents of protoplasm should
not be lost to the surroundings; it is also evident that
too ready an entrance of substances from the outside
would interfere with the stability of protoplasmic
composition. The presence of a semi-permeable bound-
ary layer appears thus to be a necessary condition for the
preservation of the normal chemical organization of the
cell. It is readily seen, for example, that the existence of
a simple diffusion equilibrium between surrounding
medium and protoplasm would prevent the latter
from acquiring the special crystalloidal content which is
characteristic of it. Hence, living cells are enabled to
survive and develop largely by virtue of being inclosed
by surface-films which are impermeable to crystalloidal
compounds of the foregoing classes.

But since these compounds do in fact gain entrance
to the cell, at least at certain times, it is clear that the
problem of cell-permeability is not a simple one. Appar-
ently we must conclude that the entrance or exit of
substances by simple diffusion is in most cases a different
phenomenon from their entrance or exit under physio-
logical conditions. The processes of absorption and
secretion are in fact special activities, requiring the
performance of work by the cell. The distinction
between a passive or purely physical permeability and
an active or physiological permeability thus seems a
necessary one.

The conditions of passive permeability are of interest
chiefly because of the light which they throw upon the
physical and chemical nature of the substances composing

PROTOPLASMIC STRUCTURE iii

the plasma membranes. The most significant general
fact is that apparently all substances with solubilities
or solvent properties characteristic of organic compounds
(rather than of water-soluble compounds or water) enter
cells with special readiness. A relation between the
presence of organic solvents in protoplasm and the
permeability of the plasma membrane is thus indicated.
Overton, who first investigated in detail this connection
between organic solubility and power of penetration,
drew the conclusion that the plasma membrane consisted
essentially of the so-called "lipoid" compounds, espe-
cially lecithin and cholesterol, which are universally
present in protoplasm. He showed that all members of
homologous series, such as alcohols, ethers, esters, normal
and substituted hydrocarbons, ketones, aldehydes,
amides, and similar compounds, readily enter living
cells; such compounds dissolve, or are dissolved by, the
lipoids or solutions of lipoids in organic solvents; and
their ready entrance is a result of this solubility. On
the other hand, sugars and polyatomic alcohols (pentites,
hexites, etc.), with molecules containing many hydroxyl
groups, are highly soluble in water, but not in lipoids,
and do not enter cells readily. In Overton's original
experiments, acid and basic dyes also showed a relation
between lipoid-solubility and power of penetration; but
the conditions are complex in this case and many excep-
tions to this rule are now known. ^ In the case of neutral
salts of alkali and alkali earth metals (especially Na,
K, and Ca) there is also Kttle or no evidence of penetra-

^Cf. Ruhland, Jahrh. wiss. Botanik, XL VI (1908), i; cf. also
Hober's discussion, Physikalische Chemie der Zelle und der Gewebe,
pp. 426 ff.

112 PROTOPLASMIC ACTION AND NERVOUS ACTION

tion in balanced solutions, a fact corresponding to the
lipoid-insolubility of these compounds. In pure solutions
(pure NaCl solution) the salts alter the properties of the
plasma membranes and secondarily may penetrate;^
but this fact is in no way inconsistent with Overton's
view, which applies to the unaltered membrane. A
definite correlation between lipoid-solubility and power
of penetrating the living plasma membrane may be
said to have been established by Overton's work and
succeeding studies of the same kind; and this generaliza-
tion is an important one, since it indicates (as do many
other facts) that the lipoids play an essential part,
apparently in association with the other chief colloidal
compounds of protoplasm, the proteins, in the formation
of membranes and probably of the other solid structural
elements of cells.

Three chief methods have been employed in determin-
ing the permeability of cells to dissolved substances:
(i) the plasmolytic or osmotic method,^ (2) the partition
method,^ and (3) the electrical conductivity method.'*
To these may be added methods dependent on the use
of indicators, either those normally present in cells,^

^ Cf. Osterhout, Science, XXXIV (191 1), 187.

2 Overton, Arch. ges. Physiol., XCII (1902), 115.

3 Hedin, Arch. ges. Physiol., LXVIII (1897), 229.

4Tangl and Bugarszky, Zentr. Physiol., XI (1897), 297; G. N.
Stewart, ibid., p. 332; Osterhout, Science, XXXV (191 2), 112, and
later papers; also Osterhout's recent book, Injury, Recovery and Death
in Relation to Conductivity and Permeability, Philadelphia, 1922.

s Harvey, Internal. Z. physik.-chem. Biol., 1 (1914), 463; Crozier,
Journal of Biological Chemistry, XXIV (1916), 255, and Jour. Gen.
Physiol., IV (1922), 723; Haas, Journal of Biological Chemistry, XXVII
(1916), 225. Cf. also Jacobs, American Journal of Physiology, LIII
(1920), 457.

PROTOPLASMIC STRUCTURE 113

or dyes like neutral red^ which may be introduced from
outside. These methods are especially valuable in
studying the permeability to acids or alkalis. Per-
meability to dyes may usually be studied by direct
observation.

The method of plasmolysis, first employed systemati-
cally by Overton, is based on the production of osmotic
effects (entrance or exit of water) when the cell is placed
in solutions having a different osmotic pressure from
that of the cell-contents (aniso tonic solutions). In
hypertonic solutions of substances which do not readily
traverse the plasma membrane the cell shrinks, in
hypotonic solutions it swells, while in isotonic solutions
its volume remains unchanged. The problem is to
determine the relative degree of permeability to differ-
ent substances, some of which may traverse the mem-
brane, but with unequal readiness. To do this the
behavior of the cell is observed in a hypertonic solution
of the substance under examination. It is clear that
when the dissolved substance in the external medium
is quite unable to penetrate the membrane, it exerts
pressure against the latter (by the continued impacts
of the molecules), and since this pressure is greater than
that exerted by the dissolved molecules within the cell,
water is extracted (or expressed) from the latter until a
permanent state of equilibrium is reached in which the
osmotic pressure of the cell-contents equals that of the
surrounding solution. The cell is then permanently
shrunken. When, however, the dissolved substance

^ Bethe, Arch. ges. Physiol., CXXVII (1909), 219; Warburg,
Z. physiol. Chem., LXVI (1910), 305; Harvey, Journal of Experimental
Zoology, X (1911), 507; Chambers, Jour. Gen. Physiol., V (1922), 189.

114 PROTOPLASMIC ACTION AND NERVOUS ACTION

penetrates gradually, its water-abstracting effect is only
temporary, since its concentration, at first greater
outside than inside the cell, is eventually equalized by
difi'usion; the cell then tends to resume its original
water-content. In the case of a solute which penetrates
rapidly (with a readiness like that of water) no effective
inwardly directed pressure can be exerted against the
membrane and no osmotic effect is produced.

Using plant cells (Spirogyra and others) and moder-
ately hypertonic solutions of various substances, Overton
found that in solutions of sugars, amino-acids, and neutral
salts the plasmolysis was permanent; in solutions of
glycerine, glycol, urea, and similar compounds the
degree of plasmolysis was less than in sugar solutions,
and after the initial shrinkage, water gradually re-
entered the cell; while in solutions of alcohols and many
other organic substances (of the same osmotic pressure
as the effective sugar solutions) plasmolysis was entirely
absent. Similar differences were typical of a large
number of other compounds. He therefore divided
soluble substances into three groups, according to their
ability to penetrate the living plasma membrane:
(i) Those to which the plasma membrane is completely
or nearly impermeable, including sugars, polyatomic
alcohols (from erythrite up), soluble amino-acids, neutral
salts of alkali and alkali earth metals; (2) those which
penetrate the membrane, but slowly and with var^dng
degrees of resistance, including glycol, glycerol, and
certain amides such as urea; and (3) those which enter
without encountering any evident resistance; here
belong a variety of organic compounds of the groups
cited above. These general conditions were found by

PROTOPLASMIC STRUCTURE 115

Overton and other investigators to be characteristic
both of animal cells (blood corpuscles, muscle cells,
egg cells, etc.) and plant cells of the most varied kinds.

In the partition method, as used by Hedin and others,
the distribution of dissolved substances between the
cells and the solution is measured directly (by cryoscopic
determinations), and its results agree closely with those
of the plasmolytic method. In general it has been found
that the above-cited conditions of permeability are highly
characteristic if not universal in living protoplasm.^

Since in general the lipoid-soluble substances which
penetrate living protoplasm are also highly surface-
active, the conclusion may be drawn that either lipoid-
solubiKty or surface-activity (or both) is a property
favorable to penetration (Overton, Traube). The
penetration of one substance through another may
depend on mutual solubility; the cases of the rubber
membranes used in Flusin's experiments, the water-
soaked bladder partitions employed by Nernst, and the
palladium partitions of Ramsay's experiments with
nitrogen and hydrogen may be cited as illustrations.^
Overton explains the permeability of the plasma mem-
brane to lipoid-soluble substances as an expression of the
solubility of these substances in the lipoids of the mem-
brane; in general he finds a parallelism between the
lipoid-water partition-ratio of a given substance and
its ability to penetrate cells; this is illustrated by the
behavior of substitution-products and of the members of

^ Cf. Hamburger's Osmotischer Druck ti. lonenlehre for an account
of comparative investigations in this field.

2 Flusin, Afin. de chim. et phys., XIII (1908), 480; Nernst, Z.
physik. Chem., VI (1890), 37; Ramsay, Z. physik. Chem., XV (1894),
518. Cf. p. 144 below.

ii6 PROTOPLASMIC ACTION AND NERVOUS ACTION

homologous series. The contrast between NH4OH
(lipoid-soluble) and KOH (Hpoid-insoluble) in their
abiHty to enter Hving cells is a striking one;^ and most
investigators who have studied the penetration of sub-
stances (like acids and bases) whose behavior can be
determined with accuracy, have found a general parallel-
ism between the rate of penetration and the lipoid-
solubility of the various compounds. This parallelism,
however, is not exact, indicating the presence of other
factors.^

Impermeability to neutral salts in balanced solution
is an especially important property of the plasma mem-
brane, since it renders possible permanent differences
of salt-content between protoplasm and surroundings.
This means differences of ionic content, and is probably
a condition of the normal electrical polarization (''physio-
logical polarization") of the membrane, since any
partition separating two solutions of different ion-content
is typically the seat of an electrical potential difference.
The results of the mineral analysis of cells, as well as
those of the plasmolytic and partition methods just
described, show clearly the inability of neutral salts to

* Cf. Warburg, loc. cit.; Harvey, loc. cit.; Gray, Proceedings of the
Royal Society, B, XCIII (1922), 104 (cf. p. no); Jacobs, Jour. Gen.
Physiol., V (1922), 181. A similar contrast exists between organic
acids (lipoid-soluble) and strong acids; cf. Loeb, Biochem. Zeitschrift,
XV (1909), 254; Bethe, loc. cit.; Gray, loc, cit. Jacobs' results especially
show the extraordinary ease with which carbon dioxide and ammonia
penetrate Hving protoplasm.

'Harvey, Intern. Z. physik. chem. Biol., I (1914), 463; Crozier,
loc. cit. Miss Collett's studies on the toxicity of acids to infusoria show
a similar general relationship between organic solubility and toxicity
{Journal Experimental Zoology, XXIX [19 19], 443, and XXXIV [192 1],
67, 75)-

PROTOPLASMIC STRUCTURE 117

penetrate the unaltered living cell. Paine^ has shown
experimentally that NaCl, (NH4)2S04, and Na2HP04
in n/io solution do not enter yeast cells appreciably
even after hours of immersion; and numerous analyses
have shown that the specific salt-content of many
living cells (blood corpuscles, muscle, etc.) is quantita-
tively or even qualitatively entirely different from that
of the medium. For example, sodium salts seem to be
almost completely absent from vertebrate muscle cells.^
This result seems incompatible with more than a very
limited permeability of the plasma membrane to these
substances. Either the salts do not diffuse across the
membrane, or some active physiological factor is at
work which opposes or compensates the effect of diffusion
and maintains the salt content of the protoplasm at a
certain norm. The nature and proportion of the salts
present in any species of cell are characteristic of that
cell and apparently represent a constant feature of its
chemical organization; this is illustrated, for example, in
the analyses of the salt-content of mammalian blood
corpuscles by Abderhalden and others.^ In the cor-
puscles of the horse, pig, and rabbit there is little or
no Na and an abundance of K; in the ox, sheep, goat,
dog, cat, the amount of Na is greater than that of K.
Inorganic phosphates are always much more concen-
trated in the corpuscles than in the serum, which is
always rich in Na and poor in K. Voluntary muscle
cells are rich in K salts and phosphates and poor in
Na salts.

^ Paine, Proceedings of the Royal Society, B, LXXXIV (191 2), 289.
^ Urano and Fahr, cf. chap. viii.
3Hober, loc. cit., p. 370.

ii8 PROTOPLASMIC ACTION AND NERVOUS ACTION

Hober {op. cit., p. 371) cites certain observations of
Warburg made in 1911^ indicating that the erythrocytes
of the goose are influenced in their oxygen- consumption
by alkaH-earth salts only after the plasma membrane
has been destroyed by freezing and thawing. Apparently
these salts cannot pass the intact plasma membrane.
On the other hand, alcohols and urethanes check oxida-
tions in the intact erythrocytes; these substances can
penetrate. It may be noted that these observations
are consistent with the view that the nuclear surface
is a chief factor in the oxidation-processes of these cells.^

The relative or complete impermeability of blood
corpuscles to the ions of the surrounding salt solution is
also indicated by the low electrical conductivity of
these cells. Stewart, Tangl, Bugarszky, and others
have shown that the electrical conductivity of blood is
due almost entirely to the plasma. Low electrical
conductivity is in fact now known to be a highly constant
and characteristic peculiarity of cells during life, and
the evidence indicates that the high resistance is chiefly
if not entirely a property of the plasma membranes.
The conductivity of the cell as a whole appears to vary
directly with the permeability of the plasma membrane
to crystalloidal solutions. All conditions that increase
general permeability (action of cytolytic substances or
unbalanced salt solutions or of poisons, high tempera-
tures, or other lethal agents) also increase electrical
conductivity. According to Osterhout, the most exact

^Warburg, Z. physiol. Chem., LXX (191 1), 413-

'In the nucleated erythrocytes of the frog, the indophenol test
shows active oxidation at the nuclear surface; cf. R. S. Lillie, Journal of
Biological Chemistry, XV (1913), 237.

PROTOPLASMIC STRUCTURE 119

measurements of permeability are those given by elec-
trical conductivity. The conductivity of a living tissue
is a measure of its permeability to ions; and while it is
conceivable that the permeability to other substances,
such as non-electrolytes like sugar, may vary inde-
pendently (within certain limits) of the permeability to
ions, the advantages of estimating permeability quan-
titatively are such that the conductivity method must
be regarded as the one to be preferred wherever it can
be applied.

Osterhout has shown that by means of the con-
ductivity method the permeability of plant tissues
under varying external conditions can be readily and
accurately determined; also that permeability can be
varied at will, reversibly, in either direction, especially
under the influence of neutral salts and lipoid-solvent
compounds.^ In pure NaCl solutions the permeability
of Laminaria fronds is increased, to a degree depending
on the duration of exposure and the temperature; on
replacing the tissue in sea water the permeability returns
to or toward the normal, the degree of possible recovery
depending upon the extent of the change produced by
the NaCl solution.^ If the permeability has been
increased beyond a certain limit, its reversal is impossible
and the plant is dead. He has suggested, therefore,
that the property of "vitality" may be measured by
determining the electrical resistance.^ Isotonic CaClz
solutions have the opposite kind of effect and at first
decrease permeability; the antagonism between Na and

' Osterhout, loc. cii., and Science, XXXVII (191 2), 3; also "Quanti-
tative Researches in Permeability," The Plant World, XVI (1913), 129.

2 Osterhout, Jour. Gen. Physiol., Ill (1920), 145.

3 Osterhout, Science, XL (1914), 488.

I20 PROTOPLASMIC ACTION AND NERVOUS ACTION

Ca salts is thus explained; when the two salts are
present in appropriate proportions (20 NaCl+i CaCla)
conductivity is unaltered, and the tissue remains living
for days, while in the pure solution of either salt toxic
action and death soon result. The changes of permea-
bility accompanying normal physiological processes have
also been measured by the conductivity method in cer-
tain cases, especially by McClendon and Gray in the
fertilization of sea-urchin eggs.^

The question of the condition of the salts in the living
protoplasm arises here; and this question is of consider-
able general importance, since it has been held by certain
investigators that these salts are present chiefly or
entirely in a combined or adsorbed state and are hence
not free to act as conductors. As we shall see later in
dealing with the phenomena of stimulation and trans-
mission, the electrical conductivity of the internal
protoplasm appears to be a necessary factor in its
normal activity; and any evidence that this conductivity
is what we should expect it to be from the known salt-
content of protoplasm is of interest. The contention
that the salts in protoplasm are non-ionized, or that the
ions are in some manner rendered immobile, is incon-
sistent with the physico-chemical observations relating
to the behavior of salts in the presence of proteins, lipoids,
or other colloids. According to Bugarszky and Lieber-
mann, the addition of even large quantities of protein
to salt solutions affects the ionic concentration only
slightly;^ Michaelis and Rona^ have shown by '^com-

^ McClendon, American Journal of Physiology, XXVII (19 lo), 240J
J. Gray, Journal of the Marine Biological Association, X (1913), 50.

* Bugarszky and Liebermann, Arch. ges. Physiol., LXXII (1898), 51.
5 Michaelis and Rona, Biochem. Zeitschrift, XIV (1908), 476.

PROTOPLASMIC STRUCTURE 121

pensation dialysis" that the salts in serum (containing
10 per cent protein) are freely dialyzable. Pauli and
Samec have also shown that alkali salts are not more
soluble in serum than in water.^

There is the possibility of the formation of com-
binations with the amino-acids present in the proteins;
but the salts of such weak acids would theoretically be
almost completely hydrolyzed; i.e., a stable combination
with protein in which the salt is completely and firmly
bound is scarcely conceivable. Any protein-salt com-
binations thus formed would hydrolyze, and the products
of hydrolysis would diffuse out through the membrane if
the latter were permeable; and further hydrolysis would
proceed until an equilibrium was reached in which a
large proportion of salt was present in the free dissolved
state."^

General chemical theory thus indicates that only a
very small proportion of the salt in the cell can be in a
state of permanent combination with protein; hence the
characteristic difference between the salt-content of the
cells and that of the medium cannot be thus explained.
It is probable, therefore, that the semi-permeability of
the plasma membrane is an essential factor in making
possible this difference.

If it were possible to measure the electrical con-
ductivity of the cell interior apart from that of the
plasma membrane, the question could be answered at
once. According to the view presented above, the chief

* Pauli and Samec, Biochem. Zeitschrift, XVII (1909), 235.

2 In other words, the proteins and other compounds present in the
cell could not hold more than a very small proportion of the salts in a
combined and indiffusible form.

122 PROTOPLASMIC ACTION AND NERVOUS ACTION

^ o o — ^—

■^g'^

s.bsmsismJ

A

T

barrier to the movement of ions is at the semi-permeable
plasma membrane. The increased conductivity observed
at death, or after cytolysis by saponin or other com-
pounds, favors this view, since semi-permeability is then
lost; i.e., there is a general parallelism between the ability
of salts, sugars, and other soluble compounds to pass
the plasma membranes and the electrical conductivity

— a fact indicating that the
chief condition rendering
living cells such poor con-
ductors of electricity is
the impermeability of the
membranes to ions. From
these considerations we
should conclude that the
ions within the cell are free
to diffuse within the space
inclosed by the plasma
membrane. But the case
requires to be tested by
experiment, for it might
be held, in spite of the fore-
going considerations, that
the electrolytes are in some
manner chemically combined in living protoplasm and
are set free only at death.

Hober has attempted to measure the conductivity of
the cell interior, using in part methods suggested to him
by Nernst. A device of the plan shown in Fig. i was first
employed.' A rapidly alternating current is induced
in the circuit containing the two balanced condensers

^ Hober, Arch. ges. Physiol., CXXXIII (1910), 237.

-R.

Fig. I. — I, inductorium; C, condenser;
G, spark-gap; S, coil (self-induction) of pri-
mary circuit; 53, coil of secondary circuit; A,
adjustable condenser; B, condenser with
space between plates for insertion of vessel
(F) containing suspension of cells; B, detec-
tor in bridge; Ri, Ri, resistances (from Hober,
loc. cit.).

PROTOPLASMIC STRUCTURE ' 123

A and B. When the capacities of the condensers are
the same and the resistances of the two halves of the
circuit (on either side of the bridge) are the same (or

when their ratios are equal; i.e., when "5^ = 7^)) no

current passes through the detector; but if the capacity
of one condenser (A) is changed, the current passes until
the capacity of the other (B) is made the same. Now
the capacity of a condenser is increased if a conducting
layer is introduced into the dielectric between the
plates, and to a degree which is proportional to the con-
ductivity of the layer. Hober's method, therefore,
consists in introducing between the plates of one of the
condensers (B), after the system has been brought into a
balanced condition, a glass vessel containing a suspension
of living cells (blood corpuscles) in isotonic sugar solution.
Such a suspension does not conduct electricity (in the
usual Kohlrausch sense), yet it increases the capacity
of the condenser and so allows a current to flow across
the bridge. This result shows that the suspension con-
tains an electrically conducting fluid; this can only be
in the interior of the cell, since the suspension-medium
is sugar solution, which is a non-conductor. The
addition of saponin (which destroys the membrane and
increases the Kohlrausch conductivity) was found not
to change the capacity, indicating that the conductivity
of the cell-contents is the same whether the semi-
permeable membrane is present or not. If a glass vessel
containing a salt solution is placed between the condenser
plates, a similar increase of capacity is shown; and by
comparing the effects produced by salt solutions of
known conductivity with those produced by suspensions

124 PROTOPLASMIC ACTION AND NERVOUS ACTION

of cells, the internal conductivity of the latter can be
estimated. The measurements cannot be made very
exact; they indicate, however, that the internal con-
ductivity of the corpuscle is of the order of that of a o.i n
KCl solution (between o.i n and o.oi n).

Hober also experimented with another method some-
what different in principle.^ If high-frequency oscilla-
tions are induced (lo^ per second) in a circuit containing
a condenser and a detector (spark-gap), and the beaker
{B, Fig. 2), encircled by a coil of wire forming part of

?=— ?

Fig. 2. — C, condenser; V, beaker containing suspension of corpuscles; G, spark-
gap; S, coil in which current is induced (Hober, loc. cit.).

the circuit, is filled with a conducting solution, the oscilla-
tions are damped. Hober again found that cells sus-
pended in sugar solution produce this effect, and to
the same degree whether they are intact or cytolyzed
with saponin, although the Kohlrausch conductivity
is, of course, much greater in the latter case. The
low Kohlrausch conductivity of intact cells is thus
apparently due to the inclosing plasma membranes;
the internal conductivity of the protoplasm is high.
Again by comparing the effects produced by cell-
suspensions with those produced by salt solutions of

' Hober, Arch. ges. Physiol., CXLVIII (1912), 189.

PROTOPLASMIC STRUCTURE 125

known conductivity, Hober found that the suspension
gave the same effect as a NaCl solution of a concentra-
tion similar to that of blood plasma (i.e., between o.i
and 0.4 per cent). Further accuracy is not possible.

Hober also used the method of measuring the con-
ductivity of cells by means of rapidly oscillating cur-
rents.^ According to theory, we should expect that the
higher the oscillatory frequency the less difference would
the presence of the membrane make. Under the condi-
tions, frog's muscle, which had been washed thoroughly
in sugar solution, behaved as if it had a conductivity
between o.i and 0.2 NaCl; the greater the frequency
of alternation the greater the conductivity. This is
. readily understood if we reflect that when the carriers
of the current (ions) have to pass through only very
short distances, they are not impeded in their movements
by the membrane.

From the results of the experiments above Hober
reaches the following conclusion:^ ''It can be regarded
as certain that the blood corpuscles possess a very con-
siderable internal conductivity, even when the external
conductivity is minimal; i.e., that free ions are present
in their interiors, and that these are prevented from reach-
ing the outside only by the presence of a barrier to
diffusion"; i.e., the plasma membrane.

The observations cited by Kite^ and others, as indi-
cating the impermeability of the internal protoplasm
to diffusing substances, are probably to be explained as
indicating the rapidity with which a semi-permeable

^ Hober, Arch. ges. Physiol., CL (1913), 15.

2 Hober, Physikalische Chemie der Zelle und der Gewebe (i9i4),p. 385.

3 Kite, Biological Bulletin, XXV (1913), i.

126 PROTOPLASMIC ACTION AND NERVOUS ACTION

barrier is formed at any free surface of living protoplasm.'
This property of forming fresh semi-permeable surface
films at exposed protoplasmic surfaces has long been
known. Nageli^ describes experiments with the root
hairs of Hydrocharis; by crushing these structures
under a cover-glass the protoplasm may be expressed in
the form of separate rounded vacuolated masses. Each
of these shows the same osmotic properties as the original
entire protoplast; i.e., shrinks in hypertonic solutions,
resists the entrance of dyes (during the living state),
and in general behaves as if it were surrounded with a
semi-permeable membrane. Such an experiment might
also be regarded as indicating the impermeability of all
parts of the protoplast to dissolved substances. But it
can be simulated by the simple experiment of breaking
or cutting up a large drop of an insoluble liquid, such as
chloroform, in a protein-containing medium; each result-
ing droplet remains separate and exhibits the same
properties as the original droplet, and the effect can be
shown to depend on the rapid formation of a thin protein
adsorption-film at the surface of each newly formed
droplet. In a somewhat similar manner, although the
conditions are more complex, living protoplasm forms
films at surfaces which are freshly exposed by cutting
or other injury; this property is shown only during life
and is presumably a manifestation of the normal property
of construction and repair, which is dependent on meta-
bolic synthesis, as we have seen. This is indicated by an
experiment of Pfeffer's,^ in which root hairs are placed in

* Of. Chambers, Jour. Gen. Physiol., V (1922), 189.
*Nageli, Pflanzenphysiologiscke Studien, Zurich (1885).
^ Osmotische Untersuchujigen (1877), p. 136.

PROTOPLASMIC STRUCTURE 127

a weak solution of acid, which kills the cells. If then
they are placed in a weakly hypotonic solution containing
dye, the latter immediately enters as soon as the cell
swells; i.e., the continuity of the plasma membrane is
then no longer automatically maintained (as in living
cells) when the membrane is stretched or ruptured.

Presumably in living protoplasm both metabolic
and purely physical factors (adsorption) take part in
the formation of new surface-films. There is evidence
that the protoplasmic surface-films undergo a change
in their physical properties soon after they are formed;
this is well shown in the freshly cut surfaces of sea-urchin
eggs. Kiister found that the protoplasm of plant
cells could be broken into fragments by strong plas-
molysis; these fragments at first readily unite or cohere,
but later lose this property.^ In the coalescence of frag-
ments of inert inorganic material a similar behavior has
been observed; freshly formed surfaces reunite readily,
but not older surfaces ; apparently the progressive deposi-
tion or adsorption of foreign materials at the surfaces
alters their properties and prevents fusion.^ There is
also evidence that the formation of new surface-films
plays an essential part in the normal return of irritable
cells to the resting state after stimulation; during the
period of recovery of excitability (refractory period) the
cell-surface is apparently the seat of progressive changes
of this kind (see below under refractory period).

Hober cites the case of myxomycetes possessing a
clear surface hyaloplasm and a granular interior ;3 when

'Kiister, Ber. deutsch. hotan. Ges., XXVII (1909), 589.

' For an account of these phenomena cf. Bancroft's Applied Colloid
Chemistry, chap, v, on coalescence.

3 Hober, op. cit. (1914), p. 64.

128 PROTOPLASMIC ACTION AND NERVOUS ACTION

the cell swells in hypotonic solution the thickness of the
hyaloplasm remains constant; apparently this thick-
ness is regulatively maintained by the transformation of
material derived from the internal protoplasm. Cham-
bers' observations on marine eggs show many interesting
instances of the formation of films at cut surfaces or at
the surfaces of protoplasmic fragments; regions where
the protoplasm is broken down by mechanical or other
injury soon become delimited by films bounding them
from the adjoining unaltered protoplasm.^ In the
formation of artificial vacuoles by the injection of
solutions into egg cells through micro-pipettes, films
with semi-permeable properties are formed about the
introduced droplets.^ The composition of the salt
solution is an important factor in the formation of such
vacuoles; Chambers has recently shown that pure solu-
tions of NaCl diffuse into the protoplasm without
forming films, while if sufficient CaCla is present, each
droplet of solution surrounds itself with a definite film
and forms a vacuole. A recent study by Seifriz^ of
the physical properties of protoplasm, as exhibited under
micro-dissection, gives many interesting details on film-
formation by living protoplasm under various con-
ditions.

De Vries showed in 1885'' that the normal vacuoles
of plant cells are surrounded by membranes having the
same osmotic properties as the plasma membranes
(those inclosing the entire protoplast). In his experi-

^ Chambers, loc. cit.

2 Kite, loc. cit.; Chambers, Jour. Gen. Physiol. , V (1922), 189,

3 Seifriz, loc. cit.

^Jahrb. Tviss. Botanik, XVI (1885), 465.

PROTOPLASMIC STRUCTURE 129

merits the cell was placed in a 10 per cent solution of
KNO3 containing some eosin to serve as indicator of
permeability; the outer protoplasm dies in one or two
hours and becomes colored, but the vacuole remains at
first clear and uncolored, showing impermeability of its
membrane to the dye. Later the membrane becomes
permeable to the dye, presumably as a result of death-
changes, and the vacuole contents become colored.
Isolated vacuoles show osmotic properties similar to
those of the whole protoplast. The observations of
Kite and Chambers on artificial vacuoles in sea-urchin
eggs illustrate the same phenomenon, the limiting surface-
film of the vacuole having apparently the same properties
as the surface-film of the entire cell.

The evidence just cited shows the error of regarding
the properties of continuous protoplasm as identical
with those of partitioned protoplasm. At boundary
surfaces of whatever kind the living substance exhibits
special properties, in particular a high resistance to the
diffusion of water-soluble substances including ions.
On the other hand, the internal protoplasm appears (at
least in many cases) to be freely penetrable to diffusing
substances of low molecular weight and to ions; hence
it possesses a considerable electrical conductivity. This
latter conclusion is of great importance for the theory of
stimulation and conduction, as will be seen below.

As additional evidence for the permeability of con-
tinuous or unpartitioned protoplasm to water-soluble
substances, we may cite the type of distribution shown
by many diffusible substances in cells. Many such sub-
stances are concentrated in special regions or structures,
in which they appear to be held in chemical or other

I30 PROTOPLASMIC ACTION AND NERVOUS ACTION

combination; a good illustration is the water-soluble red
pigment of Arbacia eggs, echinochrome, which is con-
tained chiefly in minute round chromatophores scattered
throughout the protoplasm. That this material is free
to diffuse is shown, however, by the fact that cytolytic
action, even temporary, causes its rapid diffusion into
the surroundings; and. complete loss of pigment may
thus result. The localization or condensation of the
pigment in the chromatophores indicates apparently
that it is present there in some form of loose chemical
combination or adsorption; but in any such case an
equilibrium between the adsorbed and the dissolved
compound must exist (Cads = KCs{f). Presumably the
dissolved portion of the pigment is homogeneously
distributed throughout the protoplasm, hence the
chromatophores adsorb (or combine) equal quantities
and are similar in appearance. But any local decrease
in the concentration of dissolved pigment, due to diffusion
through the altered plasma membrane, disturbs the
adsorption equilibrium and leads to the liberation of the
adsorbed pigment, which may thus be completely lost
from the cell. The conditions are similar in any other
case of reversible adsorption.

Hermann's observations on the difference between the
transverse and the longitudinal conductivity of nerve
fibers also indicate the higher conductivity of non-
partitioned protoplasm as compared with partitioned.
He found that the current encounters several times the
resistance when the fibers are placed transversely between
the electrodes, as compared with fibers arranged length-
wise, with equal distances between the electrodes and
equal sectional areas of tissue. The same is true for

PROTOPLASMIC STRUCTURE 131

muscle.^ This result indicates again that the inter-
posed plasma membranes offer a high resistance to the
movement of ions, while the resistance of the continuous
protoplasm is relatively slight.^ ♦

^Hermann, Arch. ges. Physiol., V (1872), 223. The character of
the intercellular partitions and their arrangement will, of course, affect
the inner conductivity of the cell, but regarding these conditions we have
little information at present.

^ Compare the conclusions of Stewart (/. Pharmacol, and Exper.
Therapeutics, I [1909], 49); also Chambers (1922), loc. cit.

CHAPTER VII

GENERAL CONDITIONS DETERMINING THE
PROPERTIES OF PROTOPLASMIC MEMBRANES

Overton's and later researches have shown that the
permeabiHty of plasma membranes to lipoid-solvent
substances is a universal property, that the permeability
to water-soluble substances of low molecular weight, not
soluble in organic solvents, is relatively very slight, and
that the ability of substances to penetrate most forms of
protoplasm has a close dependence on their relative solu-
bility in the two classes of solvents; i.e., on their par-
tition-ratios between organic (oil-like) solvents and water.
The permeability for the two classes of substances,
water-soluble and '^organo-soluble/' thus depends on
different conditions. It is further significant that the
permeabiHty for hpoid-soluble substances is m_uch less
variable than that for the lipoid-insoluble substances
and water; the latter form of permeability shows wide
variations in the same cell under different physiological
conditions, while the former appears to undergo little
change. The characteristic semi-permeability of the
living plasma membranes thus relates to the lipoid-
insoluble group of substances; this fact, when con-
sidered in connection with the universal permea-
biHty to the Hpoid-soluble group, suggests that the
normal semi-permeabiHty depends on the presence in
the membranes of water-insoluble compounds pos-
sessing the solvent properties of organic solvents.
The characteristic water-insolubiHty of the surface-

132

PROPERTIES OF PROTOPLASMIC MEMBRANES 133

layer of protoplasm is also intelKgible on this point of
view.

When we say that a cell or tissue is permeable to a
given substance, we state merely the ability of the
substance to penetrate; the method of penetration is
not stated. Thus a dissolved substance may penetrate
a partition by diffusing through the interspaces, or by
dissolving in or combining chemically with the substance
of the partition. In the case of ions there may be an
apparent penetration resulting from a change in the
electrical conditions at the surface of the partition. For
example, when a current is passed by zinc electrodes
through a bath of a zinc salt solution divided into two
compartments by a zinc partition, it is not strictly
correct to say that the zinc ions penetrate the partition.
The effect is the same as if they did, but in reahty zinc
ions are deionized and deposited as zinc on one face,
while from the other face the metal passes into solution
as zinc ions. There is apparently a penetration in such
a case, but not in reality; and it is possible that under
certain conditions the passage of a current through a
plasma membrane may similarly depend on the chemical
combination of ions on the one face simultaneously
with their release from the other. The physical and
chemical conditions of these various types of permeability
require careful examination.

The factors determining the penetration of dissolved
substances through living plasma membranes are various,
and for the purpose of the present discussion they may
be grouped under the following heads: (i) structural
conditions, including thickness, density (water-content),
state of dispersion of structural colloids, size of interstices

134 PROTOPLASMIC ACTION AND NERVOUS ACTION

or pores, (2) chemical conditions, including special
composition and solvent properties of constituents, (3)
physical conditions of other kinds, such as electrical
polarization at the faces of the partition or other surfaces,
electrical endosmose effects, filtration effects, and (4)
factors dependent on the Hving condition of the mem-
brane, including variation of permeabiHty due to meta-
bolic or other changes; the last-named factors are
apparently the ones chiefly responsible for the active
types of material transport in and through cells in
absorption and secretion (factors of ''physiological

permeability")-

A complete discussion of these various factors need
not be attempted here, especially since the whole subject
of permeability has recently been carefully reviewed
in the well-known textbooks of Bayliss and Hober.
Certain aspects of the problem of permeability require
special consideration, however, especially the dependence
of permeabiHty on physical conditions, and the nature
of the factors controlling the normal or physiological
variations of permeability. The most characteristic
pecuHarity of the Kving protoplasmic membranes,
especially of irritable cells, is that their properties vary
with both the external and the internal conditions, and
are subject to regulative control of a highly definite
kind. Such properties cannot be derived from any
simple static type of structure, or explained on the basis
of the pecuUarities of the colloidal compounds composing
the membrane. We must recognize that the distinctively
vital factors, those depending on the metaboHc activity
of the cell, are probably the most important of all, and
they are the least understood at present. The simpler

PROPERTIES OF PROTOPLASMIC MEMBRANES 135

physico-chemical or non- vital factors can, however,
be shown in some cases to have definite relations to
certain characteristic physiological effects; e.g., in the
action of salts and lipoid-solvent compounds on Hving
cells. These are the substances through whose action
the physiological properties and activities of cells may
most readily be altered in a reversible manner; and this
action is referable in many cases to a direct alteration
of the plasma membranes.

PERMEABILITY AND STRUCTURAL CHARACTER

OF MEMBRANES

Semi-permeability requires a certain closeness of
physical texture; i.e., density of structural material;
apparently also the structure-forming substances must
be water-insoluble. In all semi-permeable artificial
membranes water-insolubility is essential; the most
perfect examples are the precipitation-membranes of the
ferrocyanides and other insoluble salts of heavy metals.
That the chief compounds composing the plasma
membranes are also water-insoluble is shown by the
characteristic insolubihty of Hving cells in their normal
aqueous media.

The relation between structural density and permea-
bility in artificial membranes is well shown in the ''ultra-
filters" devised by Bechhold for the^ separation of various
colloids from their suspension-media.^ These filters
consist of disks of gelatine hardened in formalin. Bech-
hold found that the higher the colloid-content of these
partitions, the more numerous were the colloidal sub-

^ Bechhold, Colloids in Biology and Medicine, EngUsh translation,
or third Gennan edition, 1920.

136 PROTOPLASMIC ACTION AND NERVOUS ACTION

stances which were held back in filtration experiments;
for example, disks of 10 per cent content prevented the
passage of nearly all the colloids employed. From such
facts we should expect that a still higher degree of
impermeability (to crystalloids) would require a higher
degree of structural density; i.e., lower water-content.

Experience with artificial semi-permeable membranes
of copper ferrocyanide bears out this expectation; this
is well seen in Morse's studies of these membranes,
described in his book, Aqueous Solutions.^ He found
that for the formation of good semi-permeable membranes
an extremely fine porosity in the supporting porcelain
cells was necessary; '^ excessive fineness of texture is
absolutely indispensable to the correct measurement of
osmotic pressure" (p. 15). A good semi-permeable
membrane is thus essentially a fine-textured structure of
water-insoluble material; hence it resists the passage
of water as well as of dissolved substances. Morse also
considers that the colloidal character of the precipitate
is an important factor in semi-permeabihty. Even
under the best conditions a high degree of semi-permea-
bility is difficult to obtain with precipitation-membranes,
and this property is subject to change; it is affected by
temperature (cf. pp. 85-86) and especially by electrolytes
(cf. pp. 91-92) which cause rapid deterioration in the
membrane. LiCl was the least harmful of the electro-
lytes investigated (cf. p. 214) and Morse gives determina-
tions of the osmotic pressure of this salt.

Since fineness of texture is favorable to semi-permea-
bihty (i.e., to the production of osmotic effects), one
might expect that any insoluble porous partition would

^ Morse, Aqueous Solutions, Carnegie Institute Publications (1914).

PROPERTIES OF PROTOPLASMIC MEMBRANES 137

show osmotic action (semi-permeability) if only its
pores were sufficiently minute. In fact, Thomas Graham
observed many years ago (1854) that certain fine-grained
porcelains had definite osmotic action. Recently the
pore-diameters of osmotically acting porcelain disks
have been investigated by Bigelow and Bartell/ and
well-marked osmotic effects were found when the pores
had a diameter of the order o. 2-0.3 5/x.^

In general we may conclude from such facts that in
living semi-permeable membranes the degree of porosity
is low; i.e., the interspaces between the water-insoluble
colloidal particles forming the structural material are
at least as small as in porcelain and probably smaller,
since Bartell's membranes are not completely semi-
permeable. The physical conditions are probably closely
comparable with those existing in well-supported precipi-
tation-membranes of copper ferrocyanide, such as Morse
employed. In plasma membranes, however, variations
in the subdivision of the colloidal constituents may cause
variations in the size of the interspaces, and hence in the
permeability. The progressive deterioration to which
artificial colloidal membranes are subject in the pres-
ence of electrolytes is apparently prevented in Hving
membranes by compensatory factors dependent on me-
taboKsm; presumably any interruptions of continuity are
at once automatically repaired by the formation or
deposition of new structural material (see below under
stimulation) .

^Bigelow, Journal of the American Chemical Society, XXIX (1907),
1675; Bartell, Journal of Physical Chemistry, XV (191 1), 659, and XVI
(1912), 318.

^ Bartell. loc. cit. (191 2).

138 PROTOPLASMIC ACTION AND NERVOUS ACTION

Such fine- textured membranes must resist the passage
of water as well as of dissolved substance. In Morse's
experiments many days were often required to reach
osmotic equihbrium. There is also evidence that many
living plasma membranes offer a high resistance to the
passage of water: i.e., are relatively water-impermeable.
This statement may seem surprising in view of the fact
that the passage of water into and out of living cells in
anisotonic solutions is usually rapid; but the membranes
are extremely thin and the ratio of the surface to the
inclosed volume is very large, so that relatively rapid
entrance of water is quite consistent with a very low
specific permeability to the Hquid.

The loss of semi-permeability at death is associated
with an increase of permeabihty to water as well as to
dissolve substances. This is shown in some experiments
of Bernstein who, with another problem in mind (the
conditions under which water is held in cells) , determined
the relative rates of evaporation of water from living
and dead tissues.^ Bernstein used the method (intro-
duced by Liebig) of measuring the ''force of imbibition"
of membranes. A tube widened at one end (thistle tube)
is fixed vertically with its narrow end dipping into
mercury; the tube is completely filled with water and
the upper expanded end is closed by the membrane under
examination. As water evaporates through the mem-
brane the mercury rises in the tube, showing the develop-
ment of a pressure; the rate of evaporation through the
membrane is thus indicated. Bernstein used fine tubes
ending above in equally sized funnels which were closed
(A) with living membrane and (B) with dead membrane

^ Bernstein, Electrohiologie, Braunschweig (1912), pp. 167 £f.

PROPERTIES OF PROTOPLASMIC MEMBRANES 139

(e.g., killed by heat). Results of the following kind
were obtained with frog skin:

(A) Living (B) Dead

After 4 h ht— 23 (mm.) 52 (mm.)

6h 85 160

8h 180 285

10 h 256 347

(both dead)

23 h 401 400

It is clear that the rate of evaporation is much greater
through the dead membrane. Similar experiments with
plant tissues (leaves) gave similar results; von Mohl
(in 1847) and Naegeli (in 1861) had already called
attention to the fact that frozen plant tissues (leaves,
fruits) dried more rapidly than Hving.^ Bernstein experi-
mented also with thin sheets of muscles (abdominal
obhque of frog) , both living and killed with chloroform,
and found that the rate of evaporation through the living
muscle was about half of that through the dead; the
difference was greatest during the first hour, when the
living tissue was relatively normal.

Bernstein's conclusion was that living tissue has
greater water-binding power than dead, and he regarded
the electrically polarized condition of the plasma mem-
branes as the chief factor hindering the outward passage
of water. Apparently a potential difference of the value
of ca. 0.1 volt exists normally between the outer and
the inner surfaces of the plasma membrane; this gradient
is positive externally and negative internally. Resistance
is thus offered to the passage of the positively charged
water outwardly across the membrane. This explana-

^ Cf. Bernstein, op. cit., p. 170.

I40 PROTOPLASMIC ACTION AND NERVOUS ACTION

tion, while ingenious, does not seem sufficient; in muscle
and other living cells immersed in anisotonic solutions
water seems to pass with equal readiness in either direc-
tion through the membrane; for example, Arhacia eggs
in hyper- and hypotonic sea water shrink and swell
(respectively) at about the same rates/ Hober refers
the difference observed by Bernstein to the greater
turgor of the living cells ;^ this explanation also seems
doubtful, since turgor is either absent or slight in verte-
brate tissues. The simplest as well as most probable
explanation is that the permeabiUty to water increases
with death, along with the permeability to other sub-
stances. As the plasma membrane loses its semi-
permeability, w^ith the associated fineness of texture, it
also loses its relative impermeability to water.

Permeability to water is one of the little studied
properties of cells. Yet it is an important property
which appears to be constant for a particular cell under
definite conditions. There is evidence that it varies
with the physiological state and activity of the cell,
and in certain cases, especially in gland cells, the indica-
tions are that it is under nervous control. Thus when
the chorda t^onpani is stimulated, the submaxillary gland
secretes a copious watery saliva; similarly the sweat-
glands and the kidney secrete actively under certain
conditions, but not under others. In most cases the
secretory substances leave the cell in aqueous solution;
and although the factors are complex, there seems to
be little doubt that the transport of the secretion across
the cell-boundary is associated with an increased permea-

' R. S, Lillie, American Journal of Physiology, XLV (1918), 406.
* Hober, op. cit., p. 255.

PROPERTIES OF PROTOPLASMIC MEMBRANES 141

bility to water as well as to dissolved substances. The
character of the bioelectric variation accompanying
secretion also indicates this.

Antropoff^ has studied the influence of the water-
permeability of osmometer membranes on the rate of
osmotic transfer of water. He reaches the general
formulation :

i.e., the rate of passage of water into an osmometer
-5^ j at any time is proportional to the permeability of

the membrane to water (a) and to the osmotic pressure
of the solution (P) less the hydrostatic or other pressure
(Pi) resisting the transfer. The rate of osmotic transfer
of water is thus proportional to the product of the
effective osmotic pressure into the permeability to water.
This formulation defines the conditions for perfect
membranes; i.e., those which are permeable to water
and impermeable to solute.

In the Arbacia egg the permeability to water is
altered in a remarkable manner as a result of fertilization.
A quantitative measure of this permeability may be
obtained by measuring the rate at which water enters
or leaves the egg in dilute or concentrated sea water.
This rate is found to be increased about fourfold after
fertilization, indicating that this process is associated
with a marked increase in permeability to water. If
mixed fertilized and unfertilized eggs are placed in dilute
or concentrated sea water, the former undergo change
of volume much more rapidly than the latter, and

^ Antropoff, Z. physik. Chem., LXXVI (1911), 721.

142 PROTOPLASMIC ACTION AND NERVOUS ACTION

within a minute or less the two can easily be distinguished
by their difference in size. The semi-permeability of
the plasma membrane is not impaired, although the
permeability to water is increased. The conditions of
this phenomenon are probably complex, and determined
by metabolic factors of unknown nature. The change
of permeability is progressive and occupies a considerable
time, some 20 minutes elapsing (at 20°) before it nears
its final stage; it is arrested by anaesthetics (chloral
hydrate, ure thane, alcohols) and by cyanide in higher
concentrations.'

If the impermeability to water in this cell is due to
the presence of water-insoluble substances (lipoids or
cholesterol) in the surface-film, the change above would
indicate that these are altered or removed in part; i.e., a
change in the chemical composition of the surface layer
is to be assumed. The nature of this change is unknown ;
but Lyon's observation that the iodine-combining power
of the egg is decreased after fertilization'' may indicate
a decrease in certain unsaturated compounds which
would otherwise take up the iodine — possibly cholesterol
or unsaturated lipins (lecithin). The permeabiHty of
fertilized eggs to water can be artificially modified in
certain ways; it is decreased by anaesthetics (chloral,
ure thane, alcohols),^ a fact also indicating a dependence
on the lipoid-content of the protoplasmic surface-film.

Chambers finds certain differences in the behavior of
fertilized and unfertilized echinoderm eggs in micro-

^ R. S. Lillie, American Journal of Physiology, XL (1916), 249;
XLIII (1917), 43; XLV (1918), 406.

2 Lyon and Shackell, Science, XXXII (1910), 249.

3 R. S. Lillie, American Journal of Physiology, XLV (1918), 427.

PROPERTIES OF PROTOPLASMIC MEMBRANES 143

dissection;^ the surface of the unfertilized egg is less
easily cut or torn with a needle, but exhibits less power
of repair than that of the fertilized egg. This latter
difference is probably to be correlated with the increased
rate of metabolism following fertilization; oxygen-
consumption, COz-evolution and heat-production are
then greater,^ also the susceptibility to KCN poison-
ing and other forms of chemical injury. Facts of this
kind illustrate the close correlation existing between the
physical state of the plasma membrane and the physi-
ological properties and metabolic activity of the cell.
Other examples of this correlation will be described
later, especially in relation to stimulation-processes,
which are intimately associated with changes in the
membranes.

PERMEABILITY AND SOLVENT PROPERTIES OE
MEMBRANE CONSTITUENTS

It was first pointed out by Nernst in 1890 that the
solvent properties of the substances composing a mem-
brane may determine the latter's permeability to dis-
solved substances, and hence the production of osmotic
effects. Thus when benzol is separated from ether by a
partition consisting of bladder membrane soaked in water
(in effect a layer of water), the ether, because of its greater
solubility in water, passes the partition more rapidly than
the benzol, and a pressure is set up on the benzol side
of the partition.^ The membrane is impermeable to

^ Chambers, Proceedings of the Society of Experimental Biology and
Medicine, XVII (i9i9),4i.

2 Cf. Shearer, Proceedings of the Royal Society, B, XCIII (1921),
213,410.

3 Nernst, Z. physik. Chem., VI (1890), 37.

144 PROTOPLASMIC ACTION AND NERVOUS ACTION

benzol, which is ahnost insoluble in water, while ether
is readily soluble (to the extent of about 12 volumes
per cent) and penetrates the partition. Experiments
illustrating the same principle may be performed with
gases; thus if a tube, containing air, closed below with
a water-soaked membrane and connected above with a
manometer, is placed in a vessel of ammonia or hydro-
chloric acid gas, the pressure rapidly rises in the tube
because of the penetration of the water-soluble gas
through the water-layer. Ramsay's well-known experi-
ment with a tube filled with nitrogen and separated
from a hydrogen atmosphere by a palladium partition
(at 350° to 400°) illustrates the same general phenomenon;
hydrogen but not nitrogen penetrates the partition,
hence the pressure rises within the tube.'

In all of these cases the permeability of the partition
depends on its consisting (in part) of some material
which acts as a solvent to the penetrating substance.
Nernst called attention to the possibility that similar
conditions might exist in living cells; i.e., that the
solubility of substances in constituents of the proto-
plasmic membranes might be the condition of their
selective absorption by cells; and this form of explanation
was later adopted by Overton who reached the conclu-
sion, as a result of his studies on permeability already
described, that the protoplasmic surface layer consisted
chiefly of compounds with solvent properties similar to
those of typical organic solvents.

Later, chiefly because of experiments indicating a
parallelism between the penetration of various acid and
basic dyes into living cells and the solubility of the same

^ Ramsay, Z. physik. Chem., V (1894), 518.

PROPERTIES OF PROTOPLASMIC MEMBRANES 145

dyes in solutions of lecithin and cholesterol,^ Overton
referred the characteristic permeability of the plasma
membrane specifically to the presence of lipoids. This
evidence is in itself scarcely conclusive, especially since
other factors are now known to be of importance in the
penetration of dyes into cells, and many exceptions have
been found to the rule that lipoid-solubility connotes
ready penetration. The state of colloidal subdivision
has been shown to be an important factor,^ and the
negative electrification of the cell-surface must also
play a part by favoring the adsorption and penetration
of the positive particles of the basic dyes, which Overton
found to penetrate most readily. But other evidence
of various kinds supports Overton's conclusion in the
main;^ and there can be little doubt that the lipoids
form essential constituents of plasma membranes, even
although other compounds, especially proteins, may be
of equal importance; e.g., as furnishing a structural
support to the lipoids. The influence of lipoid-solvents
on the permeability and activity of cells, the character-
istic cytolytic action of these compounds, the influence
of salt-solutions on oil-water emulsions, the peculiar
relations of cholesterol to the mechanical properties of
the plasma membranes, the properties of artificial lipoid-
impregnated membranes, and many other facts all
indicate the important role of lipoids in determining the
properties of plasma membranes. It is not to be con-

^ Overton, Arch. ges. Physiol. y XCII (1902), 115.

* Ruhland, loc. cit.

3 Especially the results with strong and weak acids and bases, above
cited. Cf. the discussion in chap, viii, pp. 428 £f ., of Hober's Physikalische
Chemie der Zelle.

146 PROTOPLASMIC ACTION AND NERVOUS ACTION

eluded that these membranes are simple continuous
sheets of lipoid; the very fact that their properties are
complex and vary from species to species is inconsistent
with any such simple view, and the existence of specific
cytolysins may indeed be taken as proof that proteins
form an essential part of their composition. In fact,
as already pointed out, the plasma membrane is not to
be regarded as a simple passive layer of colloidal or
other material, but rather as a special living structure
with a characteristic metabolism of its own, and with
both its physical and chemical properties modified in
correspondence with its situation at the cell-boundary.
All that can safely be maintained is that its properties
are intimately dependent on the properties of its lipoid
constituents, hence vary with changes in the physical
and chemical state of the latter. It will be unnecessary
to review in further detail the large body of experimental
fact indicating the presence of lipoids in the surface-films
of cells; this evidence is discussed at length with full
references to the literature in Hober's and Bayliss'
textbooks.

PERMEABILITY AND PHYSICAL, ESPECIALLY
ELECTRICAL, CONDITIONS

Of late years electrical conditions have been shown
to be of great importance in determining the permeability
of artificial partitions (parchment, porcelain, and other
substances), and from general principles it seems certain
that such factors must play a corresponding part in
the protoplasmic membranes. In artificial membranes
two factors have been shown to be of importance: (i)
the potential-difference between the solutions in contact

PROPERTIES OF PROTOPLASMIC MEMBRANES 147

with the opposite faces of the partition, and (2) the
potential-difference between the soKd or colloidal
material composing the partition and the fluid occupying
its interstices or pores/ The influence of these electrical
factors is shown with especial clearness in the phenomena
of electrical endosmose, and in the related phenomenon of
negative or anomalous osmosis. In both phenomena an
electrified layer of fluid (occupying the pores of the
partition, hence in contact with the surface of the
structural material) is situated within the electrical
field between the two surfaces of the partition; the
fluid is accordingly transported across the partition by
electrostatic attraction in one or the other direction, so

long as the electric conditions remain unchanged. The

work involved in the transport is electrical; in negative
osmosis the electrical field is maintained by the unequal
diffusion-rates of the ions of the solution bathing the
partition; in electrical endosmose by the external
current traversing the partition.

It will be evident that under these conditions the
permeability of the partition to electrified material,
whether water, colloidal particles, or ions, will be different
in the two directions. Water, for example, will diffuse
outward in an osmometer, i.e., from the more to the less
concentrated solution, when the electrical diffusion field
is oriented in such a way that the charged layer of fluid
in the pores is attracted outwardly with sufficient force
to overcompensate the purely osmotic effect. This
condition is met when a stronger solution of HCl is

^ Girard, Compt. rend., CXLVI (1908), 927; CXLVIII (1909), 1047;
/. de Physiol, et de Path, gen., XII (1910), 471; /. de Chim. et de Phys.
XVII (1910), 383.

148 PROTOPLASMIC ACTION AND NERVOUS ACTION

separated from a weaker solution by a porcelain partition;
the solution on the more dilute side is then positive,
because of the diffusion field; the solid substance of
the partition is positive and the water in the pores is
negative; accordingly the latter is drawn toward the
more dilute solution. Anomalous osmosis of this kind
has long been known; Graham (1854) observed, for
example, that K2SO4 showed positive osmosis in alkaline
solution and negative in acid when a bladder membrane
was used; NaCl, on the contrary, showed positive osmosis
in acid solution and was indifferent in neutral solution.^
These changes in the direction of transport are now
recognized as depending on the influence of the ions on
the charged condition of the structural surfaces. Many
surfaces are rendered positive by H ions and polyvalent
cations; the adjacent water-layer, being then negative,
moves in a corresponding direction in the potential-
gradient between the two surfaces of the membrane.
Water may thus move in one or the other direction
according to the electrolyte content. The influence of
ions on the direction of transport is shown with especial
clearness in the experiments of Perrin^ and others on
electrical endosmose. The influence of ions on anomalous
osmosis has recently been studied by Loeb, using gelatine-
permeated membranes of collodion; the effect varies
with the Ph of the solution and becomes minimal at the
isoelectric point of gelatine. Isoelectric membranes can,

^ Graham, Phil. Trans., CXLIV (1854), 117. For a recent account
of negative osmosis cf. Bartell, Journal of the American Chemical Society y
XXXVI (1914), 646; Bartell and Hocker, ihid., XXXVIII (1916),
1029, 1036; Bartell and Madison, Journal of Physical Chemistry^
XXIV (1920), 444, 593-

' Perrin, loc. cit.

PROPERTIES OF PROTOPLASMIC MEMBRANES 149

however, become charged and exhibit anomalous osmosis
under the influence of trivalent ions.^

The transport of the ions of salts, and consequently
the permeability of a given membrane to a diffusing salt,
are similarly affected by the electrical state of the
partition, and this influence has recently been studied by
Girard, using bladder membranes. If the polarization
of the membrane (P.D. between its opposite faces) has
a certain orientation, the penetration of a salt like
MgCli or Na2S04 in the one direction is facilitated, in
the other direction hindered.^

The application of the principles above to the case
of the plasma membrane is somewhat uncertain, since
the structure of the latter is in many respects different
from that of fixed porous membranes of macroscopic
dimensions. Yet variations in the P.D . across the plasma
membrane must have a corresponding effect on the
permeabiHty; this effect, however, is probably accom-
panied in living protoplasm by other effects, such as
chemical effects depending on the electrode-Hke action
of the membrane, and effects on colloidal dispersion.
There is no doubt that the permeabihty of many plasma
membranes, especially of irritable cells, is very sensitive
to changes of electrical condition; thus the turgor
motors of plants {Mimosa, Dioncea.) depend for their
action upon variations of permeability, which are
readily induced by the electric current. The passage
of a current through a muscle or nerve, in such a way as
to decrease locally the resting polarization of the cell-

^ Cf. J. Loeb, Jour. Gen. Physiol., IV (1922), 463, and earlier refer-
ences there given.

^ Girard, Jour, de Physiol, et de Path, gen., loc. cit.

I50 PROTOPLASMIC ACTION AND NERVOUS ACTION

surface (i.e., cause depolarization), causes stimulation,
which, as will be shown below, is attended with an
increase of permeability. We must conclude that
electrical factors are of great importance in determining
the properties of living plasma membranes; variations
in the external electrical conditions occasion correspond-
ing variations in the membrane, and with these are
connected various physiological effects. The polar
disintegration shown by various cells through which
currents are passed also indicates a direct action on the
membrane; the character of this action is determined by
the direction of the current. The law of polar stimulation
is the expression of similar conditions in irritable tissues.^
The physiological effects of salts and electrolytes are
undoubtedly to be referred largely to changes in electrical
conditions, as shown by the numerous parallels between
the action of salts in purely physical phenomena like
electrical endosmose or change of colloidal aggregation,
and their action on vital activities of various kinds.

^ Cf. chapter on stimulation.

CHAPTER VIII

RELATION OF THE INORGANIC S.ALTS OF THE
MEDIUM TO THE PHYSIOLOGICAL PROCESSES
IN PROTOPLASM

The relation of the inorganic salts present in the
external medium to the properties and activities of
living cells has been the subject of much investigation
since Ringer's time, and the present brief account will
be confined to the more general and fundamental rela-
tions of this kind, especially those indicating that the
chief basis of the physiological action of the salts of
the medium is their action upon the protoplasmic surface
layers. Since much of the pioneer work in this field is
due to Overton, and since many of the results of his
studies are applicable to living protoplasm in general,
I shall first give a somewhat detailed summary of his
earlier experiments on the action of salt solutions on
the muscle and nerve of frogs. ^

Overton first examined carefully the well-known
effect, reversible loss of irritability in isotonic solutions
of indifferent non-electrolytes such as sugar, using small
muscles; e.g., sartorius, cutaneus pectoris, and foot
muscles. He found that all indifferent non-electrolytes,
independently of their special composition (dextrose,
sucrose, lactose, erythrite, mannite, alanin, taurin, and
asparagin), produce this effect. The action is not
poisonous, since the addition of a small proportion of an
isotonic solution of sodium chloride to the non-electrolyte

* Overton, Arch. ges. Physiol., XCII (1902), 346; CV (1904), 176.

i^i

152 PROTOPLASMIC ACTION AND NERVOUS ACTION

solution prevents the loss of irritability; the inference is
therefore justified that the effect depends upon a with-
drawal of electrolytes, especially sodium chloride, from
the tissue. That the electrolytes thus removed come
almost entirely from the interstitial spaces of the tissue,
and not from the interior of the cells, was later proved
by Urano and Fahr.^ The important conclusion follows
that the presence of electrolytes (salts) in the external
medium is essential to the normal irritability of the cell.

Overton discusses the question whether the removal
of sodium salts acts by preventing the conduction of
stimulation or by deranging the contractile mechanism
of the cell. Biedermann had previously shown that
after incorpoiation of sufficient water in hypotonic salt
solution, a muscle may lose the power of contraction
without losing that of conducting stimuli.^ By immers-
ing a portion of a sartorius in isotonic sugar solution,
Overton showed that stimuli are not transmitted through
the salt-free muscle. It is known that a muscle deprived
of irritability in sugar solution will shorten in solutions
of chloroform or other cytolytic substances; presumably,
therefore, the contractile mechanism is structurally
intact, but fails to act in the absence of electrolytes
because of the failure of conduction. Without the power
of transmitting stimuli, the muscle is unable to contract
as a whole.

The least concentration of NaCl required for the
maintenance of irritability was determined by using
mixtures of isotonic sugar solution (6 per cent) and

' Urano, Z, Biol., L (1907), 212; LI (1908), 483; Fahr, Z. Biol.^
LII, 72.

» Biedermann, Ber. Akad. Wiss. Wien, XCVU (1888), loi.

INORGANIC SALTS 153

NaCl solution (0.7 per cent). No significant decline in
irritability was found until the salt-content fell below
0.15 per cent. At o.i per cent irritability was distinctly
less than normal, and, with further decrease in concentra-
tion, it declined rapidly, becoming zero at about 0.07
per cent. In mixtures containing less than 0.07 per
cent NaCl irritability was lost as rapidly as in pure
sugar solution.^ The time required for complete loss of
irritability thus represents the time required for diffusion
to reduce the NaCl of the intercellular spaces to this
concentration.

Sugar-treated muscles rapidly regain irritability in
solutions of all Na salts. The nature of the anion
associated with the Na was found to be indifferent,
provided it was not too toxic; with most salts the
minimal concentration for the maintenance of irritability
was about the same as with NaCl. Apparently, there-
fore, it is the Na ion, and not the undissociated molecule
or the anion, which is responsible for the maintenance
of irritability. A definite function, that of preserving
the normal irritabihty of muscle and nerve, is thus to be
ascribed to the Na salts in blood plasma; their role is
not merely osmotic, as formerly supposed.^ This rela-
tion of Na ions to irritability and contractility is a
specific peculiarity of muscle cells; the conditions in
other contractile forms of protoplasm are often widely
different; thus the contraction of cilia, spermatozoa

^ This effect apparently varies with oxygen tension. Pond has
recently shown that in solutions saturated with oxygen the NaCl content
may be reduced to less than .05 per cent without loss of irritability:
Jour. Gen. Physiol. ^ III (192 1), 807.

2 Cf. also J. Loeb, "On the Production of Rhythmical Contractions
in Muscles by Ions," Festschrift fur Pick (1899), p. loi.

154 PROTOPLASMIC ACTION AND NERVOUS ACTION

and protozoan structures like the stalk of Vorticella
is independent of Na ions in the medium ; cilia, in fact,
often exhibit prolonged and vigorous activity in media,
like isotonic KCl or MgCl? which rapidly and completely
abolish irritability in muscle and nerve. ^

Since analysis shows little or no sodium in the
interior of the muscle cells, the conclusion follows that
the essential action of the Na ions is exerted upon the
semi-permeable surface layer or plasma membrane and
that penetration into the internal protoplasm is unneces-
sary. Overton, however, points out that it is not
necessary to assume impermeability to these ions under
all conditions; it is possible that under some conditions,
e.g., stimulation, there may be an exchange between
the Na ions in the medium and other cations (e.g., K)
present in the muscle cell. This suggests that impermea-
bility to Na ions is a characteristic of the muscle during
the resting state only, a view implying that changes of
permeability are an essential factor in stimulation.
Similar views were expressed by Bernstein and Brtinings
at about the same time.^

Experiments on the substitution of other cations for
Na showed that lithium salts were the only ones capable
of maintaining irritability in the same manner as Na
salts. Pure isotonic solutions of LiCl (0.435 P^^ cent)
are injurious to muscle, but a mixture of this solution
with an equal volume of isotonic sugar solution has
indifferent properties like those of an NaCl solution.
The least concentration of LiCl required for maintaining

^ The cilia of many marine animals, e.g., Arenicola and Mitylus,
remain active for hours in pure solutions of K and Mg salts.

' Bernstein, Arch. ges. Physiol., XCII (1902), 521; Briinings, ihid.^
XCVIII (1903), 241, and C, 367.

INORGANIC SALTS 155

irritability is ca 0.05 per cent, a value comparable with
that found for NaCl. Lithium chloride, nitrate,
sulphate, phosphate, and acetate all showed similar
action. All salts of the other alkali cations (NH4, K,
Rb, and Cs), in concentrations equivalent to 0.07 per
cent NaCl, promptly destroyed irritability. NaCl solu-
tions containing a little K and Ca (Ringer's solution)
were more effective in reviving irritabiHty than pure
NaCl solutions; irritability, however, was not sustained
by solutions containing Ca and K in the same propor-
tions, but sugar in place of NaCl. Na and Ca salts,
especially when both are present in the solution, antago-
nize the toxic action of potassium salts; this is readily
shown by using concentrations of KCl, which, acting
in pure solution, rapidly destroy irritabiHty.

The addition of traces of CaCL and SrCla to isotonic
sugar solutions was found to delay the loss of irritabiHty,
and a slight revival was observed when sugar-muscles
were returned to weak solutions of Ca, Sr, Ba, or Mg
salts in isotonic sugar solution; but no restorative action
was observed in pure isotonic solutions of these salts.

The results of experiments with nerve-trunks were
on the whole similar to those with muscle,^ although on
account of the structure of nerve a much longer immer-
sion in sugar solution (in the cold) was necessary in
order to abolish irritabiHty completely. The minimal
concentration of Na salts for preserving irritability
proved to be about the same as in muscle; on nerve as
well as on muscle K salts have a characteristic paralyzing
action, which is also antagonized by Ca and Sr in the
presence of Na salts.

^ Overton, loc, cit. (1904).

156 PROTOPLASMIC ACTION AND NERVOUS ACTION

An important section of Overton's work has reference
to the influence of salts on the transmission across the
myoneural junctions and through reflex arcs. Locke^
(1894) had found that when the frog's sartorius, with
nerve attached, was placed in a pure isotonic NaCl
solution the tissue within fifteen to twenty minutes
lost irritabihty through the nerve for single induction
shocks, while it remained directly irritable for some
hours; the addition of a little CaCla (0.02 per cent) to
the solution restored indirect irritability in a few minutes;
a second return to pure NaCl again aboHshed stimulation
through the nerve, and the effect could be again reversed
by CaCla. Muscles perfused with salt solution show
the same phenomenon;^ and other cases of innervation,
such as inhibition of the heart through the vagus, are
similarly dependent on the salts of the medium.^

Overton^ found that the addition of KCl to the NaCl
solution greatly accelerated the junctional paralysis;
salts of Rb and Cs acted similarly, also salts of NH3
and organic ammonium compounds, which have long
been known to exhibit this curare-like action. When the
concentration of NaCl was 0.7 per cent, the addition of
.05 per cent KCl was found to shorten the period of
irritabihty through the nerve to an eighth or a tenth
of its duration in the pure solution; with a lower concen-
tration of NaCl less KCl was required. This blocking
effect of potassium is antagonized by calcium; there is,

* Locke, Zentralbl. f. Physiol., VIII (1894), 166.

^ Gushing, American Journal of Physiology, VI (1901), 77.

3 Cf. Hober, op. cit., p. 539; Howell, American Journal of Physiology,
XV (1906), 280; Howell and Duke, ibid., XXXV (1907), 131.

4 Loc. cit.

INORGANIC SALTS 157

in fact, enough K in blood plasma to destroy all myoneural
transmission, were it not for the Ca also present. Cal-
cium is the only metallic ion in plasma (other than Na)
required for myoneural transmission; in mixtures of
NaCI and CaCL indirect irritabihty is preserved almost
as long as in serum. Sr also antagonizes this action of K,
but Ba and Mg do not. A similar blocking of myo-
neural transmission is caused by BaCl.; this action is also
antagonized by Ca. Overton found that in a mixed
solution of sodium and calcium chlorides of the same
concentration as in serum (0.7 per cent NaCl plus 0.02
to 0.03 CaCla) the addition of 0.05 to 0.06 per cent KCl
completely paralyzed the nerve-endings without affecting
the direct irritability of the muscle; in order to abolish
the latter 0.15 per cent KCl was required; when more
CaCL was added to the solution more KCl was required
to prevent transmission. In the absence of K a mere
trace of Ca is all that is necessary; in a K-free NaCl
solution one part of CaClz in 20,000 maintained indirect
irritability for twenty-four hours and one part in 50,000
for twelve hours; even one part in 100,000 had a percep-
tible effect.

Although the nerve-trunk retains its power of conduc-
tion for many hours in pure solutions of Na and Li salts,
transmission through reflex arcs is quickly prevented
by lack of Ca.^ This was shown both in perfusion
experiments with intact frogs and in experiments in
which the isolated spinal cord with nerves and muscles
attached was immersed in the solution. In such a
preparation kept at a low temperature in well-oxygenated

^ Overton, Verhandl. Ges. deutscher Naturf. u. Arzte, LXXV (1903),
Theil II, 2te Halfte, p. 416.

158 PROTOPLASMIC ACTION AND NERVOUS ACTION

Ringer's solution reflex activity may continue for days.
If the cord and attached sciatic nerves are immersed
in isotonic sugar solution, reflexes completely disappear
within twenty- four to thirty-six hours; on returning the
cord to Ringer's solution they return in a few hours.
In perfused frogs, replacement of the blood by sugar
solution rapidly causes disappearance of reflexes and
later of direct muscular irritability; perfusion with pure
NaCl solution then restores direct muscular irritability,
but not indirect or reflex, while Ringer's solution restores
all three. The quantity of Ca necessary for restoring
reflexes (as well as indirect irritability) is very slight.

In the observations above most of the fundamental
phenomena of salt action are illustrated — the necessity of
salts for cellular activity, the toxic action of pure solu-
tions, ion-antagonisms, and especially the remarkable
role of calcium in maintaining the normal structural
and functional relationships between cells. With regard
to this latter phenomenon, Overton cites Herbst's
observations on the action of calcium in promoting the
coherence of blastomeres,^ and also calls attention to the
presence of this element in the middle lamella of plant
tissues, where, according to Mangin, it is present as a
compound, Ca-pectate (or pectinate), which is necessary
for intercellular coherence. When this compound is
removed (by weak acid) or when it is substituted by the
Na salt (e.g., in pure NaCl solution) the cells fall apart.''
Apparently some Ca compound is necessary for the

' Herbst, Arch. Entwicklimgsmech., IX (1900), 424.

' Compare the recent observations of Hansteen on plant tissues;
again pure NaCl solutions cause a disintegration due to loss of intercellular
coherence: Jahrb. wiss. Bolanik, XLVII (1910), 289; LIII (1914)} 53*^-

INORGANIC SALTS 159

intimate union between the nerve end-plate and muscle
cell, or for the normal properties of the synaptic junc-
tions. Overton suggests that when the tissue is trans-
ferred to pure NaCl solution, the Ca in this compound
is replaced by Na, producing a compound of greater
water-absorbing properties, which swells and interrupts
the union. Such an effect is reversed by a return to
Ca-containing solutions.

Some of the more general inferences to be drawn from
these and recent experiments of a similar kind are as
follows: Since irritability disappears in isotonic sugar
solutions, which maintain the osmotic balance without
apparently changing the crystalloid content of the cell,
it seems clear that the action of the salts of the external
medium must be superficial; i.e., is a surface-action
exerted upon the plasma membrane. This action is
probably of a twofold nature : first, the normal composi-
tion and physical properties of the colloidal . materials
composing the surface-film are preserved only when a
certain combination of ions is present in the medium;
and, second, the normal state of electrical polarization of
the membrane is also dependent on the presence of salts
in the medium as well as in the cell-interior; this effect
is important because the electrical polarization of the
membrane is a factor determining its permeability.
These two effects, however, cannot be regarded as
independent, since changes in the physical state of the
membrane must alter its permeability and (in so doing)
its electrical polarization; and, conversely, changing the
electrical polarization influences the permeability.

The essential conclusion, however, is that salts may
exert physiological action without penetrating into the

i6o PROTOPLASMIC ACTION AND NERVOUS ACTION

interior of the cell, solely by altering the state of the
plasma membrane, and there is much additional evidence
that this is the case. The analytical results of Urano
and Fahr^ show that in normal vertebrate muscle Na
is almost entirely absent from the cell interior, although,
as just shown, it is essential to the maintenance of irrita-
bility. Urano kept frogs' sartorii for several hours in
isotonic sugar solution, and found that the muscle gave
off to the solution relatively much more Na than K.
Muscles thoroughly extracted in sugar solution were found
to contain only 2 per cent or less of the normal total Na-
content, but nearly all of the K and phosphate. The
conclusion seems certain that the Na is contained almost
entirely in the intercellular spaces of the tissue, and the
K and phosphate in the cells. Muscles kept for six
hours in sugar solution lose very little ash that may not
be accounted for by the salts present in the interstitial
lymph. Fahr, in an ash-analysis of the extract of fresh
uninjured muscles in isotonic sugar solution, found that
only 6 per cent of the original K of the tissue, but 90
per cent of the original Na, was thus recoverable. If
four-fifths of the total volume of the muscle be regarded
as consisting of muscle cells, and one-fifth of interstitial
tissue and lymph spaces, the Na-content of the tissue
is completely accounted for by the salts present in the
lymph. The latter must therefore exert their physio-
logical influence through their action upon the external
protoplasmic layer or plasma membrane.

Hober's experiments on the influence of isotonic
solutions of neutral salts on the demarcation-current
potential of muscle^ also support Overton's view that

^ Loc. cit. * Hober, Arch, ges. Physiol.^ CVI (1905), 599.

INORGANIC SALTS i6i

the normal uninjured resting muscle is impermeable to
sodium salts. Solutions of sodium and lithium salts
leave the normal electrical potential of the external
muscle-surface unchanged; while salts like those of K,
Rb and NH4 (which give other evidence of penetrating
the muscle) produce an injury-current or local negativity.
Absence of penetration is indicated by failure to change
the external potential of the muscle; the alkali earth
salts and in part those of caesium are thus indifferent.
Nevertheless, they alter the properties of the tissue;
thus Mg salts have a strongly anti-stimulating or narcotic
action, and Ca and Sr salts in pure isotonic solution
similarly render muscle resistant to stimulation; all of
these effects are reversible if the exposures are not too
prolonged.

Loss of contractility in non-electrolyte solutions and
its prompt return in solutions of sodium salts — especially
if some Ca is also present — are characteristic of many
varieties of muscle. Among invertebrates the larvae of
Arenicola show this phenomenon in a striking manner;^
here also, as with vertebrate muscle, Li and Na salts are
alone capable of preserving contractility for prolonged
periods, and K salts have a paralyzing action. Mg
salts repress contractility very promptly and completely,
and the action is readily reversed, especially by solutions
of Na salts containing a little Ca.^ The resemblance of
Li to Na in its power of maintaining the normal properties
of muscle (though less perfectly than Na) seems to be
general for both vertebrate and invertebrate muscle;
for example. Mines describes this phenomenon in the

^ R. S. LiUie, American Journal of Physiology, XXIV (1909), 459.
2 Op. cit.f p. 485.

1 62 PROTOPLASMIC ACTION AND NERVOUS ACTION

molluscan heart (Pecteny and in the heart of elasmo-
branch fishes.

Much further evidence could be cited indicating that
changes in the plasma membranes of the living cells,
produced by salts or other compounds which show no
evidence of penetrating into the cell interior, may
profoundly affect the properties of irritable tissues or
organisms. An interesting example is seen in the
changes produced by isotonic salt solutions in the
sensitivity of frog's muscle to various forms of chemical
stimuH and to physical agents like heat or contact.
A curarized frog's gastrocnemius, placed for a few
minutes in a pure isotonic solution of a sodium salt
(NaCl, NaBr, NaN03, Nal, NaC103, etc.) and then
dipped into a solution containing a stimulating compound
(K salt) or a cytolytic agent (chloroform in saturated
solution), contracts much more vigorously than a
normal muscle which is dipped into the same solution
directly from Ringer's solution. The pure Na salt
solution increases the responsiveness to the stimulating
agent; i.e., induces a sensitization; this effect is readily
antagonized by CaCla. The reverse effect (desensitiza-
tion) is produced by exposure to isotonic CaCla, SrCla,
or MgCL; after a brief stay in these solutions the muscle
fails to respond to the stimulating solution or responds
subnormally. In all such cases normal irritabiUty
returns in Ringer's solution.^ Various facts indicate
that Ca compounds in the surface layer of the cell are

' G. R. Mines, Journal of Physiology, XLIII (1912), 467.

* R. S. Lillie, Proceedings of the Society of Experimental Biology and
Medicine, VI (19 10), 170; American Journal of Physiology, XXVIII
(1911), 197; cf. p. 215.

INORGANIC SALTS 163

concerned in normal irritability;^ a striking example is
the marked hypersensitivity induced in frog's muscle
and nerve by Na salts whose anions precipitate calcium
or form Ca salts of limited solubility. Muscles treated
for a few minutes with isotonic solutions of these salts
(sulphate, phosphate, tartrate, citrate, oxalate, etc.)
become highly sensitive to the contact of foreign sub-
stances or media, and contract vigorously when exposed
to air.^ These phenomena of sensitization are produced
so rapidly as to leave little doubt that they depend on
alterations in the surface-films of the cells. ^

Direct evidence that changes in the permeability of
the plasma membranes play an essential part in processes
of stimulation will be cited later. Apparently all
substances that alter the physical or chemical state of
the plasma membrane (salts, acids, alkalis, narcotic
agents) modify the stimulation-process; this influence
may be in the direction either of facilitation or of repres-
sion, according to conditions."*

The precise means by which salts produce these
effects has been much debated and is still imperfectly

^ Cf. the experiments of A. J. Clark, cited on page 185.
^ J. Loeb, American Journal of Physiology, V (1901), 362.

3 Other effects dependent on surface changes in the cells are related
to those just described; e.g., changes in the surface-films of blood
corpuscles and bacteria, affecting the critical concentrations of haemolysis
or agglutination by H ions, are produced by the addition of proteins;
cf. the recent papers of Coulter, Jour. Gen. Physiol., Ill (1921), 309, and
IV (1922), 403; Northrop and De Kruif, ibid., IV (1922), 655; also
Eggerth and Bellows, ibid., p. 669.

4 Such effects evidently imply an influence on cell metabolism.
Warburg's observations on sea-urchin eggs show that the oxygen con-
sumption may be increased several times by the addition of alkali which
shows no evidence of penetrating the cell: Z. physiol. Chem., LXVI
(1910), 305.

1 64 PROTOPLASMIC ACTION AND NERVOUS ACTION

understood. Perhaps the clearest light on the problem
has been afforded by the phenomena of salt-antagonism.
These have shown definitely, at least in certain cases,
that structural changes in those parts of the cell which
are most directly exposed to the action of the solution
(plasma membranes and other surface-structures like
cilia) are the primary condition of the effects produced.
The toxic and other effects are secondary consequences
of these structural changes. The physical condition of
the structural colloids — state of subdivision, of hydration,
of electrical polarization, etc. — is changed by the action
of the salt, with corresponding changes in the properties
of the cell-structure itself and of the chemical and other
activities controlled by it. In many cases the injurious
action of the pure salt-solution (NaCl) is referable to a
destruction of the nonnal semi-permeability of the plasma
membrane; this effect, if not soon reversed, involves
chemical and other disorganization followed by death.
Any condition preventing or retarding the alterative
action on the membrane, such as the presence of an
antagonistic salt or non-electrolyte (narcotic), has
accordingly a protective or ''anti-toxic" action.^ These
general conditions are well illustrated in a simple marine
organism much used in experimental work at Woods
Hole, the larva of the annelid Arenicola cristata. This
is a segmented trochophore larva about one-third of
a millimeter in length, having pigmented body cells
and swimming by a combination of muscular and
ciliary movements. When placed in pure isotonic

' Cf. the discussion in my paper on antagonism between salts and
anaesthetics, A^nerican Journal of Physiology, XXXI (1913), 255; cf.
pp. 275 fif.

INORGANIC SALTS 165

NaCl (or similar unbalanced solution of alkali salt)
the larvae contract strongly and the yellow pigment
begins to diffuse from the cells; by degrees the cilia
cease movement and undergo a visible breakdown or
disintegration (suggesting liquefaction or absorption of
water). In a solution of NaCl containing a little
CaClz (95 volume m/2 NaCl plus 5 vols, m/2 CaCL) all
of these immediate effects are prevented, and the larvae
retain their normal appearance and behavior for some
time and die much more gradually.^

In this case it is clear that the immediate action of
the pure salt-solution is upon the surface structures —
cilia and plasma membranes — which quickly lose their
normal structural coherence and continuity; the result
of this change in the cilia is physical breakdown, and
in the plasma membrane a marked increase of permea-
bility, hence the diffusion of soluble cell-constituents to
the exterior and the progressive disorganization. In
many other organisms and cells it can also be shown that
an abnormal increase of permeability is produced by
pure solutions of Na salts and prevented by the addition
of CaCla or other antagonistic salt.^ The experiments of
Osterhout on the electrical conductivity of Laminaria
and other plant tissues afford perhaps the clearest evi-
dence that the toxic action of pure NaCl solutions is the
result of a destruction of semi-permeabihty and that
antagonistic salts (CaCla and others) produce their anti-

^ American Journal of Physiology , XXIV (1909), 14; cf. pages 22 ff.

^ In an. earlier paper {Biological Bulletin, XVII [1909J, 188) I have
given a summary of observations showing the correlation between
permeability-increasing action and toxicity for compounds other than
salts.

1 66 PROTOPLASMIC ACTION AND NERVOUS ACTION

toxic effects by counteracting the structural change in
the membrane.^

The question of just how, in the physico-chemical
sense this result is accomplished is a fundamental one;
salt antagonism is shown by all groups of organisms from
bacteria^ to vertebrata, and is apparently a universal
phenomenon in living protoplasm. It should be noted
that other effects produced by the pure Na salt solu-
tion are also antagonized by Ca and other salts; e.g., its
stimulating action, shown in the production of twitches
in vertebrate and other muscle,^ the sensitizing action
on muscle just described, and the activation of unfertil-
ized eggs."* The fact that prevention (by Ca and nar-
cotics) of increase of permeability in various irritable
tissues and organisms is associated with prevention of
stimulation has a general interest as evidence of the
essential part played by membranes in stimulation, and
will be considered more fully later.

The problem of the physico-chemical basis of salt
antagonism has been approached in various ways and

^ Osterhout, loc. cit. Related observations are those of Ham-
burger on the action of calcium in preventing the increase in perme-
ability produced in frog's kidneys perfused by NaCl solutions containing
dextrose (cf. Hamburger, Biochem. Zeitschrift, LXXXVIII [1918], 97).
In 1904 J. B. Mac Galium had observed the antidiuretic action of calcium
salts and had related it to the action in decreasing permeability (Journal
of Experimental Zoology, I [1904J, 179; University of California Pub-
lications, Physiol., II [1905], 93). It is well known that calcium antago-
nizes cytolysis by saponin and other permeability-increasing compounds.
Chiari's observations on the formation of exudates and many other obser-
vations are related; cf. Hober, op. cit., pp. 544 ff.

^ For the case of bacteria cf. Shearer, Journal of Hygiene, XVIII

(1919), 339-

3 Cf. Loeb's article in Festschrift fur Fick, loc. cit.

4 R. S. LilHe, American Journal of Physiology, XXVII (191 1), 289;
Journal of Morphology, XXII (191 1), 695.

INORGANIC SALTS 167

on the whole most effectively by comparing the action of
salts on colloidal solutions and emulsions with their
action on living cells.' The physiological effects pro-
duced by salts and salt combinations vary with the nature
of the ions in a manner which in many cases shows a
close parallelism with the physical effects produced by
the same salts in simple colloidal systems. For example,
in the counteraction of the toxic effects of pure Na salt
solutions the results all indicate that the cation of the
antagonistic salt is the effective agent; and in this case
certain relations highly characteristic of the action of
ions on colloidal systems are shown clearly; thus with
the cilia of Arenicola all salts of bivalent heavy metals
(Co, Ni, Cd, Zn, Mn, Pb, Fe++, Cu) were found to pro-
duce their maximal antitoxic effects in concentrations of
the order m/400 to m/i6oo; while with trivalent metals
(Al, Cr, Fe^'O the physiologically corresponding concen-
trations were from 50 to 100 times less.^ This increase
in effectiveness with increase in valence is characteristic
of the action of salts on negatively charged suspensoid
systems (rule of Schulze and Hardy); and apparently
the observations above indicate that the cations produce
their effects in the living system by influencing the state
of subdivision of the suspensoid colloids forming part
of the protoplasmic structure. Other cases of rapid
increase in physiological effectiveness with increase of
valence are well known.

Such cases of antagonism are consistent with the hy-
pothesis that the normal state of the living protoplasmic

^ Cf. Hober's review in his textbook, chap, x, p. 471.

^American Journal of Physiology, VII (1904), 419; similar results
were obtained with the giU cilia of Mitylus: ibid., XVII (1906), 89.

1 68 PROTOPLASMIC ACTION AND NERVOUS ACTION

structure (e.g., a cilium or plasma membrane) requires a
certain state of subdivision of the chief structural colloids.
Apparently in the normal medium, e.g., sea water, the
influence of the various ions is so balanced as to preserve
this state; the collective effect of the negative charges
carried by the anions (CI, SO4, HCO3, etc.) is just
balanced by the collective effect of the positive charges
carried by the cations (Na, K, Ca, Mg). In the pure
solution, however, of NaCl, in which no bivalent cations
are present, the influence of the anions is insufficiently
compensated, and the colloids are altered in a manner
injurious to cell structure: i.e., a preponderant influence
of anions is the essential toxic factor in such cases. The
addition of bivalent or trivalent' cations of any kind
has under these conditions a compensating and hence
antagonistic or antitoxic influence; trivalent ions exercise
this effect in much lower dilution than bivalent ions,
and bivalent than monovalent ions. At an appropriate
concentration, the balance is restored, protoplasmic
structure remains normal, and life continues. The
reverse condition, in which the toxic action of the pure
solution results from a prepotency of cation action, and
in which accordingly the addition of small quantities of
salts with powerfully acting anions has an antitoxic
effect, is apparently also realized in some cases; e.g., the
ciliated epithelium of the molluscan gill {Mitylus) in solu-
tions of SrCla; here various Na salts (NaOH, NaBr, NaT,
NaCNS, NaaSOj show well-marked antagonistic action.^
Such purely physical explanations, while apparently
partly applicable in some cases, prove inadequate in

^ Cf. American Journal of Physiology, XVII (1906), 89; cf. p. 129;
cf. Raber, Jour. Gen. Physiol., II (1920), 541, for analogous observations
on Laminaria.

INORGANIC SALTS 169

many others, where the foregoing relation of antagonism
to valence does not appear to hold, and especially in
those numerous cases where different cations similar in
valence have widely different antagonistic or other
physiological actions.^ In such cases the special chemical
properties of the compounds formed between the ions
and the structural colloids of the cell must apparently
be taken into consideration. Thus various facts indicate
that calcium proteinates or calcium soaps (or both)
are of special importance as constituents of cell structures;
and these compounds cannot be replaced satisfactorily
by the corresponding compounds of other bivalent
elements.^ Such a conception explains why strontium,
the metal most closely resembling calcium in general
chemical properties, usually comes nearest to calcium in
antagonistic effectiveness; the other alkali earth cations
come next; while the bivalent heavy metals are effect-
ive in only a few special cases (Mn. Co, Fe++), the great
majority of these metals being too strongly toxic to act
as efhcient antagonists.^

^ Cf. Hober's discussion in his recent papers on the physiological
action of calcium; Arch. ges. Physiol., CLXVI (191 7), 531, and
CLXXXII (1920), 104.

2 The cases are numerous where calcium has a specific action which
cannot be replaced by that of other cations, even Sr; instances are the
specificity of calciimi in phagocytosis (Hamburger), in the inhibition
of swimming plates, and in the preservation "of contractility in heart-
strips. On the other hand, in its simple stabilizing or protective action,
(e.g., in cytolysis, etc.) Ca can often be replaced by other cations (cf.
Hober's papers, loc. cit.; cf. also Wiechmann, Arch. ges. Physiol., CXCV
[1922], 588).

3 Nevertheless even the most toxic cations, like Ag and Hg, may
show definite antitoxic action in certain cases; e.g., the beat of the
ctenophore swimming plate in pure isotonic NaCl solution is prolonged
several times by AgNOj and HgCL in concentrations of m/ 100,000 to
m/ 200,000 (cf. American Journal of Physiology, XVI [1906], 117).

lyo PROTOPLASMIC ACTION AND NERVOUS ACTION

The evidence, taken as a whole, indicates that the
salts act chiefly by altering the physical state of the
structural colloids of the cell; and it is to be presumed
that general physical factors (of the kind regarded as
acting in all colloidal phenomena) and special chemical
factors specific for each form of protoplasm are both
concerned in producing the total effect. In the directly
toxic or injurious action of salt solutions, a frequent, if
not invariable, factor is a destruction of the semi-
permeable properties of the protoplasmic partitions,
primarily of the plasma membranes. In physiological
salt-actions of other kinds, e.g., stimulation, sensitization,
inhibition, it is to be presumed that the physical proper-
ties of the plasma membranes undergo special modifica-
tions of a corresponding kind, but that the eft'ects do not
exceed a certain range and are therefore reversible.
The general fact that a certain combination of salts,
usually of Na, Ca, and K, is required in the external
medium for most forms of normal protoplasmic action
indicates the fundamental importance of the influence
of salts on the structural colloids of protoplasm. Appar-
ently the structural conditions required for the continuity
and closeness of texture necessary in semi-permeable
membranes depend on the maintenance of a definite
equilibrium between the ions in the medium and those
associated (chemically or otherwise) with the structural
coUoids. Shght changes in the salt-content imply
corresponding structural changes, with dependent physio-
logical effects.

It should be added that other general physical
conditions, also dependent on the presence of salts,
are of importance in the normal activity of protoplasm.

INORGANIC SALTS 171

such as the electrical conductivity of protoplasm and
medium and the electrical polarization of the plasma
membranes. These conditions will be considered later
under the subject of stimulation.

The special relations of the three chief cations of
the protoplasmic media, Na, K, and Ca, to protoplasmic
activity appear to depend on chemical conditions of a
kind still imperfectly understood. The differences
between the physiological actions of Na (and Li) and of K
(and Rb and Cs) in their relation to vertebrate muscle
and nerve cannot be satisfactorily explained on the basis
of the physico-chemical constants of these ions. Na
and K, though chemically closely similar, appear in
many forms of protoplasm, e.g., vertebrate muscle, to
act as physiological antagonists; in others their physio-
logical differences are relatively slight; e.g., in the case
of fish eggs (Fundulus) and sea-urchin eggs (Arbacia)
isotonic KCl solution is even less toxic than NaCl solu-
tion; both solutions are antagonized by Ca.

It is remarkable that the striated muscle cells of
vertebrata appear to be readily permeable to some K
salts (KCl, KBr, KI, KNO3) but not to others (K2SO4,
K- tartrate, K-phosphate) ;^ in this respect K salts
exhibit a striking contrast to Na salts. The special
permeabihty to KCl is shown in frog's muscle by rapid
increase in the weight of the tissue when it is immersed
in isotonic solutions of this salt; similar conditions have
been found by Seebeck^ in the frog's kidney, which, like
muscle, shows normal semi-permeability toward Na and
Li salts. The special physiological action of K is prob-

^ Overton, Arch. ges. Physiol., CV (1904), 176.
' Seebeck, ihid., CXLVIII (1912), 443.

172 PROTOPLASMIC ACTION AND NERVOUS ACTION

ably related to this peculiarity, but the nature of the
relationship is unknown. Recently Zwaardemaker has
attributed importance to the slight radioactivity of K,
on the ground of certain striking parallels between the
action of K salts and uranium salts on the frog's heart ;^
but other evidence from experiments with marine
organisms fails to support this view.^

Recently Meigs^ has investigated the osmotic behavior
of smooth muscle in solutions of salts and non-electro-
lytes, and finds a number of remarkable differences
between this tissue and striated muscle. For example,
the stomach muscle of the frog gains weight when
immersed in isotonic solutions of sugar, alanin, or
NaCl, and even in Ringer's solution. He concludes
that smooth muscle cells differ fundamentally in their
structure and mode of action from striated muscle cells,
and that they are not surrounded by semi-permeable
membranes. In the adductor muscle of the marine clam
{Venus mercenaria), sl smooth muscle which reacts
slowly and maintains its state of tension for a long time,
closely similar conditions were found. Pieces of muscle
immersed in sea water take up chloride from the latter
and increase in weight; they even fail to lose weight in
lo per cent NaCl or in sea water of twice the normal
concentration. "*

^ Zwaardemaker, Journal of Physiology, LIII (1920), 273, and
LV (1921), 33.

^ R. F. Loeb, Journal of General Physiology, III (1920), 229. A. J.
Clark fails to confirm Zwaardemaker's claim that uranium can act
as a substitute for K in restoring the heartbeat (Journal of Pharmacology
and Experimental Therapeutics, XVIII [1922], 423),

3 E. B. Meigs, Journal of Experimental Zoology, XIII (191 2), 497.

4 Meigs, Journal of Biological Chemistry, XVII (1914), 81.

INORGANIC SALTS 173

In explanation of these apparent discrepancies it
has been suggested that smooth muscle cells are easily
injured and that the normal semi-permeability had dis-
appeared at the time when the tissue was examined, but
this possibility is rejected by Meigs. It is difficult to
escape the conclusion that a continuous semi-permeable
membrane does not invest the whole cellular element in
this tissue. Possibly the contractile part of the entire
structure has a relation to the protoplasmic part similar
to that which the fibers in connective tissue have to the
cells by which they are formed. The sluggishness of
the movement and its slow reversibility suggest a
fundamentally different type of organization from
that of striated muscle cells. Semi-permeability appears
to be universal in indifferentiated cellular elements and
in the majority of specialized cells; but a differentiation
in which part of the total structure acquires non-cellular
properties is not infrequent in organisms, as illustrated
in connective tissues and skeletal structures; and the
above-cited types of contractile tissue may exemplify
this general condition.

The penetration of salts through the egg-membrane
of the sea-minnow, Fundulus, as shown in various toxic
effects and antagonisms, illustrates many conditions of
great interest, and especially indicates the importance
of the purely chemical factors in permeability. This
membrane is a dead structure, or chorion, external to
the living protoplasm of the egg, and apparently consist-
ing chiefly of a keratin-like protein. In its normal
state, it is almost completely impermeable to water or
the salts of the medium; hence the eggs will develop
either in distilled water or in concentrated sea water.

174 PROTOPLASMIC ACTION AND NERVOUS ACTION

Pure salt solutions (NaCl) or solutions of acid or alkali
alter the membrane and allow penetration. Character-
istic antagonisms are shown in these effects;^ e.g., the
penetration of heavy metal salts (cobalt and nickel) is
retarded by CaClz and their toxic action is thus pre-
vented.^ Eggs placed in pure strongly hypertonic
NaCl solutions float at first, but soon sink and die,
indicating the penetration of salt and water as the
membrane is altered; but in the same solution to which
CaCL has been added, they may float and remain
living for several days.^ Recently Loeb and Cattell have
studied the penetration of K salts and acids into the
Fundiilus egg in the presence of other salts (alkali and
alkah earth) in varying concentrations.'' As index of
the penetration of KCl the paralyzing action of the salt
on the heart was used; this action is reversible, so that
if eggs containing embryos whose hearts have previously
been arrested in KCl solution are placed in sea water, the
heartbeat after a time revives, indicating outward
diffusion of KCl through the membrane. The remark-
able fact is that this recovery does not occur in distilled
w^ater or in solutions of non-electrolytes; in order that
the potassium shalf penetrate the membrane, a treatment
of the latter with salt solutions is necessary. Thus
(typically) all of the poisoned hearts resume beating
within a day when the eggs are placed in sea water or

* Cf. the article by J. Loeb in Oppenheimer's Handhiich der Bio-
chemie, II, 104, for a general account of antagonisms in Fundnlus eggs.

2 A. P. Mathews, American Journal of Physiology, XII (1905), 419.

3 Loeb, Science, XXXVI (191 2), 637; Biochem. Zeitschrift, XL VII
(1912), 127.

4 Loeb and Cattell, Journal of Biological Chemistry. XXIII (191 5),
41; Loeb, ibid., XXVII (1916), 339, 352, 363; XXVIIl', 175.

INORGANIC SALTS 175

isotonic NaCl solutions, while none revive in sugar
solution. A ''salt action " is necessary to the penetration
of KCl in either direction through the membrane; all
salts, however, are not equally effective; in the case of
Na salts the effect shows an increase with increase in
the valence of the anion. These effects apparently
indicate that the penetration of ions through a membrane
consisting of colloidal material with which the ions
form compounds (proteinates) requires the presence of
other free ions with which the penetrating ion can form
alternative combinations. Otherwise it is held in
position by chemical forces and unable to move freely.
Similar conditions have been observed in the diffusion
of dyes (neutral red) from stained eggs; this dift'usion is
also facihtated by the presence of salts in the outer
medium. In order to obtain the maximum diffusion-
facilitating salt-effect a certain medium concentration
of the effective salt is required; higher concentrations
and lower concentrations have a retarding influence on
diffusion; this latter effect appears to form the basis
of the usual salt-antagonisms or protective effects shown
by calcium and other bivalent salts. ^

It is evident that changes in the physical properties
of membranes must alter the readiness with which diffus-
ing materials penetrate, and that such changes may result
from changes in the chemical composition of the
membrane-forming compounds as well as from changes in
their distribution or state of subdivision. In a membrane
consisting of protein, the formation of proteinates with
varying physical properties will change the permeabiHty
and the other properties of the membrane. Similarly

^Loeb, Journal of Biological Chemistry (191 6), loc. cit.

176 PROTOPLASMIC ACTION AND NERVOUS ACTION

if lipoids or other ester-like compounds are membrane-
components, the formation of soaps becomes a possi-
bihty; these vary in their solubilities and water-
combining properties; and variations in their proportions,
e.g., the substitution of Ca soaps for Na or K soaps, will
entail corresponding changes of permeability.

The question of whether the protein or the hpoid
components of the living plasma membranes are chiefly
concerned in the variations of permeability underlying
toxic effects or normal physiological processes is an open
one ; but in all probability both compounds play a part,
although the present evidence seems to favor the view
that the proteins are physically the more stable of the
two and form the permanent structural substratum,
while variations of permeability depend chiefly on
changes in the Hpoid components, especially the soaps.
Parallels to the physiological salt antagonisms are wxll
known in non-Hving systems containing both classes of
compounds. Of chief biological interest is the antago-
nism between Na salts and Ca salts, which is apparently
universal in Kving organisms. Protein systems contain-
ing salts of these two metals show variations in properties
when the proportions of the salts are varied; such
properties as water-combining power (swelling or hydra-
tion), viscosity, osmotic pressure, susceptibility to
alteration by organic compounds (precipitation by
alcohol), and electrical polarization of particles are
affected; Loeb's recent investigations afford instances
of salt-antagonisms affecting all of these properties.^
The inference from such facts would be that Ca and Na

^Loeb, Journal of Biological Chemistry, XXXI (191 7), 3; XXXIV
(1918), 395, 489; XXXV (1918), 497.

INORGANIC SALTS 177

proteinates must co-exist in certain definite proportions
in the cell-structures, including the membranes, if
certain biologically necessary physical properties are to
be preserved. Substitution of Na for Ca compounds
would occur in pure Na salt solutions, with resulting
structural changes which might well be injurious or fatal.
Clowes^ has recently shown that salt-antagonisms
having an even closer resemblance to biological salt
antagonisms may be demonstrated in oil- water emulsion
systems by varying the proportions of Na and Ca soaps
in the interfacial films. The number of separate drops
formed when a slightly alkaline salt solution is allowed
to flow through a stalagmometer into oHve oil is found
to vary in a remarkable manner with variations in the
proportions of NaCl and CaCla in the solution; thus
with an alkaUnity of .ooin NaOH, the following number
of drops per minute were observed with different
solutions :

Solution ^Minute'

Control (no salt) 44

Pure 0.15 m NaCl 300

Pure 0.0015 m CaClz 24

0.15 m NaCl-ho.oois m CaCL 44

0.3 m NaCl-|-o.oo3 m CaCL 43

0.45 m NaCl-ho.005 m CaCL 43

0.6 m NaCl-|-o.oi m CaCL ........ 43

The pure NaCl solution greatly promotes the
tendency toward fine subdivision of the drops; the
CaCL decreases this tendency; at a certain ratio of the
two salts (Na: Ca about 50 or 100:1) the two tendencies
counteract each other. The result depends on the

^ CloA;l^es, Journal of Physical Chemistry, XX (1916), 407.

178 PROTOPLASMIC ACTION AND NERVOUS ACTION

different solubilities of the Na-oleate and the Ca-oleate
in the two phases, the former being soluble in water
but not in oil, the latter soluble in oil but not in water.
The surface-tension conditions at the interface are
accordingly oppositely affected by the two salts, with
correspondingly opposite effects on the state of disper-
sion; at a certain ratio of concentrations the two op-
posite effects are balanced. Clowes shows that when
equal volumes of oil and alkaline salt solution (mixtures of
n/io Na OH and n/io CslCU) are shaken together, the
effect varies according to the proportions of Na and Ca
in the solution; when Na is present in excess, the oil is
dispersed as droplets in a continuous water phase, while
when Ca is in excess the water is dispersed as droplets
in a continuous oil phase. A reversal of phase-relations
may thus be accomplished in an emulsion system by
changing the salt-content; and this conclusion has
highly important biological applications, since it bears
closely on the problem of the relations between the lipoid
and the aqueous components in the living protoplasmic
system. Any system in which the oil (organic solvent
or lipoid) phase is the continuous one is permeable to
oil-soluble substances, but not to water-soluble sub-
stances which are oil-insoluble; and the general corre-
spondence of this condition with that observed in living
protoplasm suggests the possibility that the external
layer of the living plasma membrane consists (at least
during the greater part of its existence) of a continuous
layer of lipoid material, the continuous condition de-
pending on the presence of compounds with properties
like those of Ca soaps. The importance of Ca to
the semi-permeability and water-resisting properties

INORGANIC SALTS 179

of the protoplasmic surface-film, on this hypothesis, is
evident.

The possibility of a reversibility of phase-relations
under salt action^ implies the possibility of inducing
reversible changes of permeability under these conditions,
and such reversible changes are, as we have seen, a
characteristic feature of the living plasma membranes.
Clowes has in fact constructed a model in which an
emulsion consisting of equal volumes of oil and a mixed
salt solution (containing NaCl, KCl, and CaCla in propor-
tions similar to those of sea water and slightly alkaline)
is supported in the interstices of a paper partition (sheets
of filter paper) fixed in position by rings of rubber in the
interior of a U-tube.^ The electrical conductivity of
the emulsion-permeated paper is then found to vary,
when the solution in contact with it is changed, in a
manner resembling in general that shown by the parti-
tion of living plant-tissue in Osterhout's experiments.
In pure NaCl solution the conductivity is rapidly
increased; in pure CaCla it is decreased; and the changes
of conductivity are reversible if they are not allowed to
proceed too far. It is assumed that in the capillary
interstices the emulsion undergoes reversible changes of
phase of the above-described kind, the conversion of the

* For the conditions of the reversibility of .phase-relations in emul-
sions cf. Newman, Journal of Physical Chemistry, XVIII (19 14), 34;
Briggs and Schmidt, ihid., XIX (1915), 478; Clowes, ihid., XX (1916),
407; Bhatnagar, Jour. Chem. Soc. Trans., CXVII (1920), 542; also
Physics and Chemistry of Colloids, Discussion of Faraday Society and
Physical Society of London (1921), p. 27; also in Kolloid-Z., XXVIII
(1921), 206.

' Clowes, Proceedings of the Society of Experimental Biology and
Medicine, XV (1918), 108.

i8o PROTOPLASMIC ACTION AND NER\^OUS ACTION

oil from the continuous to the discontinuous phase
corresponding to an increase in conductivity and vice
versa.

It will be remembered that water-insoluble colloidal
material (Cua FeCye) held in the interstices of a support-
ing structure forms the essential composition of the semi-
permeable membranes used in the osmotic pressure
determinations of Pfeffer and Morse; in such membranes,
also, permeability is changed by the action of salts.
The conditions in living membranes, while probably
similar, in the above broad sense, to those in such
structurally composite membranes, are vastly more
complex, chiefly on account of the constant presence of
the metabolic factor. Clowes propounds the general
hypothesis that ^'variations in the permeability of the
protoplasmic membranes are attributable to the action
of electrolytes and metabolic products on delicately
balanced interfacial soap-films and emulsion systems,
and that proteins may play no part in the valve-like
mechanism controlling permeability other than to afford
a supporting filamentous or mesh-like structure."^

Such an arrangement need not be taken too literally
as an exact model of the conditions in the protoplasmic
surface-films. It seems probable, however, that in the
plasma membrane there is a combination of a relatively
permanent supporting structure, presumably protein,
with a variable emulsion-like component whose water-
insoluble phase is normally so disposed as to block,
with a considerable degree of completeness, the interstitial
spaces or capillary channels of the membrane. Varia-
bility in this emulsion, under the influence of salt solu-

^ Clowes, op. cit., p. no.

INORGANIC SALTS i8i

tions or other factors, involves variability in the perme-
ability of the partition; and as a secondary consequence
other conditions depending on that permeability, such
as electrical polarization, are affected. Such a concep-
tion implies that changes in the supporting protein
structure may also influence permeability; but it
regards the normal or physiological variations in this
property as dependent chiefly on variations in the state
of the most external water-insoluble or lipoid portion of
the protoplasmic complex. Such a conception allows
for a wide range of variability in the permeability of the
membrane. The latter is not to be regarded as a con-
tinuous lipoid sheet under one set of conditions, which
changes without transition, under other conditions, into
a state in which the lipoid becomes the discontinuous
and the aqueous phase the continuous phase. A bal-
anced state, with fluctuations on either side of a mean,
which is regulatively maintained by metabolic processes,
is rather the one to be conceived as representing the
condition actually existing during life.

A simple case of salt-antagonism, which I have
recently studied in the starfish egg,^ appears to throw
light upon the more specifically chemical conditions of
these phenomena. The fully mature starfish egg (ca.
1 60 /x in diameter) is surrounded with a layer of jelly-like
substance, of 15 to 20 fj, in diameter, consisting of
water-swollen material (of undetermined nature) sepa-
rated or secreted from the egg-protoplasm. This layer
is rendered visible by mounting the eggs on a slide, with
cover-glass, in a suspension of India ink, to which the
jelly is impermeable; it then appears under the micro-

^ Jour. Gen. Physiol., Ill (1921), 783.

i82 PROTOPLASMIC ACTION AND NERVOUS ACTION

scope as a clear halo surrounding each egg. When the
egg is washed in pure isotonic NaCl solution (0.54 m),
preferably with the aid of centrifuging, the jelly swells
and dissolves; but in NaCl solution containing a little

CaCl2 ( — is suflSicient) it remains intact. Salts of
400

several other metals (Mg, Mn, Co, Ni, Al) have been
found to have a similar effect.' The jelly layer, in the
presence of the salts of sea water and in mixtures of
NaCl and CaCU, thus possesses a certain water-
insolubility and physical consistency; these properties
are apparently dependent on the presence of a calcium
compound (possibly proteinate) with definite physical
properties (especially water-insolubility and lack of
tendency to swell) which preserves the characteristic
structure and consistency of the whole layer. In pure
NaCl solution this constituent is replaced by the corre-
sponding sodium compound which is water-soluble and
swells readily, hence the coherence and insolubility of
the layer as a whole are lost.

This process may be regarded as a model of the
essential kind of change occurring in the plasma mem-
branes of living cells in pure NaCl solution. The
surface of the starfish egg is in fact physically altered
by NaCl solution in a characteristic manner; unfertilized
eggs placed in the pure solution cohere in clumps or
agglutinate; many eggs also form fertilization-membranes
(when returned to sea water) and show evidence of
partial activation by cleaving and in some cases develop-
ing to the blastula stage. All of these effects of the pure

^ Unpublished experiments in the Nela Research Laboratory at the
Marine Biological Laboratory at Woods Hole.

INORGANIC SALTS 183

solution are prevented in the presence of a little CaCla.
The toxic action of the pure solution and also its
membrane-forming action are correlated with a
permeability-increasing action, as in the other cases
cited above; and the correspondence with the behavior
of the jelly seems to imply that the increase of permea-
bility in the pure solution is to be referred also to the
replacement of water-insoluble Ca compounds (e.g.,
Ca proteinates or soaps), on which the properties of
the plasma membrane depend, by soluble Na compounds.
There is much independent evidence that this role
of calcium compounds — i.e., of determining the properties
of the surface layers of cells — is a general one; we may
thus understand the importance of Ca to all normal
cell-processes (stimulation, etc.) which depend on changes
in the surface layers. The peculiar relation of Ca to
the coherence of blastomeres and of plant cells has
already been mentioned; according to the earlier work
of Mangin, Ca compounds (''Ca-pectate") in the middle
lamella of plant cells are essential to the structural
coherence of cellular tissues; when the Ca is replaced
by Na, the cells tend to fall apart. Similar phenomena
are also well known in the epithelial tissues of animals;
e.g., in the ciliated epithelium of Mitylus the cells swell
and fall apart in pure solutions of many Na and K salts,
and this effect is prevented by Ca.^ Recently the
changes occurring in plant tissues in pure NaCl solutions
have been studied in much detail by Hansteen.* The

* R. S, Lillie, American Journal of Physiology, XVII (1906), 89; cf.
p. 122.

* Hansteen (Hansteen-Cranner), Jahrh. wiss. Botanik, XL VII
(1910), 374; LIII (1913-14), 536; Ber. deutsch. Botan. Ges., XXXVII
(1919), 380. Also his recent book, Zur Biochemie u. Physiologie der
Grenzschichten lebender Pflanzenzellen, Kristiania (1922).

1 84 PROTOPLASMIC ACTION AND NERVOUS ACTION

general effects are swelling, loss of consistency or turgor,
and disintegration of the tissue, accompanied by loosen-
ing of intercellular coherence; these effects are all
prevented or greatly decreased in the presence of a
little Ca. Hansteen believes that water-insoluble Ca
compounds are essential constituents of living protoplasm
in general, and that they are present especially in the
protoplasmic surface layers and other solid structures;
he also cites evidence indicating that these compounds
are lipoid in nature. According to his conception,
pure alkali salt solutions attack the surface layers of
cells because they alter the water-insoluble Ca-lipoid
compounds there present, converting them into water-
soluble compounds and secondarily inducing absorption
of water and structural breakdown. Such effects are
apparently of the same nature as the disintegration of
the cilia of marine animals (Arenicola) and the general
loss of semi-permeability of plant and animal cells in
pure salt solutions, already described. They are con-
sistent with the general view that in the formation of the
solid or permanent (water-insoluble) protoplasmic struc-
tures, Ca compounds play an essential part. Hansteen
found that the presence of more than the normal concen-
tration of Ca salts in culture solutions favored profuse
branching and the formation of an abundance of root
hairs in seedlings; Wiechmann, in Hober's laboratory,
has recently confirmed this result, and has found further
that Sr, Ba, and a few heavy metals (Mn, Ni, Co) act
similarly to Ca, while Mg is ineffective.^ It would
seem that certain necessary physical properties of
protoplasmic structures, such as rigidity, water-
^ Wiechmann, Arch. ges. Physiol., CLXXXII (1920), 99.

INORGANIC SALTS 185

insolubility, and impermeability to water-soluble sub-
stances (like sugar and salts), require the presence of
Ca compounds. These compounds impart the necessary
structural stability to the whole protoplasmic complex.
A fact of interest in relation to this question is Meigs's
recent observation that an approach to semi-permeability
can be imparted to certain artificial colloidal membranes,
otherwise highly permeable, by depositing insoluble Ca
salts (phosphate) in their substance.^

These facts and considerations are consistent with
the view that Ca compounds, e.g., Ca soaps, play a
similar part in the surface layers of protoplasm, and the
recent interesting experiments of Clark^ with the frog's
heart lend support to this general conception. He finds
that the heart, after being weakened by prolonged
perfusion with Ringer's solution, rapidly regains its
vigor if perfused with Ringer's solution to which serum,
serum-Hpoids, lecithin, or soaps of higher fatty acids,
have been added. During perfusion with pure Ringer's
solution the heart loses to the solution some material
which has a similar reviving action when perfused
through other exhausted hearts. In order that these
substances, or soap, should exhibit this beneficial action,
calcium must be present. He concludes that the benefi-
cial action of the soaps is associated. with the adsorption
of a water-soluble Ca soap or similar compound upon the
surface of the muscle cells, and puts forward the hypothe-
sis ''that the activity of the heart is dependent upon the
semi-permeabihty of the cell to electrolytes, that this is

^ Meigs, American Journal of Physiology, XXXVIII (1915), 456.

* A. J. Clark, Journal of Physiology, XL VII (1913), 66; also Clark
and Daly, ihid., LIV (1921), 367.

i86 PROTOPLASMIC ACTION AND NERVOUS ACTION

dependent on the presence of Ca and lipoids at the
surface of the cells, and that during perfusion the heart
loses Hpoids and becomes more permeable to electro-
lytes." This conception of semi-permeability as depend-
ent on the presence of lipoids, and of the state of the
Hpoids as determined by the salts of the medium, is in
harmony with much recent work.^ It also affords a
point of view from which it is possible to understand
why the physiological effects of salts and of lipoid-
solvent compounds should have so much in common.

^Blackman, "The Plasmatic Membrane and Its Organization,"
New Phytologist, XI (191 2), 180; Clowes, loc. cit; Czapek, Oberfldchen-
spannung der Plasmahaut, Jena (1911); McDougall, Science, LV (1922),
653; Hansteen-Cranner, loc cit. See also Stiles's review of the subject
of cell permeability in New Phytologist, XX, XXI, XXII (1921-23).

CHAPTER IX

GENERAL PHYSIOLOGICAL ACTION OF LIPOID-
ALTERANT AND SURFACE-ACTIVE
SUBSTANCES

From the physiological point of view the reversible
forms of salt action are the important ones; the proper-
ties and activities of living protoplasm may thus be
modified by changing the salt-content of the medium,
and return to the normal when the original salt-content
is restored. Such reversible effects are of special
biological interest, since their essential conditions are in
all likelihood similar to those controlling the normal
variations of activity. Substances are continually being
formed in metabolism (e.g., CO2 and other acids) which
directly influence protoplasmic action. It is, therefore,
of fundamental interest to note the existence of another
large class of substances, many of which are chemically
indifferent, i.e., not readily oxidized or reduced (hydro-
carbons and their substitution-products), which have a
profound influence on protoplasm, completely reversible
within wide limits. These substances are those organic
compounds, varying widely in their chemical nature,
which have in common two general physical properties :
(i) a solvent action on, or solubiHty in, the water-insoluble
organic constituents of protoplasm (fats, lipoids, etc.);
and (2) a high degree of surface-activity, i.e., influence
on the surface-tension at the boundary between water
and non-aqueous phases. These compounds appear also
to produce their physiological effects by altering the

187

1 88 PROTOPLASMIC ACTION AND NERVOUS ACTION

structural substratum of protoplasm in a manner which
does not permanently change its properties or physical
state. Hence in their presence physiological processes
are modified temporarily in rate or character, and resume
their former conditions when the substance is removed.
The special affinity of these compounds for substances
having fat-like properties indicates that their primary
action is on the lipoid constituents of protoplasm; their
possible action on proteins, however, is also to be consid-
ered. In general they include the substances comprised
in Overton's first group (alcohols, ethers, esters, normal
and substituted hydrocarbons, etc.).

We distinguish, therefore, two chief groups of com-
pounds which by means of their reversible influence on
the structural substratum of protoplasm may modify
vital processes without affecting them permanently or
injuriously: (a) neutral salts or other electrolytes
(acid and alkali), and (b) lipoid-solvent or surface-active
organic compounds. These two groups may be charac-
terized respectively as general colloid-alterants and
lipoid-alterants. The compounds of these groups differ
somewhat sharply in their physiological action from those
compounds whose chemical effects tend to be irreversible;
the latter include most of the strong oxidizing and reduc-
ing agents and the salts of heavy metals; usually these
are not capable of modifying physiological processes with-
out permanent injury; hence they are toxic or poisonous
in small doses. Recovery from the effects of this '' poison-
ous" group depends upon the reparative activity of
the living protoplasm, just as does recovery from mechan-
ical injury, and not upon a simple reversal of the chemical
or other action of the compound.

LIPOID-ALTERANT SUBSTANCES 189

The most remarkable general physiological effect
produced by the lipoid-alterant substances is a reversible
suppression of irritability or spontaneous activity.
This effect always appears in certain definite, not too
high, concentrations of these compounds, and constitutes
the phenomenon of narcosis or anaesthesia, which is
universal in living matter.^ The power of inducing
this state seems to be independent of the special chemical
nature of the narcotizing compound; evidently this
power is connected in some manner with the general
physical properties just named; and the question first
arises whether the lipoid-solubility of these compounds
or their surface-activity is the property primarily
responsible for this characteristic action.

CORRELATION BETWEEN PHYSIOLOGICAL ACTION AND

LIPOID-SOLUBILITY

The existence of a relation between the solubility of
chemical compounds in fats and their narcotic action
was early noted, first by Bibra and Harless in 1847, ^^^
later by Claude Bernard, Hermann, Richet, Ehrlich,
and others.^ Richet propounded the rule that any
narcotizing compound has the stronger action as a
narcotic the lower its solubility in water. This is
similar to the rule of Overton and^ Meyer that the
narcotic effectiveness of a compound runs parallel with

^"We may say that everything living is sensitive and can be
anaesthetized; whatever is not sensitive is not living and cannot be
anaesthetized." — Claude Bernard, address, "La Sensibilite" (delivered
in 1876), published in his book. La Science Experimentale, Paris (1890).

^ Cf. Overton, Studien iiber die Narkose, Jena (i 901), for a historical
account of the earlier work on narcosis.

I90 PROTOPLASMIC ACTION AND NERVOUS ACTION

its oil-water partition-coefficient, a relation first studied
systematically by these investigators.'

In his investigations on permeability, Overton had
reached the conclusion that solubility in lipoids was the
chief factor determining the entrance of compounds
into cells; such readily penetrating compounds belong
for the most part to the narcotizing group; and a
detailed study of the phenomena of narcosis in tadpoles
demonstrated a close parallelism between the relative
solubilities of a large number of organic compounds in
oil and water and their narcotizing action.^ The ratio
according to which any compound is distributed between
these solvents (when the solvents are in contact and
equilibrium is reached) is a measure of its relative solu-
bility in the two; this ratio is known as the ^'partition-
coefficient." In the early members of any homologous
series of compounds the ratio of oil-solubility to water-
solubility increases progressively as the molecular
weight increases, and the same is true of the narcotizing
properties of the compounds. For example, with the
ethyl esters of the first five fatty acids, Overton found
the concentrations required for the complete narcosis
of tadpoles to be as indicated in the table (p. 191).

The narcotic effectiveness of the ester increases
regularly as its water-solubility decreases; and in
general each member of the series is from two to three
times as effective as its immediate predecessor. Rela-
tions of a similar kind were found with other series,

^ Overton, Vierteljahrschriften d. Naiurf. Gesellschajt, Ziirich,
XLIV (1899), 88; Studien iiber die Narkose (1901); H. H. Meyer,
Arch. f. exper. Path. u. Pharmakol., XLII (1899), 109.

' Studien iiber die Narkose.

LIPOID-ALTERANT SUBSTANCES 191

including hydrocarbons, alcohols, aldehydes, ketones,
ethers, and various substituted compounds. Any
increase in the oil-water partition-coefficient was associ-
ated with increase in narcotic effectiveness. Overton
accordingly drew the conclusion that v the narcotics act
by dissolving in certain oil-like or fatty constituents of
the irritable cells (in this case nerve cells); and he
identified these constituents with the lipoids, especially
lecithin and cholesterol, which appear to be always
present in protoplasm. The essential determining con-
dition of anaesthesia, according to his view, is the solution

■p.,.-- Narcotizing Concentrations Solubilities in Oil

^^^^^ (Mols. per litre) and Water

Ethyl formate .07 to .09 m Oil : water 4 : i

Ethyl acetate .03 m In 15.2 parts water;

in all parts oil
Ethyl propionate. . . .01 to .012 m In 50 parts water;

in all parts oil
Ethyl butyrate .0043 m In 190 parts water;

in all parts oil
Ethyl valerianate. . . .0019 m In 500 parts water;

in all parts oil

of the narcotic compound in these cell constituents;
when the hpoids are charged or impregnated with the
compound, they undergo a change of physical properties,
entailing corresponding alterations in the irritability of
the cell. Meyer drew independently^ a similar conclu-
sion;' he pointed out that the narcotizability of cells
seems to be related to the nature and proportion of the
lipoids present in the protoplasm; e.g., the high suscepti-
bility of nerve cells to narcosis is a correlative of their
high hpoid-content. Different narcotics act unequally
because they are distributed in unequal ratios between the

^ Meyer, loc, cil.

192 PROTOPLASMIC ACTION AND NERVOUS ACTION

lipoid and the aqueous phases of protoplasm; in general,
the greater the relative lipoid-solubility, the larger the
proportion of the anagsthetic compound which is in
solution in the lipoids when the partition-equilibrium is
reached. Hence when a compound has a very high
Hpoid-solubiHty it may exert narcotic action in extremely
dilute solution; phenanthrene, for example, was found
by Overton to narcotize tadpoles in dilutions of one part
in 1,500,000 of water.

The general conception known as the '^Overton-
Meyer theory of narcosis" may be defined as follows.
The solubility of narcotizing compounds in the cell-
hpoids forms the basis of their narcotic and presumably
other pharmacological properties. By dissolving in the
lipoids, such compounds alter the physical properties
of these essential components of the protoplasmic
system, and hence all properties of the system, especially
irritability, which are dependent on the state of the
Hpoids. Since simple solution without chemical combi-
nation is the basis of this effect, the latter is readily
reversed by allowing the compounds to diffuse away.

The general conclusion that selective solubiHty is
the essential basis of narcotic action does not, however,
necessarily follow, since the same reasoning would
apply to other physical effects which are reversible under
similar conditions; e.g., effects dependent on surface-
activity, involving a lowering of surface-tension at the
protoplasmic phase-boundaries and a concentration of
the narcotizing compound at these surfaces. Probably,
however, the case is not one of alternatives; if lipoids
are present in the system, any substances which are
soluble in these com_ponents must inevitably dissolve

LIPOID-ALTERANT SUBSTANCES 193

according to the partition-ratios. Any tendency to
concentrate at surfaces (e.g., the general cell surface or
the surfaces of other hpoid-containing protoplasmic
structures) would be favorable to such solution; i.e.,
would render it more rapid and greater in degree than
it would be otherwise, since the partition-equilibrium
would then be between the solution in contact with the
surface of the lipoid particle and the solution in the
interior of the particle.

Meyer's experiments on the influence of temperature
on the critical anaesthetizing concentrations of certain
compounds^ gave further indications that the solubility
of these compounds in the cell-lipoids is the essential
factor in the physiological effect. He chose six com-
pounds whose oil-water partition-ratios vary considerably
with temperature, and determined the minimal concen-
trations required to anaesthetize tadpoles at the two
temperatures 3° and 30°. These concentrations are
given in the following table :

An^cfViofi^ Critical Concentration Oil-Water Partition-

Ansestnetic ^^^ Anaesthesia Coefficients

At 3° At 30° At 3° Atao"

(A) Salicylamide m/1300 m/600 22.23 ^4

Benzamide m/soo m/200 0.67 0.43

Monoacetin m/90 m/70 0.099 0.066

(B) Ethyl Alcohol m/3 m/7 0.026 0.247

Chloral Hydrate. . . m/50 m/250^ oo53 0.236

Acetone m/3 m/7 o. 146 o. 235

In the first three compounds (A) the relative Hpoid-
solubiHty decreases with rise of temperature, and the

^ Meyer, Arch. f. exper. Path. u. PharmakoL, XL VI (1901), 3'38.
Recently Mary E. Collett has investigated the effects of variation of
temperature on the narcotic action of various compounds on marine
organisms; cf. Proceedings of the Society for Experimental Biology and
Medicine, XX (1923), 259.

194 PROTOPLASMIC ACTION AND NERVOUS ACTION

critical narcotizing concentration increases; in the last
three (B) the conditions are reversed. Tadpoles com-
pletely anaesthetized at 30° in m/250 chloral hydrate
revive and become active on cooling the solution; on
warming they again become inactive. Such an effect
is difficult to explain except on the basis of the greater
lipoid-solubility of the anaesthetic at the higher temper-
ature, since adsorption is in general increased by lowering
the temperature. Meyer therefore concludes that the
anaesthetic produces its effect by dissolving in the cell-
lipoids. Such experiments seem to indicate clearly that
the solvent action of the protoplasmic lipoids is a main
factor in anaesthesia; and since anaesthesia has close
affinities with normal variations of irritabiUty, such as
sleep and fatigue, they also point to the conclusion that
under normal conditions the lipoids are important in the
cell largely because of their peculiar properties as solvents.

The relation of the solvent properties of lipoids to
permeability has already been considered. The Upoids
thus represent the organic solvents of the cell, and
apparently the properties of the protoplasmic system
vary according to the nature and concentration of the
substances which they hold in solution. This is a
conclusion of much general interest, apart from its
special relation to anaesthesia, since variations in the
proportions of water-soluble to lipoid-soluble substances
no doubt occur constantly in living protoplasm. The
question of why such variations alter irritability, spon-
taneous activity, and metaboHsm will be considered
more fully later in connection with stimulation.

In many cases it has been shown that the organic
anaesthetics accumulate in cells in greater concentration

LIPOID-ALTERANT SUBSTANCES 195

than in the medium, and this fact is usually interpreted
as favoring the partition theory of narcosis. Pohl^
found that the blood corpuscles and brain of deeply
narcotized dogs contained from three to four times as
much chloroform as the serum; Hedin^ investigated
cryoscopically the distribution of alcohols, aldehydes,
ketones, esters, and ether between corpuscles and
plasma, and found that most compounds, especially the
more highly lipoid-soluble, collected in higher concentra-
tion in the cells than in the plasma. More recently
Warburg and Wiesel^ have made similar observations on
the blood corpuscles of birds, using alcohols, ketones,
urethanes, thymol, formaldehyde, and HCN. All of
these compounds, in appropriate concentrations, were
found to decrease oxygen consumption, and the tendency
to concentrate in the cells ran closely parallel with this
effect. Thus the lower alcohols (up to butyl alcohol)
were the least effective in reducing oxygen consumption,
and showed correspondingly a somewhat lower solubility
in the protoplasm than in the medium; methyl ure thane,
diethyl urea, and acetone behaved similarly; amyl
alcohol and isobutyl urethane were more effective, and
were about equally distributed between cells and medium;
while the most effective substances — phenyl-methyl
ketone, thymol, phenyl urethane — ;all showed decided
concentration in the cells. When solutions were used
that decreased the oxygen consumption by 50 per cent,
phenyl-methyl ketone was found to be about twice,
phenyl urethane three times, and thymol nine times as

^ Pohl, Arch, exper. Path. u. Pharmakol., XXVIII (1891), 239.

^Hedin, Arch. ges. Physiol., LXVIII (1897), 229.

3 Warburg and Wiesel, Arch. ges. Physiol., CXLIV (191 2), 465.

196 PROTOPLASMIC ACTION AND NERVOUS ACTION

concentrated in the cells as in the medium. Formal-
dehyde and HCN also underwent concentration in the
cells. The series, methyl alcohol < butyl alcohol <amyl
alcohol < phenyl -methyl ketone < phenyl ure thane <
thymol represents the order both of increasing physio-
logical action and of increasing concentration in the
cells.

On the whole the foregoing studies of the distribution
of narcotics between cell and medium appear to favor the
partition theory of the action of these compounds. It
should be pointed out, however, that the order of relative
adsorption is in general the same as that of Hpoid-
solubihty. The probabiKty, as already pointed out, is that
both solution and adsorption are factors in the total effect.

CORRELATION BETWEEN PHYSIOLOGICAL ACTION
AND SURFACE-ACTIVITY

The relation between the narcotic or other physio-
logical action of organic compounds and their influence
on surface-conditions has recently received much investi-
gation. The majority of compounds of the lipoid-
alterant group have a marked influence in lowering the
surface-tension at the boundary-surfaces between their
aqueous solutions and air or other adjoining phase;
at the same time, in accordance with the Gibbs-Thomson
rule, they undergo increase of concentration (or adsorp-
tion) at such surfaces. In homologous series both the
influence on surface-tension and the degree of adsorption
increase progressively with increase in molecular weight;
and the view that the physiological action is determined
by this surface-action, i.e., by the condensation of the
compounds at the protoplasmic phase-boundaries, rather

LIPOID-ALTERANT SUBSTANCES 197

than by solution in the Hpoids has been supported by
many recent investigators, especially Czapek, Traube,
and Warburg/

The general fact that the physiological action of
homologous compounds increases progressively with
increase in molecular weight has long been noted.
Richardson,^ in 1869, in a study of the pharmacological
action of alcohols, called attention to this rule, which
applies also to the effects on lower organisms; thus,
according to Regnard,^ the first six alcohols have equal
effects in suppressing the growth of yeast in the
following concentrations :

Ai^^T,^! Volumes

^I'^^^o^ (per cent)

CH3OH 20

aHsOH 15

C3H70H 10

C4H9OH 2.5

CsH„OH I

C6HX3OH 0.2

In a series of papers beginning in 1904, Traube has
directed special attention to the parallelism between
surface-activity and physiological action.'* For example,

^ Czapek, Oberflachenspanmmg der Plasmahaui, Jena (191 1) ; Traube,
"Theorie der Narkose," Arch. ges. Physiol, CLIII (1913), 276; CLX
(1915), 501. Cf. also "Theorie des Haftdrucks und Lipoidtheorie,"
Biochem. Z., LIV (1913), 305, and other papers there cited. Warburg,
see below.

2 Richardson, "Physiological Researches on Alcohols," Medical
Times and Gazette, VIII (1869) (cited from Czapek, loc. cit.).

3Regnard, Compt. rend. Soc. Biol., X (1889), 124. Warburg and
Wiesel obtained similar results with the series of ure thanes {loc. cit.).

4 Traube, Arch. ges. Physiol., CV (1904), 541, 559; further refer-
ences in the papers cited above.

1 98 PROTOPLASMIC ACTION AND NERVOUS ACTION

the surface-tensions (against air) of 0.25 m aqueous solu-
tions of alcohols decrease in regular order as follows:

Alcohol Surface-Tension

(Mgms. per mm.)

CH3OH 7.05

C2H5OH 6.73

C3H7OH 2.89

C4H9OH 4-49

CsHxrOH 3.05

(H.0) (7.3)

The order of relative adsorption by charcoal and other
adsorbents is similar; if the degree of adsorption runs
parallel with the lowering effect on surface-tension, and if
equal adsorption corresponds to equal physiological
action, we should expect that solutions of the same
surface-tension (isocapillary solutions) would have the
same physiological effect. Czapek and Traube have in
fact demonstrated a close parallelism between the influ-
ence of a large number of compounds on air- water surface-
tension and their physiological action. Traube has
called attention to the fact that in a homologous series
of compounds the degree of activity, both physical and
physiological, increases very generally about three times
with each increase in molecular weight. According to
this rule the isocapillary concentrations of the successive
compounds of the series should diminish in geometrical
progression, with one-third as exponent, as the molecular
weight increases. He gives the following determinations
for the series of alkyl acetates :

Ester and Concentration Capillary Height

CH3COOCH3.. (m) 58.1m

C.Hs (m/3) 58.0

C3H7 (m/9) 57.7

i-QH, (m/27) 58.8

i-CsH„ (m/8i) 59.9

(H,0) (91.5)

LIPOID-ALTERANT SUBSTANCES 199

In a considerable number of cases the degree of physio-
logical action has been shown to follow a similar rule;
the observations of Fiihner on haemolysis (cited below)
are a good example.

If physiological activity is in fact a function of capil-
lary activity, solutions of equal surface-tension should
exhibit equal narcotic action or otherwise produce equal
effects in protoplasm; and Czapek has brought forward
evidence that this is very frequently the case.' Using
a large ninnber of surface-active organic compounds, he
determined the surface-tensions of those solutions which
had equal effect in liberating tannin from plant cells
(chiefly the leaves of Echeveria) ; this effect depends on
a permeability-increasing action analogous to that
accompanying cytolysis. In general he finds that solu-
tions of a concentration just sufficient to cause exosmosis
of tannin have very nearly the same surface-tension
against air; viz., about two-thirds that of pure water;
according to his hypothesis, the surface-tension of the
protoplasm becomes zero in such solutions and an
effective surface of separation ceases to exist. Kisch^
also finds that iso capillary solutions of alcohols have equal
effects in liberating invertase from yeast and molds
and in inhibiting the growth of yeast cells; and H.
Zuckerkandl^ has observed a similar relation in the
protoplasmic streaming of plant cells. According to
Traube and others, haemolysis follows the same rule.^

^ Czapek, loc. cit.

^Kisch, Biochem. Zeitschrift, XL (1912), 152.

3 H. Nothmann-Zuckerkandl, Biochem. Zeiischrlft, XLV (1912), 412.

'•Traube, loc. cit.; Fiiliner and Neubauer, Zentralhl. f. Physiol.,
XX (1906), 117; Arch, exper. Path. u. Pharmakol., LVI (1907), 333.

200 PROTOPLASMIC ACTION AND NERVOUS ACTION

The following table summarizes some of the results of
Czapek and Kisch with alcohols.

Critical surf ace -tensions (water = i ) of solutions causing

Alcohol A. Exosmosis of Tannin from B Inhibition of

Cells of Echeveria Gro\Yth of Yeast

Methyl 0.7 0.51

Ethyl 0.67 0.48

N-propyl. . . . 0.675 ca. 0.49

I-propyl 0.69

N-butyl 0.69

I-butyl o. 665 ca. 0.5

I-amyl o . 665 o . 49

Czapek's ascription of the effects which he observes
to a definite or critical lowering of the surface-tension
of the plasma membrane is, however, of doubtful validity,
since there is no necessary parallelism between the
influence of a given substance on the surface-tension
at a water-air interface and its influence on the tensions
at other interfaces. This has recently been pointed out
by Lorant;^ for example, in comparing the tensions
exhibited by various liquids in contact with air and with
water, respectively, Lorant finds the following:

S.T. of CHCI3/ water is about 6%-4%>S.T. of CHCyair

S.T. of CCiywater is about 65%>S.T. of CCVair

S.T of CeHe/ water is about 33% > S.T. of CeHe/air

S.T. of CeHsNO^/water is about 4i-42%< S.T. of CoHsNO^/air

S.T. of CeHixOH/ water is about 77%< S.T. of CeHixOH/air

Lorant also made observations on the surface-
tensions between various organic fluids (e.g., ether) and
salt solutions. Usually the influence of neutral salts on
surface-tension was in the direction of an increase. Of
the different anions CI has the greatest effect, and I and

'Lorant, Arch. ges. Physiol., CLVII (1914), 211.

LIPOID-ALTERANT SUBSTANCES 201

CNS the least; with ethyl ether and nitrome thane, the
chlorides, sulphates, and bromides increased the inter-
facial tension, while the iodides and thiocyanates
decreased it. Similar conditions were found with
CHCI3 and CCI4, but in this case iodide also somewhat
increased the surface-tension.

It would appear that the physical relations (of
adhesion, mutual solubility, etc.) between water and
the organic compound, as well as between the latter and
the non-aqueous protoplasmic phase or structure (e.g.,
membrane) are of importance in the physiological effect.
According to Traube the narcotic action of organic
compounds is determined by what he calls their "Haft-
druck" (''adhesion- tension"), i.e., special attraction to
or affinity for water ;^ the tendency of any compound
to pass out of aqueous solution and concentrate in the
surface layer between water and the other phase — i.e., to
undergo adsorption — is in general the greater the less
its affinity for water. This is one manner of interpreting
the relation noted by Richet and others between water-
insolubility and narcotic action; but since solubility in
water and solubiUty in organic solvents — e.g., in the
esters of higher fatty acids which form the organic sol-
vents of protoplasm — have similarly reciprocal relations,
this consideration does not enable us to decide whether a
solution-effect or an adsorption-effect is the essential fac-
tor in the physiological action. The recent investigations
of Langmuir and Harkins on adsorption^ indicate, how-

^ Cf. Traube, "Theorie des Haftdrucks und Lipoidtheorie," Bio-
chem. Zeilschrift, LIV (19 13), 305.

^ Langmuir, Journal of the American Chemical Society, XXXIX
(1917), 1848; XL (1918), 1361; Harkins, Clark, and Roberts, i6JJ.,XLII
(1920), 700; Harkins and Cheng, ibid., XLHI (192 1), 35.

202 PROTOPLASMIC ACTION AND NERVOUS ACTION

ever, that there is no fundamental difference between
these two processes; the affinity for water seems depend-
ent usually on the terminal or polar group of the organic
compound (COOH, NH2, OH, etc.), and an adsorbed
compound may be one in which part of the molecule has
an affinity for (equivalent to solubility in) water, while
the other part has not, but is attracted more strongly by
the other phase. In such cases the position at an inter-
face may be the chief position of equilibrium, and the
predominant effect may be adsorption, with limited solu-
tion in either phase. Such a view implies that the tran-
sition from adsorption to partition is a continuous one,
and explains why highly surface-active compounds usually
have high lipoid- water partition-coefficients. Such com-
pounds will enter into solution in the non-aqueous phase,
provided this is also a solvent. They may, however, con-
dense at the surface of material in which they do not dis-
solve, and in so doing influence chemical action at such
surfaces. Traube calls attention to the fact that the cat-
alytic action of finely divided non-solvent materials like
carbon and platinum may be thus influenced; and he
places narcotics in the class of ''anti-catalysers";^ i.e.,
they are regarded as decreasing the catalytic and hence
the chemical activity of living matter by some form of
surface-action, e.g., by occupying the interfacial positions
(where chemical activity appears to be greatest) in the
heterogeneous protoplasmic system and displacing the
chemically reactive compounds.^ This view, while
partial, may well be correct in certain cases, although it

' ''tJber Katalyse," Arch. ges. Physiol., CLIII (1913), 309.

' Compare Warburg, Biochem. Zeitschrijt, CXIX (1921), 134; see
footnote, p. 206.

LIPOID-ALTERANT SUBSTANCES 203

probably does not cover the entire range of phenomena
included under narcosis.

INFLUENCE OF ORGANIC NARCOTICS ON THE
CHEMICAL REACTIONS IN PROTOPLASM

The fact that narcotic compounds arrest spontaneous
activity, and in general act as depressants of vital
processes and of irritability, shows that they interfere
with the energy-yielding chemical reactions of proto-
plasm. The essential problem relates to the means by
which this effect is produced, whether it is primary or
secondary; i.e., there are the alternative possibilities:
(i) that the primary action may be a modification of the
structural conditions on which the chemical reactions
depend; and (2) that the reactions themselves may be in-
fluenced directly; e.g., by some form of anticatalytic action.

Warburg and his associates have made an extensive
study of the influence of narcotizing compounds on the
oxygen consumption of Kving cells, and the results of
this work show many striking parallels with those already
described. The cells used in the various determinations
included sea-urchin eggs, erythrocytes (chiefly of birds),
yeast, lymphocytes (from thymus), spermatozoa (of
fishes), liver cells (of frog and mouse), and bacteria
(Vibrio, Staphylococcus, Bacillus Typhi) .^ In all cases
the rate of oxygen consumption was decreased, reversibly,
in the presence of a sufficient concentration of anaesthetic.
The facts point in general to some kind of physical rather
than specifically chemical interference with the oxidation
reactions. Thus the effect produced by a particular
compound is largely independent of the chemically

' For a summary of this earlier work of Warburg, see Milnch. med.
Wochenschr., LVIII (191 1), 289; also Warburg and Wiesel, loc. cit.

204 PROTOPLASMIC ACTION AND NERVOUS ACTION

characterizing group; e.g., two nitriles may be of very
unequal effectiveness — require different concentrations
to produce the same degree of depression, although
possessing the same polar group — and the same is true
of other compounds. In any series the depressant
action on oxidation is greater with the higher members
of the group; and the relative effectiveness of the
different compounds is closely similar to that observed
in experiments on narcosis. For example, the several
alcohols were found to lower the oxygen consumption
of birds' erythrocytes by about 50 per cent in solutions
of the following concentrations;^ Overton's determina-
tions of the minimal anaesthetizing concentrations of the
same compounds for tadpoles'" are cited for comparison:

Concentration of Solutions

Concentrations (Molecular)

Depressing

2 CONSUMP-

Required for Anaesthesia

Alcohol

TION BY

50%

of Tadpoles

By weight

Molecular

Methyl

i6

Sm

0.52-0.62 m

Ethyl

7-3

1.6 m

0.27-0.31 m

Propyl

^

0.8 m

0. II m

N-butyl

1. 1

0.15 m

0.038 m

I-butyl

I.I

0.15 m

0.045 III

Amyl

0.4

0.045 m

0.023 m

As an example of experiments with bacteria {Vibrio
Metschnikovii) the following series may be cited; to
diminish oxygen consumption by about half the following
concentrations of ure thanes were required:

Methyl urethane o. 67 m (5%)

Ethyl 0.4 m (3.5%)

Propyl 0.097 Di (1%)

Isobutyl o . 043 m (o. 5%)

Phenyl 0.003 "i (0.05%)

*Z. physiol. Chem., LXIX (1910), 452.
* Stndien iiher die Narkose, p. loi.

LIPOID-ALTERANT SUBSTANCES 205

The relative effects of these compounds on the anaer-
obic growth of yeast were similar; the following solutions
produced about the same degree of inhibition :

Methyl urethane 8%

Ethyl urethane 4%

Propyl urethane 2%

Isobutyl urethane 1%

Phenyl urethane o. 1%

These results^ are similar to those of Regnard with
alcohols, cited above.

Usui, working under Warburg's direction,^ found also
a decrease in the oxygen consumption of vertebrate
tissues (liver, central nervous system) under the influence
of narcotic compounds (alcohols, ketones, urethanes,
methyl urea, and phenyl urea) ; but in order to produce
marked depression of oxidations much higher concentra-
tions were required than in normal reversible narcosis,
and the effect was imperfectly reversible. This result
is interesting as indicating that anaesthesia is not neces-
sarily associated with a decrease of intracellular oxida-
tions, as Verworn and others have supposed; in fact,
Warburg, Winterstein, Loeb and Wasteneys, and others
have shown in a number of instances that anaesthesia
when perfectly reversible does not necessarily involve
a decrease in oxygen consumption.^ Diminished oxida-
tion is to be regarded rather as a secondary consequence
than as a cause of narcosis. Apparently the chemical

I

Warburg and Wiesel, loc. cit.

^Usui, Arch. ges. Physiol., CXLVII (191 2), 100.

3 Warburg, Z. physiol. Chem., LXVI (1910), 305; LXX (1911),
413; Winterstein, Biochem. Z., LXI (1914), 81; also Winterstein's
book on narcosis; Loeb and Wasteneys, Journal of Biological Chemistry,
XIV (1913), 517; Biochem. Zeitschrift, LVI (19 13), 295.

2o6 PROTOPLASMIC ACTION AND NERVOUS ACTION

effect is secondary to some physical modification pro-
duced in the protoplasm by the narcotizing compound.^

PHYSICAL CHANGES PRODUCED BY LIPOHD-SOLVENT
COMPOUNDS IN PROTOPLASM

Certain definite changes in the physical properties of
protoplasm, analogous in many respects with those
produced by salts, have been observed in various cases
to accompany the action of narcotizing compounds;
these changes indicate that underlying narcosis there
are definite modifications of the structural conditions in
protoplasm; and presumably it is to such modifications
that the changes in physiological properties and activity
are to be referred. As we have seen, the distinctively
vital processes are controlled by structural conditions;
structural change impHes physiological change.

' For a more detailed discussion of the relation of narcosis to oxida-
tion processes see my review, "The Theory of Anaesthesia" {Biological
Bulletin, XXX (1916), 311, also American Yearbook of Anesthesia, I, i).

According to Warburg (cf . his recent article on the physical chemistry
of cell-respiration, Biochem. Zeitschrift, CXIX [1921], 134) the proto-
plasmic oxidations occur at the surface of the solid cell structures, which
adsorb the water-soluble oxidizable compounds; narcotics influence
oxidations by changing the physical and chemical character of the surfaces.

He expresses his general conclusions on the conditions of proto-
plasmic oxidations as follows: "Two chief means are employed by the
cell to diminish the chemical resistance at the regions of oxidation;
namely, adsorption and the catalytic action of heavy metals .... Cell
respiration is a capillary process occurring at the iron-containing surfaces
of the solid cell-constituents. By adsorption at these surfaces the inert
organic compounds become capable of reacting with O2 just as do amino-
acids at the surface of charcoal. This view does not explain respiration
in the physical sense, but classes it with general phenomena of the

inorganic world Narcotics check the cell-oxidations by occupying

the surfaces and thereby displacing the oxidizable compounds. The same
action is exhibited by different narcotics when the same fraction of the
active surface is occupied by the narcotic" (pp. 152, 153).

LIPOID-ALTERANT SUBSTANCES 207

Changes of permeability, of viscosity, and of resist-
ance to the action of cytolytic or other injurious condi-
tions are the most evident physical effects produced by
lipoid-alterant compounds in living protoplasm. During
narcosis there appears very generally to be a decrease of
permeabihty, an increase in the resistance to structural
breakdown or cytolysis, and an increase of protoplasmic
viscosity. From the general nature of these changes it
would seem that the structural substratum of the Uving
matter assumes temporarily a denser or physically more
stable condition. In any event it is clear that the
modification is in such a direction as to interfere with
stimulation, a process, which (as we shall see later)
involves structural changes in the irritable system.
Hence what may be described as a stabilization, decrease
of susceptibiHty to structural change, is indicated as in
all probabiHty the essential physical condition underlying
narcosis; but any such general term gives little indication
of the detailed nature of the physical modification
produced in the living protoplasm. The manner in
which narcosis modifies stimulation-processes will be
considered in more detail later; in the present section
we shall merely describe briefly those changes in the
physical state of protoplasm which appear to have some
bearing on the question of the nature of the conditions
determining narcosis.

The changes observed in the larva of Arenicola are
simple and instructive; normal larvae transferred from
sea water to pure isotonic NaCl solution undergo stimula-
tion and marked increase of permeabihty as shown by
loss of pigment; on the other hand, larvae which are
first placed in a solution of a magnesium salt, or in sea

2o8 PROTOPLASMIC ACTION AND NERVOUS ACTION

water containing a suitable anaesthetic (ether, chloretone,
alcohol) in the narcotizing concentration, and are then
transferred to NaCl solution (preferably containing
anaesthetic), show no such effect; there is no immediate
loss of pigment and Httle or no stimulation. The
breakdown of cilia in the NaCl solution is also prevented.'
A protective (antitoxic) or stabilizing action is associated
with the narcotizing action in this organism; and
essentially similar conditions have been found in the
eggs of the sea-urchin Arhacia.^ Analogous observations
have been made on other cells by a number of investiga-
tors. Arrhenius and Bubanovic found that the break-
down of blood corpuscles in hypotonic media was
hindered by anaesthetics;^ similar observations have more
recently been made by Linzenmeier and Runnstrom;
haemolysis and agglutination by foreign proteins may
also be diminished by anaesthetics."* These '^ stabiliza-
tion" effects are observed in the concentrations corre-
sponding to the anaesthetizing range; higher concentra-

^R. S. Lillie, American Journal of Physiology, XXIX (1912), 372;
XXXI (1913), 255.

2 R. S. Lillie, American Journal of Physiology, XXX (1912), i.

3 Publications of Nobel Institute (1913), No. 32, cited from Hober's
Physikalische Chemie der Zelle u. der Geivebe, p. 466.

4 See Linzenmeier, Arch. ges. Physiol., CLXXXI (1920), 169, and
CLXXXVI (1921), 272; for similar observations on bacteria cf. Vor-
schiitz, ibid., CLXXXVI (1921), 290; Runnstrom, Biochem. Zeitschrift,
CXXIII (192 1), I. See also the observations of Traube {Biochem.
Zeitschrift, X [1908J, 371) and Clowes {Proceedings of the Society of Experi-
mental Biology and Medicine, XI [1913], 8). These changes of properties
resulting from the modification of surface-films have an interesting
relation to those produced by addition of proteins to suspensions of
bacteria and blood corpuscles, and recently investigated by Northrop
and de Kruif {Jour. Gen. Physiol., IV [1922], 655), Eggerth and Bellows
{ibid., p. 669), and Coulter {ibid., p. 403).

LIPOID-ALTERANT SUBSTANCES 209

tions produce irreversible structural change or cytolysis.
Such facts indicate, in general, that in the narcotizing
solutions the plasma membranes become more resistant
to alteration. Hober's observations on the action of
anaesthetics in hindering the production of injury-
currents in muscle by potassium salts illustrate the same
condition; the local negativity (an index of increase of
permeability) develops much more slowly in the presence
of ether, urethane, and other narcotizing compounds
than in the pure solution/

A decrease of permeability to water-soluble diffusing
substances and ions, and also in some cases to water, is
an effect of a related kind. The entrance of water-
soluble dyes into plant cells {Spirogyra) is retarded
during anaesthesia;^ neutral salts may also produce
this effect both in animal and plant cells.^ Decreased
permeability to ions is indicated by decreased electrical
conductivity; this has been demonstrated by Osterhout
in Laminaria, by McClendon in sea-urchin eggs, and by
Joel in blood corpuscles. "* In Laminaria a reversible
decrease of conductivity is found only in moderate
concentrations of ether and other anaesthetics, corre-
sponding to the anaesthetizing concentrations; stronger
solutions cause marked and irreversible increase of

^Hober, Arch. ges. Physiol., CVI (1905), -^99.

^ Lepeschkin, Ber. deuisch. botan. Ges., XXIX (191 1), 349.

3 Szucs, Jahrh. wiss. Botanik, LII (191 2), 85. Cf. also the recent
observations by Miss Irwin, Jour. Gen. Physiol., V (1923), 223, 727.
Mg salts decrease the rate of penetration of dyes into Arenicola larvae
{American Journal of Physiology, XXIV [1909], 26).

4 Osterhout, Science, XXXVII (1913), in; The Plant World, XVI
(1913), 129; McClendon, Popular Scientific Monthly (1915), p. 569; and
Joel, Arch. ges. Physiol., CLXI (1915), 5.

2IO PROTOPLASMIC ACTION AND NERVOUS ACTION

conductivity, indicating a destructive or cytolytic effect.
In fertilized sea-urchin eggs, anaesthetics (chloral,
hydrate, alcohols, urethane) decrease the permeability
to water, as shown by the decreased rate of shrinkage
in hypertonic sea water containing the anaesthetizing
compound.' The penetration of acids into the pigment-
containing mantle cells of nudibranchs is also retarded
by anaesthetics.^

Changes of protoplasmic viscosity, as indicated by
changes in the readiness with which cell structures are
mechanically displaced (by centrifuging), have also been
observed, but the character of the change appears to
vary in different forms of protoplasm. In plant cells,
according to the observations of Heilbronn^ on seedHngs
and F. Weber^ on Spirogyra, ether in the anaesthetizing
concentrations increases the viscosity of the protoplasm;
in lower concentrations, on the other hand, it decreases
viscosity. This result agrees with the observations
of Ewart^ and others who find that weak solutions of
anaesthetics accelerate protoplasmic streaming while
stronger solutions retard or arrest it. In sea-urchin
eggs L. Heilbrunn^ has recently found that various
anaesthetics in concentrations sufficient to prevent cell-

^R. S. Lillie, American Journal of Physiology, XLV (191 8), 406;
cf. p. 427.

^ Crozier, Jour. Gen. Physiol., IV (1922), 723.

sHeilbronn, Jahrb. wiss. Botanik, XLIV (1914), 357.

4 F. Weber, Biochem. Zeitschrijt, CXXVI (192 1), 21; Ber. deutsch.
hofan. Ges.y XL (1922), 212.

s Ewart, On the Physiology and Physics oj Protoplasmic Streaming in
Plants, Oxford (1903). Cf. also the observations by Demoor and others
cited in Czapek's Biochemie der Pflanzen, Jena (1913), p. 161.

•^L Heilbrunn, Biological Bulletin, XXXIX (1920), 307.

LIPOID-ALTERANT SUBSTANCES 2ii

division cause decrease of viscosity, i.e., facilitate the
displacement of granules by the centrifuge. In some
cases, however, Heilbrunn found effects of the opposite
kind; and he distinguishes two kinds of anaesthesia, in
which protoplasmic viscosity is respectively increased
and decreased. The significance of such changes in
relation to the functional activity of the cell is not clear.
They show, however, that the structural conditions
within the protoplasmic system are modified reversibly
by the lipoid-solvent group of compounds and that the
concentrations required for this effect correspond to
those which produce narcosis.

Apparently the most general inference to be drawn
from the foregoing facts is that one constant accompani-
ment of narcosis is a modification, in the direction of
greater stability or impermeability, of the physical state of
the plasma membrane of the irritable cells; and there is
good reason to believe that the physiological effect of
narcotic compounds depends on this effect; this conclusion
will receive further support when the subject of stimula-
tion is discussed. The chief locus of action of anaesthetics
thus appears to be the same as that of salts, which is
evidently superficial, as already pointed out. Antago-
nisms between salt action and anaesthetic action are in
fact readily demonstrable in many cases. ^ Salts like
NaCl in pure solution tend to disintegrate the plasma
membranes, and the addition of an anaesthetizing
compound to the solution frequently retards or prevents

^ See my series of papers on antagonisms between salts and anaes-
thetics, American Journal of Physiology, XXIX (1912), 372; XXX, i,
andXXXT (1913), 255; dlso Journal of Experimental Zoology, XVI (1914),
591. Hober's observations (above cited) on the effects of anaesthetics
in retarding the production of injury-currents in muscle by salts furnish
other examples of this phenomenon.

212 PROTOPLASMIC ACTION AND NERVOUS ACTION

this action in a manner resembling that of an antagonistic
salt like CaCla. Clowes has shown that in the physical
drop-systems which he studied — alkaline NaCl solution
flowing from a stalagmometer through oil — anaesthetics
produce an effect closely comparable with that of CaCla;
both actions are to be referred to changes in the properties
of the interfacial films formed between the oil and the
aqueous solution.^ The presence of a compound (Ca
soap, or fat-solvent compound) which is more soluble
in the oil phase than in the water phase, modifies the
conditions at the boundary in the same manner in both
cases, and produces the same physical effect in the
system. The parallelisms observed by Clowes between
the biological and the physical phenomena may be
interpreted as indicating that the conditions in living
protoplasm are of a closely analogous kind.

The foregoing effects of anaesthetics on the physical
properties of protoplasm do not, however, enable us to
decide whether the solvent action or the adsorbent
action is the chief factor in the narcotic effect, or whether
both are equally concerned. Certain widely general
biological phenomena do not appear to be entirely
consistent with Traube's theory that surface-activity is
the essential factor in all cases of narcosis. These are:
(i) the great differences observed between the narcotizing
concentrations of the same compound in different cells,
tissues, and organisms; (2) the fact that weak solutions
of many narcotic compounds have a sensitizing or
accelerating influence on many cell-processes;^ (3) the

'Clowes, Journal of Physical Chemistry, XX (1916), 407; cf.
pp. 434 fif.

* Cf. the instances cited in my review of the theory of anaesthesia
{Biological Bulletin, loc. cit.).

LIPOID-ALTERANT SUBSTANCES 213

general relation, pointed out by Overton and Meyer,
between the Kpoid-content of tissues and their suscepti-
bility to narcotic action; and (4) the anaesthetizing action
of compounds, such as Mg and K salts, wliich are without
surface-activity in the foregoing sense. Traube's theory
in fact, in emphasizing the importance of a single physical
factor, seems to disregard other possible factors, and on
the whole to underestimate the complexity of the
physiological conditions.

Hober,^ Vernon,^ and others have pointed out
various exceptions to the rule that isocapillary solutions
have equal physiological action. This rule cannot be true
in any precise sense, since an organic compound may
affect the conditions at the various interfaces in a complex
system Kke protoplasm, and at an air-water interface,
quite differently; and other factors, including viscosity,
solubiUty in the protoplasmic phases, and specific
chemical affinities enter to modify the simple surface
conditions. In cases where the conditions are simple,
the increase from compound to compound in a homolo-
gous series may be very regular; a good example is
seen in Fiihner's observations on haemolysis by solutions
of alcohols;^ the critical haemolytic concentrations for
the first five alcohols are as follows:

m.-sol.

Methyl ^ . . 7 . 34

Ethyl 3.24

Propyl 1 . 08

Butyl 0.312

Amy] o . 09

^ Hober, Physik. Chem. d. Zelle, p. 415.
' Vernon, Biochem. Zeitschrift, LI (1913), i.

3 Fuhner and Neubauer, Arch, exper. Path. u. PharmakoL, LVI
1907); 333-

214 PROTOPLASMIC ACTION AND NERVOUS ACTION

In this case, the ratio of about 3 : i between successive
members is well shown. In other cases this relation is
obscured by chemical and other factors; an instructive
instance is the relatively high toxicity of methyl as
compared with ethyl alcohol, apparently a consequence
of the special properties of the former's oxidation
products, formaldehyde and formic acid. Fiihner cites
other cases showing a similar regularity; he finds,
however, that in tissues with high lipoid-content, such
as the vertebrate central nervous system, the higher
members of certain series (alcohols) are more effective
than would be expected from this simple rule. This
discrepancy he ascribes to the larger proportion of
organic solvent (Hpoid) in such tissues; thus, in the
adult frog the divergence from the 3 : i ratio is greater
( = ca. 4:1) than in the tadpole ( = 2.9:1); this difference
is apparently referable to the increase in lipoid constitu-
ents as the central nervous system develops.^ It seems
probable that in lipoid-rich tissues the lipoid-solvent
factor becomes relatively important in comparison with
the capillary constant factor.

The relatively great solubility of many anaesthetic
organic compounds in protein-containing systems defi-
cient in Hpoid (serum, finely divided muscle, etc.) has
been attributed by Moore and Roaf^ to the formation
of chemical combinations with the protein; but since
all such systems are undoubtedly polyphasic, and since
chemical combinations (in the stoichiometric sense) of
hydrocarbons (like CHCI3, benzol, etc.) with proteins are

^ Fiihner, Z. Biol., LVII (191 2), 465.

2 Moore and Roaf, Proceedings of the Royal Society, B, LXXIV
(1908), 382; LXXVII (1906), 86.

LIPOID -ALTERANT SUBSTANCES 215

difficult to conceive, it seems more likely that an
adsorption-effect is involved, similar to that observed
in finely divided suspensions of charcoal. It is well
known that the catalytic effect of such suspensions is
markedly influenced by surface-active compounds;^ this
effect indicates an alteration in the character of the
surface, probably resulting from adsorption. Other
phenomena of a related kind, e.g., the precipitation
produced by organic solvents in protein solutions (as
observed by BattelH and Stern and Moore and Roaf),^
the liquefying action of these compounds on gelatine
gels (Traube and Kohler),^ the solidifying action on
lecithin suspensions, and the interference with the
precipitation of lecithin suspensions by electrolytes
(Koch, Hober and Gordon, and others) ,'' are similarly
referable to surface- conditions, although the special
nature of these conditions is not clear in all cases. It
is noteworthy that most of these effects are observed
at concentrations far in excess of those required to
produce reversible narcotic effects in living protoplasm.
To characterize the organic anaesthetics as negative
catalyzers, as Traube does, may place them in a class,
but does not explain their characteristic action on
living matter.

It seems certain from the physical peculiarities of
these substances that they must undergo adsorption

^ Cf. Warburg, Arch. ges. Physiol., CLV (1914), 547.

2 BatteUi and Stern, Biochem. Zeitschrift, LII (1913), 226; Moore
and Roaf, loc. cit.

3 Traube and Kohler, Internal. Zeilschr. f. physik.-chem. Biol., II
(1915), 42.

4 Cf . pp. 360 ff. of my Theory of Ancesthesia, loc. cit.

2i6 PROTOPLASMIC ACTION AND NERVOUS ACTION

at the structural surfaces in protoplasm, and also dissolve,
in accordance with their partition-coefficients, in the
organic solvents of protoplasm. It is not a case of two
incompatible processes; both occur simultaneously, and
each contributes to the total effect. Possibly the
reversible effects characteristic of low concentrations of
anaesthetic substances are the expression of solution in
the lipoids, while with higher concentrations the specific
structural compounds of the protoplasm, the proteins,
are affected through coagulation or other changes due
to adsorption, and irreversible effects result. The
reversible effects — stimulation or sensitization in very
weak, inhibition or anaesthesia in stronger, solutions —
are the expressions, respectively, of facilitation and
hindrance of the normal metaboHc processes underlying
stimulation and automatic activity; i.e., the influence
of the cell-structure on the metabohc reactions is modi-
fied, and the whole behavior of the protoplasmic system
is altered correspondingly.

If the influence of structural conditions on cell-
metabohsm is to be included under the class of heterogene-
ous or contact catalysis, as many of the foregoing facts
indicate, it is evident that a consideration of this type
of catalysis and of the manner in which it is influenced
by substances of the foregoing kind becomes essential in
the further analysis of the conditions in living protoplasm.

CHAPTER X

CATALYSIS IN RELATION TO THE CHEMICAL
PROCESSES IN LIVING MATTER

The chemical reactions in protoplasm are under the
control of structure, as we have seen, and their velocity
is decreased and, in the case of the most characteristically
vital reactions, the specific syntheses, is reduced to zero
when protoplasmic structure is destroyed. If we class
this influence of structure as catalysis, the case becomes
one of heterogeneous catalysis, in which the reacting
substances are predominantly substances in aqueous
solution. Organic solvents, however, are also present,
represented chiefly by the lipoids; and, as in all cases of
heterogeneous catalysis, the interfacial relations are
undoubtedly of primary importance. The permanent
structural elements are chiefly protein in composition,
probably associated with lipoid; and this fact favors the
inference that the interfaces between the solid protein
structures of the cell and the adjoining more fluid phases
are the site of the biologically essential reactions, and
especially of the syntheses. The fact that surface-
active substances as a class interfere so strongly with
these reactions favors this interpretation.

The distinctive syntheses of living matter are those
of proteins. These are the syntheses on which specific
growth depends, and growth is the fundamental vital
activity. It thus appears probable that growth is
chiefly a result or expression of synthetic reactions
occurring at such interfaces; and the general suscepti-

217

2i8 PROTOPLASMIC ACTION AND NERVOUS ACTION

bility of protoplasmic processes, including growth, to
electrical influences seems to imply that the electrical
conditions existing at these interfaces are an essential
factor in the control of these reactions.

The general subject of the catalysis of substances
in aqueous solution in heterogeneous systems has thus
an intimate bearing on the fundamental biological
problem which we are considering; and a brief review
of the more relevant facts in this field is essential to the
further analysis of the conditions in living protoplasm.
It must be remembered, however, that the theory of
catalysis is still in many respects incomplete, and that
many reactions in living protoplasm appear to be
determined by other than purely catalytic conditions —
using the word catalysis in the accepted sense of an
acceleration in which the catalyzer undergoes no perma-
nent change in the reaction. Induced reactions probably
play an important part; and there are apparently also
cases where the catalyzer acts by introducing a factor
necessary to those special physical conditions' — e.g.,
flow of electric current through the bioelectric circuit —
which control the reaction.

The general parallel between the conditions deter-
mining chemical reaction- velocities in general, and those
determining the flow of an electric current through a
circuit, has often been dwelt upon.^ The quantity of
material transformed in a reaction, or of current flowing
through the circuit is determined: (i) by the intensity
of a physical condition, called electrical or chemical

^ Cf. Moore, Recent Advances in Physiology and Biochemistry,
pp. 45 ff.; Mellor, Chemical Statics and Dynamics, p. 25; van't Hoff, Vor-
lesungen, I, 172, 178; Bredig, Ergehnisse der Physiol., I (1902), 137.

CATALYSIS AND BIOCHEMICAL PROCESSES 219

"potential," whose expression is a furtherance of the
change in question; and (2) by the resistance to this
change. The general formula C = P/R describes the
general conditions, where C signifies either the rate of
chemical change (under determined conditions of con-
centration, temperature, etc.) or the intensity of the
current flowing through the circuit, P the potential,
signifying a function of "chemical affinity" in the one
case, or the electrical pressure or "voltage" of the
circuit in the other, and R the resistance to either the
chemical change or the flow of current. In the case of
a chemical reaction occurring at an electrode (electroly-
sis), where the quantity of chemical change, e.g., of
copper deposited as metal at the cathode, is proportional
to the quantity of current flowing through the circuit
(Faraday's Law), the factors determining the flow of
current are the same as those determining the rate of
chemical change, and chemical resistance becomes
identical with electrical resistance. In such a case any
condition decreasing the electrical resistance or increasing
the electrical potential increases the velocity of the purely
chemical change at the electrode.

At present it is customary to describe a catalyst as
a substance which in some manner, without itself under-
going permanent alteration, decreases the resistance
to the interaction of other substances in the reaction-
system. On such a definition any substance which
decreases the electrical resistance in a circuit would
catalyze the chemical reactions occurring at the elec-
trodes. Such an effect might not ordinarily be classed
as catalytic; but since our interest is not in defining
the significance to be attached to terms, but in ascertain-

220 PROTOPLASMIC ACTION AND NERVOUS ACTION

ing the physico-chemical conditions under which chemical
reactions actually are accelerated in systems of the kind
under consideration, we must note as especially signifi-
cant the fact that reactions occurring under electrical
influence at surfaces (especially metallic surfaces) may
be influenced in their velocity by the contact of materials
which change locally the electrical state of the surface.
The rusting of iron in water or salt solution is a good
instance of this type of effect; the reaction may be
greatly accelerated by placing another metal, e.g.,
copper or platinum (which itself does not undergo
change) in contact with the iron. The apparently
catalytic effect in this case is due to the formation of
an electrical circuit between the two metals, the iron
becoming anodal and hence freeing Fe ions with increased
rapidity; these can then react to form carbonate or
hydrate with the anions present in the solution. Another
simple and striking demonstration of a '^catalytic"
action of this kind is made by placing in a solution of
KjFeCye (containing a little NaCl to allow a soluble
zinc salt to be formed) two similar strips of metallic zinc,
one of which is marked with a lead pencil or bound with
a small piece of copper or platinum, while the other is
free from such contact. In a few hours a luxuriant
^'growth" of filaments and tubules of zinc ferricyanide
is formed from the first strip, while the second remains
almost unaltered. The carbon, or the noble metal,
acts ''catalytically" in this reaction because it furnishes
a surface of lower solution-tension, which formes the
cathode of the local electric couple; and since these two
areas are in metallic connection and immersed in the
electrolyte solution, a current flows which enables

CATALYSIS AND BIOCHEMICAL PROCESSES 221

Zn ions to enter the solution more rapidly and hence
accelerates the formation of the structure-forming pre-
cipitate of zinc ferricyanide.^

There is reason for believing that the remarkable
chemical activity of living matter, as well as its sus-
ceptibility to electrical influence and to stimulation,
is largely dependent on physical conditions which are
fundamentally of the kind just described. For example,
during stimulation the excited and the unexcited areas
of the reactive protoplasmic surface — the surface of the
irritable cell, neurofibril, or other structure concerned —
are at different electrical potentials; apparently the
current flowing between these two areas produces chem-
ical effects which secondarily determine the propagation
of the state of excitation and hence the distinctively
physiological effect or response. There is a close analogy
here to the case of local circuits in metals immersed in
electrolyte solutions; these circuits also form the con-
dition for the transmission of chemical effects. This
general condition will be considered more fully under
the subject of stimulation; at present it is sufficient to
call special attention to it as probably forming a highly
important factor in the catalytic or quasi-catalytic
action of Hving protoplasm. Here we use the term
"catalytic" simply as a designation for the remarkable
property shown by living protoplasm of enabhng reac-
tions to occur, at a relatively high velocity, which are
absent or inappreciable in dead protoplasm.

The most familiar form of catalysis observed in
living organisms, and the one showing the closest parallels

^ Cf. my two papers on precipitation growths from metals, Biological
Bulletin, XXXIII (191 7), 135, and (with E. N. Johnston) XXXVI (1919),
225.

2 22 PROTOPLASMIC ACTION AND NERVOUS ACTION

with the usual inorganic types of catalysis, is that
dependent on the activity of enzymes. Enzymes are
constant constituents of protoplasm, and their presence
accounts for many characteristic features of its chemical
performance. Enzyme-action, however, is obviously
responsible for only a portion of the metaboHc reactions,
especially the destructive or disintegrative ones, which
are largely hydrolytic. Although certain types of syn-
thesis are accelerated by enzymes under certain condi-
tions (dehydrolytic synthesis of esters, carbohydrates,
and apparently polypeptides), others cannot be thus
accounted for; e.g., photosyn theses, synthesis of fat
from protein or carbohydrate, or other syntheses involv-
ing the expenditure of much energy. Moreover, the
responsiveness of protoplasm to stimulation is not thus
explained, since enzymes show no such instantaneous
and marked acceleration of their action, under electrical
or mechanical influence, as is shown by Hving protoplasm.
As we shall see later, changes in protoplasmic structure
seem to be primarily responsible for the immediate
chemical effects following stimulation.

Enz3anes are simply colloidal catalyzers of complex
and specific chemical constitution. A part of their
catalytic activity presumably depends on their colloidal
state; i.e., a state of subdivision making surface-condi-
tions of preponderant importance in their chemical
behavior. The general conditions of heterogeneous
catalysis thus apply to enzyme action; in addition there
are special conditions referable to the specific stereo-
chemical configuration of the enzyme molecule.

It is well known that finely divided material of various
kinds (material with large surface-extent) is often very

CATALYSIS AND BIOCHEMICAL PROCESSES 223

active in catalyzing chemical reactions; this is especially-
true of certain forms of carbon and of metals like
platinum. The action of platinum is especially well
known; it increases with the state of subdivision, i.e.,
the extent of surface, hence colloidal platinum is a very
effective catalyzer. Other metals have similar proper-
ties, although usually less marked.

Usually in such heterogeneous catalyses the accelera-
tion of reaction-velocity is regarded as a result of in-
creased concentration at the surfaces. Faraday (1839)
suggested this explanation for the action of platinum
in catalyzing the combination of hydrogen and oxygen.
In general, when considering any special case of hetero-
geneous catalysis, three independent processes with
different rates are taken into account: (i) the rate of
diffusion of the dissolved substrate to the active surface;
(2) the rate of adsorption at the surface; and (3) the
rate of chemical combination. The rate of reaction is
Limited by the rate of the slowest of these interdependent
processes. In most cases the reaction- velocity (F) is
regarded as determined by the concentration (C)
attained at the surface and by the specific velocity-
constant (K) of the reaction (i.e., V = KC), since adsorp-
tion is rapid and also diffusion (when the distances are
small). The rate of chemical change is increased (cata-
lytic effect) because the concentration of the reacting
molecules is increased in this part of the system.^

It is evident, however, that other factors frequently if
not usually enter dependent on the special chemical
nature of the reacting compounds. Many inorganic

^ Cf. Hober's Physik. Chemie der Zelle, pp. 702 ff., for an account of
the general conditions of catalysis in heterogeneous systems.

224 PROTOPLASMIC ACTION AND NERVOUS ACTION

catalyses are referable to the formation of intermediate
compounds, and the same is undoubtedly true of many
enz3ane-reactions. The specificity of enzymes and other
facts in their behavior indicate that chemical union
often occurs between the enz3rme and the substrate
molecules, and that it is the combination thus formed
which breaks down rapidly, yielding the products of
hydrolysis and the free enzyme, which then repeats the
chemical cycle of combination and hydrolysis with fresh
molecules of substrate.

Apparently many cases of heterogeneous or contact
catalysis are referable to simple increase of concentration
due to adsorption. But in the case of metals and other
conducting substances the possibility of an additional
factor, the formation of local electrical circuits between
different portions of the active surface, is also to be
considered. In either case the essential condition is
some form of surface-action.

Enzymes are colloidal in their condition, indiffusible,
precipitable, readily adsorbed by indifferent adsorbents,
and, according to Bayhss, their mode of action is also a
surface-action. The clearest proof of this is that emulsin,
lipase, urease, and trypsin exert their action in alcohoHc
media of such a strength that the enzyme is insoluble
and can be filtered off.^ BayHss regards the adsorption
of the substrate on the enzyme phase as the first step
in the process; the chemical reaction then follows. In
some cases the adsorption- compound of enzyme with
substrate is separable; e.g., starch- amylase, fibrin-
pepsin, and trypsin with caseinogen.^ A close union or

^ Cf. Bayliss, Principles of General Physiology, p. 325.
' Bayliss, op. cit., p. 326.

CATALYSIS AND BIOCHEMICAL PROCESSES 225

adhesion of the enzyme to the substrate is characteristic;
and the influence of electrolytes on enzyme-processes
is probably in large part to be referred to their influence
on adsorption.^

In the simple adsorption type of catalysis the acceler-
ating effect depends on the concentration attained at the
interface. Close adhesion of the reacting compound
to the adsorbent surface is important since this impHes
a high concentration at the surface. Hence a corre-
spondence between the molecular configuration of the
adsorbing surface and of the adsorbed compound is
favorable to adsorption as well as to chemical combina-
tion. The importance of such conditions is seen in the
growth of crystals, in which, according to Marc, the
dissolved molecules are abstracted from the mother-
liquid and deposited on the surface of the crystal by a
process identical with adsorption.^ Slow growth is
favorable to the formation of large crystals, because
time is then allowed for the regular orientation of the
surface-molecules thus deposited. Organic growth
apparently also depends on the apposition of newly
formed molecules to the similarly constituted molecules
already laid down in the soKd state as structure; and
this consideration may explain why, in living organisms,
where definiteness of form and of .structural characters
is essential, the rate of growth is slow. Probably no
essential distinction is to be drawn between adsorption
and chemical combination; in adsorption the surface
molecules of the adsorbent are alone concerned because

' Cf. Bayliss, "Adsorption as a Preliminary to Chemical Reaction,"
Proceedings of the Royal Society, B, LXXXIV (191 1), 81.

' Marc, op. cit.

226 PROTOPLASMIC ACTION AND NERVOUS ACTION

of the solidity of the adsorbing phase and the remoteness
of the internal molecules from the sphere of reaction.

SPECIAL FACTORS IN THE CATALYTIC ACTION OF

PROTOPLASM

Since surface-relations are all-important in hetero-
geneous catalysis, all conditions modifying the composi-
tion, electrical polarization, or other characters of the
active surfaces influence the catalytic activity of the
system. Hence surface-active substances as a class
have a marked effect on such catalyses, and their influ-
ence on the chemical activity of Hving matter is un-
doubtedly in large part referable to this effect. A brief
review of the action of these substances on the catalytic
and other properties of heterogeneous systems will
indicate the nature of the factors.

One of the most complete recent studies of the anti-
catalytic action of surface-active substances on enzyme
action is that of Warburg and WieseP on the zymase-
containing ''press- juice" of yeast. All substances of
the anaesthetic class retard the alcohoHc fermentation
caused by this enzyme, although the non-hving enzyme
requires higher concentrations than the hving cell for
the same proportional degree of retardation. In homolo-
gous series the concentration necessary for a given retar-
dation decreases in the usual manner with increase of
molecular weight. The following orders of relative
action were found for different compounds. Alcohols:
methyl < ethyl < propyl <isobutyl; urethanes: ethyl <
propyl <isobutyl; nitriles: aceto<propio<isovalero;
ketones : acetone < methyl-propyl < methyl-phenyl. It

* Warburg and Wiesel, loc. cit.

CATALYSIS AND BIOCHEMICAL PROCESSES 227

was also noted that all of these compounds in sufficient
concentration caused precipitation in the press-juice, and
that the orders of relative precipitating effectiveness and
anticatalytic action were the same; this order is also
that of relative narcotic action. This parallelism
between precipitating action and narcotic action recalls
Claud Bernard's hypothesis that a partial coagulation
of protoplasmic constituents is the essential condition
of narcosis/ With the living cell, however, much lower
concentrations are required to stop fermentation than
with the enzyme solution, so that the parallel between
the inactivation of the structureless enzyme solution and
the inhibition of fermentation in the Hving cell is not
complete. This difference may indicate the importance
of the vital organization as such, or it may depend on the
presence of special compounds (Kpoids) in the living cell.
Warburg and Wiesel found, however, that dried yeast
cells (extracted with ether and acetone) exhibited a
well-marked fermentative action, which was arrested
by narcotic compounds in somewhat high concentrations.
Meyerhof found a closely similar anticatalytic action
of the same compounds in solutions of yeast invertase;^
and in this case also the effect was associated with a
precipitating action; similar observations on oxidase-
containing tissue-extracts have been made by Battelli
and Stern.^ Vernon^ also observed a general inhibitory
action of narcotics on tissue-oxidases ; the effect was

^ Claude Bernard, Leqons sur les Anesthesiques et sur rAsphyxie,
Paris (1875), P- 154.

* Meyerhof, Arch. ges. Physiol., CLVII (1914), 251.

3 Battelli and Stern, Biochem. Zeitschrift, LII (1913), 226.

4 Vernon, Journal of Physiology, XLV (191 2), 197.

2 28 PROTOPLASMIC ACTION AND NERVOUS ACTION

imperfectly reversible and may be regarded as destructive
rather than simply anti-catalytic. Narcotics also check
heterogeneous catalysis in purely inorganic systems;
e.g., the oxidation of oxaHc acid by charcoal (Warburg)^
and the decomposition of H2O2 by colloidal platinum
(Meyerhof),'' and the same order of relative action is
again seen.

The foregoing association of a precipitating action
with an anticatalytic action indicates an alteration of
surface-conditions, but precipitation as such is not a
necessary accompaniment of this action. Many organic
compounds (alcohols) precipitate solutions of proteins
and other colloidal compounds, but usually the con-
centrations required for precipitation far exceed the
anticatalytic concentrations.

Other effects, also dependent on surface-action,
have an intimate bearing on the present problem. Of
special interest is the action of narcotic compounds on
suspensions of lecithin. Changes in the viscosity, gel-
forming properties, and precipitabiUty of lecithin emul-
sions are characteristic. Many lipoid-solvents (alcohols,
etc.) increase the viscosity of these emulsions to a greater
degree than can be accounted for by the increase in the
viscosity of the aqueous phase.^ In somewhat concen-
trated emulsions (10-12 per cent) the addition of ether
even causes gelation, so that the test tube can be inverted
without spilling; this effect is also caused by alcohols
(n-propyl up to capryl), esters (ethyl formate, acetate,

^ Warburg, Arch. ges. Physiol., CLV (1914), 547.

'Meyerhof, Arch. ges. Physiol., CLVII (1914), 280.

3 Handowsky and Wagner, Biochem. Zeitschrift, XXXI (1911), 32;
A. Thomas, Journal of Biological Chemistry^ XXIII (1915), 259.

CATALYSIS AND BIOCHEMICAL PROCESSES 229

propionate, and nitrate), ethyl ether, EtCl, EtBr,
chloretone, paraldehyde, CCI4, hydrocarbons hke benzol,
toluol, and xylol, but not by urethanes and lower
alcohols.^ In a lecithin- containing system such as
protoplasm this influence might act anticatalytically
by slowing diffusion- velocities; any chemical process
whose rate depended on the rate of diffusion would thus
be retarded. Increased hindrance to the movement of
ions would be shown in decreased electrical conductivity.
Loewe^ has in fact found that artificial membranes
impregnated with lecithin exhibit an increased electrical
resistance in the presence of anaesthetics; in a system
Uke hving protoplasm such an effect would retard
chemical reactions dependent on electrochemical con-
ditions. The increase in viscosity is probably to be
referred in part to the formation of adsorption-films at
the surface between the lecithin particles and the water.^
It is possible also that changes in the relative volumes
occupied by the colloidal particles and the aqueous
phase may play some part; presumably the lipoid-soluble
compound concentrates in the lecithin in accordance

... . . . lipoid-solubiHty, , .

with its partition-ratio, — : , , .,. and in so

^ water- solubihty

doing enlarges the particles; this may explain why the

higher alcohols are more effective in causing gelation than

the lower alcohols, which are highly water-soluble.

As the emulsion particles enlarge, their freedom of

movement becomes less, contacts are more frequent,

and coherence to a gel is favored.

^ Cf. my review, The Theory of AncBsthesia, op. cit., p. 362.
2 Loewe, loc. cit. 3 Cf . Ibid.

230 PROTOPLASMIC ACTION AND NERVOUS ACTION

It should be noted that in the case of some other
colloidal organic compounds the formation of gels may
be prevented instead of promoted by the addition
of surface-active substances; this effect was observed
by Schryver^ in the gelation of Na-cholate in the presence
of various surface-active organic compounds. He found
a retardation in the rate of gelation, the effect running
in general parallel with capillary activity and narcotic
action. This phenomenon is analogous to protective
action in colloidal precipitation.

The protection of suspensions of lecithin against
precipitation by electrolytes is also an effect character-
istic of many surface-active compounds. Hober and
Gordon^ found that suspensions containing ether, chloro-
form, chloral, or amyl alcohol were less readily precipi-
tated by alkaH-earth cations (Ca, Ba) than the control
suspensions; i.e., were stabihzed; and they character-
ized this action as ''narcotization of the plasma mem-
brane colloid lecithin." Koch and MacLean^ found
that the stabilizing effect was not uniform with different
anaesthetics; some compounds so act, but others are
indifferent, while still others further precipitation,
especially the lower alcohols and paraldehyde, which are
highly water-soluble. My own observations on the
precipitation of lecithin by CaCla and HCl confirm this
result, but they show that at appropriate concentrations
the great majority of anaesthetics have a stabiHzing
effect. The compounds examined included alcohols

^ Schryver, Proceedings of the Royal Society, B, LXXXVII (1914),
366.

2 Hober and Gordon, Hofmeisters Beitr., V (1904), 432.

3 Koch and McLean, Journal of Pharmacology and Experimental
Therapeutics, II (19 10), 249.

CATALYSIS AND BIOCHEMICAL PROCESSES 231

from n-propyl up, esters, urethanes, hydrocarbons,
chloroform, nitromethane, ethyl ether, chloretone, phenyl
urea, and others.^

In general these effects in lecithin suspensions are
referable to several factors, of which the chief probably
are solution of the compound in the colloidal particles,
formation of adsorption-films which change the electrical
polarization or other physical constants of the particles,
and increase of viscosity. The total effect in most cases
is increase in the physical stabihty exhibited by the
system in the presence of conditions tending to alter
the state of aggregation.

The protective influence exerted by proteins and other
surface-active colloids in the precipitation of metallic
hydrosols by electrolytes is apparently a closely related
phenomenon. Its conditions have been studied care-
fully by Zsigmondy,^ who finds wide variations in the
effectiveness of different compounds; e.g.., gelatine is
highly effective as compared with peptone. He assigns
to each protective colloid a characteristic ^'gold number" :
this number defines the quantity of the colloid required
to prevent the precipitation of a standard gold suspension
by a definite concentration of NaCl. In this case the
protective effect undoubtedly depends on the formation
of adsorption-films, which prevent coalescence or flocking
of particles. Adsorption-films of soap, protein, or
similar substances play an analogous part in the forma-
tion of emulsions, as already pointed out, the suspended
droplets being thus prevented from fusing. In a similar
manner these substances prevent sedimentation in finely

^ Cf. my The Theory of Ancesthesia, p. 361.
* Zsigmondy, Colloids and UUramicroscopy.

2^2 PROTOPLASMIC ACTION AND NERVOUS ACTION

■0

divided suspensions of water-insoluble materials (chalk,
Ca-phosphate, etc.).

In his textbook^ Hober has described various other
instances of protective action and has discussed briefly
the general biological significance of this phenomenon.
In h\dng organisms, where water-insoluble materials
constitute an indispensable element in the formation of all
permanent structure, protective action is of great impor-
tance. Many otherwise insoluble materials are thus
kept in a permanent state of fine dispersion or pseudo-
solution. It is well known that uric acid and cholesterol
are present in serum in concentrations far higher than
correspond to their solubiHty in water; presumably
they are held in suspension as ultra-microscopic particles
by the protective action of the serum proteins. Pauli
and Samec^ have shown that gelatine, serum albumin,
and other proteins may keep large quantities of insoluble
calcium salts (sulphate, phosphate, carbonate) in appar-
ent solution. The silver salts in photographic plates
and the Ca-phosphate in milk are other instances of
insoluble materials kept in fine dispersion by protective
colloids. The suggestion has been made that the forma-
tion of pathological concretions in higher animals
(gallstones, uric acid deposits) is an expression of defi-
ciency in protective colloids. It is probable that in the
formation of normal structures, such as bone, by sepa-
ration of insoluble salts as a finely divided and struc-
turally regular deposit, the protective action of the proto-
plasmic colloids is a necessary factor; presumably in
the absence of this factor the particles would be flocked

' Page 344.

2 Biochem. Z., XVII (1909), 235.

CATALYSIS AND BIOCHEMICAL PROCESSES 233

by the salts present and the uniform and gradual building
up of regular structure would be impossible.

All of the above-described effects depend on the
formation of interfacial films which alter the physical
properties of the surfaces. Apparently also the chemical
or adsorptive and hence catalytic properties of the
surfaces are secondarily altered by the deposition of
such foreign materials; and the foregoing evidence indi-
cates that the an ti catalytic action of surface-active
compounds depends on a surface-concentration of this
kind.

Anticatalytic action is, however, also well known in
homogeneous solutions and even in gases, so that this
form of explanation does not always apply. Numerous
instances of anticatalysis in homogeneous solutions are
cited by Traube in his Theorie der Narkose;^ Bigelow's
observations^ on the oxidation of Na2S03 by oxygen
furnish many striking illustrations; he found that this
reaction was depressed by a large number of organic
substances; for example, a trace of mannite (about
.0014 per cent) decreased the reaction velocity by 50
per cent; benzyl alcohol and aldehyde, isobutyl alcohol
and ethyl alcohol were even more effective in checking
the reaction. It is interesting to note that the alcohols
showed increasing effectiveness with increasing molecular
weight, and that the effect decreased with the number of
hydroxyls in the compound (e.g., monohydroxylic alco-
hols > dihydroxyhc > trihydroxyHc, etc). Aromatic
compounds were more effective than chain compounds.^

^ Arch. ges. Physiol., CLIII (1913), 279.

^Bigelow, Z. physik. Chem., XXVI (1898), 423, and XXVII, 585.
3 Many other similar cases are cited in Traube's paper (the work of
Veley, Titoff, Young, and others).

234 PROTOPLASJVnC ACTION AND NERVOUS ACTION

Anticatalysis in homogeneous solution has the appear-
ance of being a different phenomenon from the forms of
anticatalysis considered above, and its conditions are
obscure. Possibly the nearest analogies are with photo-
catalysis; e.g., a catalytic substance may play a role
analogous to that of a photochemical sensitizer; it is
clear that interference with the action of the latter would
arrest the photocatalytic action. Just as the chemical
effect of Hght is influenced by th'e presence of substances
with special Hght-absorptive properties, as in sensitized
photographic plates, so also catalysis by chemical com-
pounds may be influenced by other compounds having
special chemical relations with the catalyzer. Such
considerations are perhaps vague, but since many of
the conditions controlHng reaction-velocities are imper-
fectly understood, it seems best not to attempt further
definiteness at present.

CHAPTER XI

ELECTRICAL AND OTHER FACTORS IN THE
CATALYTIC ACTION OF PROTOPLASM

Of the conditions, other than temperature and the
presence of catalyzers, influencing reaction-velocities,
light and other forms of radiation and electricity are
the most important. Apparently all chemical reactions
are influenced by radiation of appropriate wave-length,
and the acceleration caused under these conditions is
called "photocatalysis." It differs, however, from the
chemical forms of catalysis in that energy is added to
the reacting system from without; in this respect the
conditions may be compared to those present when an
electric current is passed into a solution from an electrode ;
at the surface of transition between solution and electrode
chemical reactions are induced (electrolysis), the effect
depending directly on the quantity of electricity (i.e.,
number of electrons) transferred between the molecules
of the electrode and those of the solution. Similarly
the chemical effect of light is referred to faciHtation of the
transfer of electrons between molecules. In virtue of
its physical character as electromagnetic oscillation,
light alters the range of movement of the electrons; and
when the periodicities of electron motion and ether-
vibration correspond, this range may be increased to a
degree sufflcient to enable adjacent molecules to interact.
The electrons affected are apparently the valence or
combination electrons of the molecules concerned.
The chemical action of light is thus ultimately to be

235

236 PROTOPLASMIC ACTION AND NERVOUS ACTION

related to its general influence on electrons, an influence
shown in the photoelectric effect.

The phenomena of induced reactions, which are
almost certainly of great importance in cell-metabolism,
are probably also to be afhliated in a general way with
photocatalysis and electrolysis (which may perhaps
be called " electrocatalysis ") 5 but it is impossible here
to do more than direct attention to these possibiH-
ties, the investigation of which is the subject of
physical chemistry rather than of biology. It is
sufficiently evident that all conditions influencing chemi-
cal reaction-velocities are of fundamental biological
interest.

In considering the case of heterogeneous catalysis
(or chemical contact effects) and the influence of anti-
catalyzers, the effect of the latter on contact-potentials
should be noted. These potentials are affected by many
organic substances, especially surface-active compounds;^
the strength of the current in a battery and the rate of
the associated chemical effects may thus be decreased.
For example, in the formation of precipitation-structures
from zinc and Fe under the influence of local circuits,
the presence of alcohols, esters, and other surface-active
compounds of the anaesthetic groups has a well-marked
retarding influence.^ The concentrations required for
pronounced retardation are similar to those effective in
the above-described forms of anticatalytic action, and
the effect may be described as anticatalytic, although
its conditions are probably complex, the influence on

' Cf. the papers of Gouy, Abl, Grumbach, Loeb and Beutner cited
below.

* Unpublished observations of my own in Clark University.

ELECTRICAL AND OTHER FACTORS 237

viscosity and on adsorption entering in addition to that
on contact-potentials.

In a recent review of the facts and theories of contact-
catalysis Bancroft^ cites various instances where electrol-
ysis and electrode-potentials are altered by foreign
substances. Thus the P.D. at which O2 is freed at a
platinum surface is found to be influenced by the electro-
lytes present. With platinum electrodes oxidations
occur more readily at platinized than at smooth surfaces,
apparently because of the ''catalytic" action of the
finely divided platinum. The presence of cyanide and
other compounds reduces the rate of oxidation occurring
at an electrode under a given P.D.; for example, a
neutral solution of Na2S203 is oxidized to tetrathionate
at a platinized anode with a P.D. of 0.44 volts and a
current-density of 3X10"'* amperes per square centime-
ter. If a trace of Hg(CN)2 is added, the anode P.D. for
the same current rises to 0.48 volts. Various salts have
marked influence on the electrochemical processes at
smooth anodes.^ Gouy^ made an extensive study of the
effects of various compounds on the surface-tension
maxima in the capillary electrometer. Usually this
maximum corresponds to a minimal P.D. between the
Hg and the H2 SO4; but the P.D. and the surface-tension
are both changed by the substance added, so that the
position of the maximum is shifted, and this influence
was found to be greatest with highly surface-active
substances. Similar observations were made by Abl

^ Bancroft, Journal of Physical Chemistry, XXI (191 7), 734.

* Cf. Foerster, Elektrochemie wassriger Losungen (19 15) for further
details.

3 Gouy, Ann. de Chim. et de Phys., XXIX (1903), 145; VIII (1906),
291, and IX, 75.

238 PROTOPLASMIC ACTION AND NERVOUS ACTION

on the electromotive force of cadmium-amalgam cells/
In the case of contact-potentials between water and other
dielectrics, Grumbach^ found in general that organic
compounds which lower the surface-tension of water
decrease these potentials. With CH3OH, C2 H5OH, and
C4H9OH the effect increases in the order of increasing
molecular weight; the curves relating potential change
and concentration resemble the corresponding curves of
surface-tension and adsorption. The observations of
Loeb and Beutner^ on the contact-potentials between
organic membranes (apple skin) and electrolyte solutions
containing alcohols also show a decrease of P.D. with
the addition of alcohol. With the first three alcohols
the effect increased was in the order of Ci<C2<C3
(p. 302). Similar results were obtained with solutions of
lecithin in guaiacol. The concentrations required to
produce a decided influence on the potentials were,
however, much higher than the physiologically effective
or narcotizing concentrations.

Traube nevertheless regards the influence of surface-
active substances on the contact-potentials between the
living cell and the medium as an important factor in the
physiological effect, and MacaUum has expressed a
similar view.'* It is doubtful, however, if this can be
generally true, since the bioelectric potentials are not
necessarily decreased in anaesthesia. For example, in
pure sugar solution, which produces in muscle effects

^ Cf. Traube, "Theorie der Narkose," Arch. ges. Physiol., CLIII
(1913), 276; cf. p. 303.

2 Ann. de Chim. el de Phys. (191 1).

3 Biochem. Zeitschrift, LI (1913), 300.

4 A. B. Macallum, Surface Tension and Vital Phenomena, University
of Toronto Studies (1912), No. 8, pp. 68 ff.

ELECTRICAL AND OTHER FACTORS 239

similar to anaesthesia, the bioelectric P.D. is increased.'
The local negativity produced by anaesthetics under
certain conditions, as in AUcock's experiments,^ is
probably to be referred to the permeability-increasing
effect of the compounds in higher concentrations.

The connection betv/een adsorption and contact
catalytic action has been usually regarded as a special
case of the mass-action law. Since any increase in the
concentration of a reacting substance involves a propor-
tional increase in reaction- velocity, and since the concen-
tration of many dissolved substances is increased in the
layer of solution adjoining an interface, it follows that
in any such case there must be increased reaction-
velocity in that region. Hence if the adsorbing
surface is sufficient in area, as with fine subdivision, a
large proportion of the reacting material may be present
in the surface layer at any time, and a considerable
increase in reaction- velocity may result. Direct parallels
between adsorption and catalysis have in fact been
observed in certain cases.^

It is, however, questionable if this influence is in
itself sufficient to account for the effects observed; in
many cases the surface appears to exercise some specific
chemical influence; and certain contact catalyzers,
especially platinum, have an accelerative action which is
far greater than can be accounted for on the ground of
their adsorptive capacity alone. "*

^ Cf. Briinings, Arch. ges. Physiol., XCVIII (1903), 241. Mac-
donald's work also shows an increase of the demarcation potential with
decrease in the external electrolyte content. Cf. p. 304.

2 Allcock, Proceedings of the Royal Society ^ B, LXXVII (1906), p. 267.

3 Cf. Hober's Physik. Chemie d. Zelle (1914), p. 705.

4 Cf. Bancroft's Applied Colloid Chemistry, p. 40.

240 PROTOPLASMIC ACTION AND NERVOUS ACTION

From the physiological point of view the chief case
of specific acceleration by a finely dispersed or colloidal
catalyzer is enzyme action. There seems to be no doubt
that in many cases the enzyme acts through the forma-
tion of intermediate compounds, which break down at
high velocity yielding the decomposition products and the
original enzyme, the latter then recombining with the
substrate molecules to repeat the cycle. The formation
of the enzyme-substrate combination presupposes inti-
mate contact between the molecules of enzyme and
substrate; hence a correspondence in size and pattern
between the intercombining molecules (or parts of
molecules; e.g., zymophore groups) is required, of the
kind indicated by the ''lock and key" comparison of
Emil Fischer. That relations of this kind play a part
in adsorption is indicated by Marc's observation that a
crystalline adsorbent like BaS04 exhibits preferential
adsorption for compounds crystallizing in the same
system.^ Apparently this is a simple instance of specific
adsorption. Close contact of this kind gives the occasion
for chemical transformations (hydrolysis, etc.) which
otherwise would not occur, or at much lower velocity.
According to Bayliss^ a simple reversible adsorption
often precedes a more intimate ''chemical^' union
between adsorbent and adsorbed substance. In specific
catalyses, like those induced by enzymes, factors of
a similar kind probably enter; the union is specific
because of the similarity of molecular configuration
between enzyme and substrate, and the chemical effect
follows because the enzyme-substrate combination is

^ Cf. Marc, loc. cit.

* Bayliss, Proceedings of the Royal Society, B, LXXXIV (191 1), p. 269.

ELECTRICAL AND OTHER FACTORS 241

unstable and breaks down in the manner already
indicated.

Catalytic effects due to simple increase in concentra-
tion are accordingly to be distinguished from those which
depend on the appearance of special relations of some
kind between the molecules of the substrate and of the
catalyzer. In the latter case the formation of combina-
tions differing from the substrate in reactivity, e.g.,
velocity of hydrolysis or oxidation, becomes possible;
and when these temporary combinations break down,
setting free the catalyzer and enabling the latter to
repeat the combination, the total effect is the same as if
chemical change in the substrate alone were accelerated.
The general theory of intermediate compound formation
would thus apply to any specific adsorption-catalysis.
The adsorbent would correspond to the enzyme, and the
theory of such catalysis need not differ in principle
from that of catalysis in homogeneous solutions; e.g.,
H-ion catalysis. At present, however, there appears to
be no completely satisfactory theory of this type of
catalysis.

With regard to other conditions which may enter in
cases of heterogeneous catalysis, it has been suggested
that the increased reaction-velocities shown by many
substances (sugars, etc.) in protoplasm are phenomena
of the same kind as the differences of velocity shown by a
given reaction in different solvents, as observed by
Menschutkin and others.' Bredig compares a particle
of a solid catalytic agent with a drop of liquid, and regards
the formation of an adsorption-film as equivalent to

'Menschutkin, Z. physik. Chem., XI (1887), 611; von Halban,
ibid.y LXXXII (1913), 325. Cf. Hober, op. cit., p. 704.

242 PROTOPLASMIC ACTION AND NERVOUS ACTION

solution;^ in this case different reaction-products might
be obtained from the same reaction-mixture under the
influence of different catalytic agents, differences of
adsorption having the same effect as differences of
solubihty. Selective adsorption and selective catalysis
would thus be referred to the same conditions, and both
referred ultimately to the same conditions as selective
solubility. At present there is an increasing tendency
to regard both solution and adsorption as special cases
of chemical combination. But although protoplasm
contains solvents in which it is conceivable that certain
reactions may proceed with increased velocities, it does
not seem probable that such considerations can explain
the high velocities of the biologically more important
reactions, such as the oxidation of sugar. Many facts,
especially the phenomena of irritability, show that the
conditions determining the increase of reaction-velocities
in protoplasm are of a special kind and that the factor
of organized structure is all-important. It is not
simply a case of transferring a substance from a solvent
in which its reaction-velocity is low to one in which it
is high.

It has been pointed out by various authors that when
adsorption is highly selective, displacements of equi-
librium may occur; thus an adsorbent may change the
H-ion concentration of a solution. Such effects may
be attributed to the selective adsorption of ions (Freund-
lich) ;^ but, as we have seen, the distinction between the
adsorption of a substance and its chemical combination
with the surface molecules of the adsorbent cannot be

^ Cf. Bredig, Ergehnisse der Physiol., I (1902), 211.
' Cf. Freundlich, Kapillarchemie.

ELECTRICAL AND OTHER FACTORS 243

sharply drawn. If, in fact, the two processes are
indistinguishable, the case becomes essentially one of
alteration of equilibrium following the introduction of an
additional reagent into a reaction-mixture. Bancroft
cites instances where the same compound undergoes
different reactions under otherwise similar conditions
according to the nature of the contact-agent present;^
thus with colloidal nickel as catalyzer, alcohol forms
acetaldehyde and hydrogen, while with colloidal silica
or alumina it forms ethylene and water; and he refers
this difference in the catalytic action to the differences
in the selective adsorptive action of the two substances,
nickel adsorbing hydrogen and alumina water. The rate
and character of the reaction undergone by a given
reaction-mixture may thus vary with the character of
the catalyzer, according to the latter's special adsorbent
properties.

It has been mentioned that simple adsorption is
insufficient to account for the great activity of certain
contact catalyzers. Taylor^ points out that charcoal
adsorbs carbon monoxide and oxygen but does not
catalyze the reaction; but in the case of ethylene and
oxygen it both adsorbs and catalyzes; hence as catalyzer,
it differentiates between carbon monoxide and ethylene,
although adsorbing both. Metallic oxides, on the
contrary, catalyze the oxidation of carbon monoxide.
It would appear, therefore, that specific, presumably
chemical, relationships enter even in such simple cases.

^Bancroft, Journal of Physical Chemistry, XXI (191 7), 573;
Applied Colloid Chemistry, pp. 40 £f.

* Hugh S. Taylor, Trans. Amer. Electrochem. Soc, XXXVI (19 19),
150.

244 PROTOPLASMIC ACTION AND NERVOUS ACTION

Facts like these (the so-called ''preferential catalysis")
may be regarded as e\ddence of conditions which in
their higher developments in living organisms appear
as the highly selective specificity of many enzymes.

The recent work of Langmuir and Harkins^ has
shown that molecules assume definite orientations at the
surfaces at which they are adsorbed. This orientation is
undoubtedly a factor in the chemical effect produced;
reactive groups may thus be brought into a position in
which they more readily come into contact with other
molecules (or the reactive groups of other molecules),
and in this manner reaction is furthered. Cases of
preferential catalysis may thus be explained. The
catalytic effectiveness of phase-boundaries exhibits itself
in many remarkable ways. Taylor and Langmuir^
cite Faraday's observation that a perfect crystal of
sodium sulphate does not effloresce until its surface is
scratched or broken, when the efflorescence spreads from
the injured part over the rest of the crystal. Other
similar examples are well known; apparently the
reactivity of molecules is altered by the adjacent
molecules of reaction-product: the use of ''catalyst-
promoters" in chemical processes illustrates the same
phenomenon. Enhanced reactivity at interfaces is in
fact a very general phenomenon; and in Hving matter,
with its polyphasic constitution, the conditions are
exceptionally favorable for this type of influence.

^ Langmuir, loc. cit.; Harkins, loc. cit.

2 H. S. Taylor, "Catalysis and Catalytic Agents in Chemical Pro-
cesses," Jour. Franklin Inst., CXCIV (1922), i; Langmuir, "Chemical
Reactions at Surfaces," Trans. Faraday Soc, XVII (1921), Part III
(September); reprinted in Gen. Elec. Rev., XXV (1922), 445.

ELECTRICAL AND OTHER FACTORS 245

It is probably significant that the substances which
effect the greatest variety of contact catalyses, carbon
and the metals, especially platinum, belong in the class
of metallic conductors/ In such conductors, according
to the electron theory, there is ready transfer of electrons
from atom to atom, hence their electrical conductivity,
and other properties correlated with this peculiarity
(optical, etc.). It might be expected that such sub-
stances would also faciHtate the transfer of electrons to
or from molecules with which they are in contact, and
thus furnish the conditions necessary for chemical
reaction. In other words, factors characteristic of the
metallic state may enter in contact-catalysis; for
example, the formation of local electrical circuits between
different parts of the metallic surface, or oscillation
phenomena of a frequency corresponding with that of
the combination-electrons of the interacting substances
(resonator eff'ects). An example of the former type of
influence was seen in the simple experiment described
above, in which the contact of a nobler metal or carbon
accelerates or "catalyzes" the formation of ferricyanide
filaments from zinc. Combinations of two contact
catalyzers seem often to be more effective than either
one alone; thus Shenstone found that ''platinized
charcoal" was extremely effective in oxidizing alcohol,
''converting spirits of wine into vinegar in a few hours,"
and other cases of a similar kind are described by Bancroft
in a recent review.^ Presumably the two components
differ in their potential-difference against the medium,

^ Contrast, e.g., the lack of catalytic power in colloidal silica (cf.
Taylor, Trans. Amer. Electrochem. Soc, op. cit., p. 150).

2 Journal of Physical Chemistry, XXI (1917), 644; cf. pp. 667 fif.

246 PROTOPLASMIC ACTION AND NERVOUS ACTION

and thus local circuits arise which have chemical effects.
Rideal and Taylor also describe cases where metallic
couples have a greater catalytic effect than either metal
singly.' The readiness with wliich the metalKc phase
conducts electricity enables any local inequahties in the
metal, or in the nature or concentration of materials (e.g.,
salts and oxidizable and reducible compounds) present
in the solution, to give rise to electric currents between
different parts of the surface. These local currents may
secondarily influence chemical reactions, as in other cases
of electrolysis at metalHc electrodes. Effects of the
reverse or anticatalytic kind may result when the metal-
Hc surface is altered in such a way as to lessen the local
potential-differences or increase the resistance of the local
circuits; strongly adsorbed organic compounds may
have both of these effects, as already pointed out.

Electrochemical oxidations at platinum anodes are
subject to variations (partly mentioned above) which
are possibly referable to the existence of local circuits;
these exercise their own influence independently of the
E.M.F. appHed from without, and give rise to inter-
ference and summation phenomena of various kinds.
The possibiHty that the formation of local circuits plays
a part in the normal catalytic acti\'ity of platinum and
other metals in liquid systems seems to have been
insufficiently considered by chemists. The essential
conditions of such catalysis are perhaps best illustrated
by the decomposition of hydrogen peroxide, which is
effected by a large number of finely divided metals of
which platinum is especially active. The striking

^ Rideal and Taylor, Catalysis in Theory and Practice (Macmillan,
1919), pp. 130, 160.

ELECTRICAL AND OTHER FACTORS 247

parallels between this action and the similar action of
organic fluids and cell-extracts have long been known.
The decomposition of H2O2 by living tissues is now
regarded as due to a special enzyme, catalase. The
relation of this catalytic action of tissue-extracts to
enzyme action and fermentation was early recognized
by Schonbein, who characterized the splitting of H2O2 as
a model or prototype of all fermentative processes
(Urbild alter Gdhnmgen)^ A study of the conditions
under which it occurs in the presence of metals may thus
throw some light on the general nature of catalytic effects
in heterogeneous systems and especially in Hving proto-
plasm.

Among other metals mercury shows great activity in
decomposing H2O2. The most striking feature of the
catalytic decomposition of H2O2 in contact with a mercury
surface is that under certain conditions the process
exhibits a definite regular rhythm, closely resembhng
physiological rhythms like that of the heart beat in its
frequency, in its dependence on temperature (Qio = about
2) , and in the influence exerted upon it by chemical and
electrical conditions. This phenomenon has lately been
the subject of much investigation, specially by Bredig
and his students, and its detailed conditions have been
studied most closely by Antropoff.^ The chief result
of this study has been to show that the catalytic rhythm
is dependent on the alternate formation and dissolution
of a surface-film of oxidation-product ('^peroxidate")
formed by interaction between the mercury and the
peroxide. Since evidence from various sides indicates

^ Schoenbein, /. prakt. Chem., LXXXIX (1863), 335.
* Antropoff, Z. physik. Chem., LXII (1907), 513.

248 PROTOPLASMIC ACTION AND NERVOUS ACTION

that many vital processes (stimulation, cell-division)
are also dependent on changes undergone by surface-
films — those forming the plasma membranes and other
protoplasmic partitions — this feature of the H2O2 ca-
talysis suggests that the foregoing physiological parallel
may be more than a superficial one, and that the similari-
ties depend on a fundamental identity in the conditions
controlHng the course of the reactions in the two systems.
It is necessary therefore to examine more closely into
the nature of the conditions controUing the activity of
the Hg-H202 system.

The rhythm is best shown in 10 per cent aqueous
solutions of H2O2. The film which forms over the surface
of pure mercury in this solution is gold-brown in color,
and the conditions for the rhythmical reaction are best
when the solution is slightly on the alkaline side of
neutrahty. The evolution of oxygen occurs during the
breaking down of the film; when the film covers the
entire surface of the mercury the reaction ceases. If
the solution is slightly acid, the film is stabilized and
no evolution of gas occurs; at the appropriate degree of
alkaHnity it is alternately formed and broken down with
a regular rhythm of about ten to fifteen per minute
(at 18°) . The film shows great sensitivity to the presence
of foreign substances, including salts and surface-active
organic compounds (such as ether and olive oil); the
latter abolish the rhythm.^ It is also highly susceptible
to changes in electrical polarization. Many other
striking parallels with physiological rhythms are
described in the article by Bredig and Wilke.^

^ Cf. Bredig and Weinmayr, Z. physik. Chem., XLII (1903), 601.
^ Biochem. Zeitschrift, XI (1908), 67.

ELECTRICAL AND OTHER FACTORS 249

The fact indicating most clearly the essential nature
of the conditions governing the course of the reaction is
that the rhythm of decomposition is associated with a
parallel rhythm of electrical potential. The mercury
in contact with the H^O^ solution is always found
cathodal with reference to the calomel electrode, but
during the active phase, while O2 is being freed, it is less
so than during rest; i.e., the metal becomes anodal
relatively to the resting condition. According to
Antropoff's measurements, the inactive mercury is
about 0.12 volt more cathodal (i.e., nobler) than the
active. This difference resembles that between passive
and active iron in nitric acid solution, the inactive
mercury corresponding to the passive iron. A change
of surface-tension accompanies the change of potential,
the rounded convex surface of the mercury becoming
flatter (from decreased surface-tension) as the film^
forms; this behavior is in accordance with the Lippmann-
Helmholtz rule of electrocapillarity, according to which
the surface-tension decreases with increase of the
potential-difference across the surface. Graphic registra-
tion of the curve of potential change shows that its
course runs closely parallel with the curve of oxygen
evolution as measured by a manometer.

Close observation shows that the gas is evolved only
during those times when part of the mercury is film-
covered and part bare; further, that the evolution of gas
occurs chiefly near the boundary between the bright
and the film-covered surfaces. When a regular rhythm
is established, the reaction during each cycle is observed
to pass rapidly over the surface of the mercury in a
wavelike fashion. At the beginning of a cycle, when

250 PROTOPLASMIC ACTION AND NERVOUS ACTION

the whole mercury surface is fihn-covered and inactive
and is flattening as a result of the lowering of surface-
tension, a rupture of the film appears, usually at the
margin of the drop, disclosing the bright metallic surface
beneath. Instantly an effervescence starts at the edges
of this fissure and then sweeps over the whole surface of
the metal; a new film is then formed and the cycle is
repeated. In an inactive drop, an artificial mechanical
rupture will often initiate a reaction which similarly
spreads rapidly over the whole surface.

Bredig found that a stationary film formed over an
inactive surface of mercury is quickly dissolved by render-
ing the metal cathode;^ in this process of dissolution
oxygen is freed. The essential condition of the transmis-
sion thus becomes clear. When a film-covered and a
bright area of the mercury surface adjoin each other,
e.g., after a local rupture of the film, a local electrical
circuit is formed between the two, the film-covered
area being the cathode of the local circuit. The current
of this circuit has the effect of dissolving the film, by
cathodic reduction, for a certain distance (estimated at
1-3 milKmeters) from the boundary, oxygen being freed
in the process; and by repetition of this effect at each
new boundary as soon as it is formed the effect spreads
rapidly over the whole surface. The Hg-HjOz pulsating
catalysis thus in reality represents an intermittent
electrolysis of H2O2 under the influence of the temporary
local circuits formed during the alteration or removal
of the surface-film.

Rhythmical chemical processes at the surfaces of
metals immersed in electrolyte solutions containing

' Of. Bredig and Wilke, op. cit., p. 69.

ELECTRICAL AND OTHER FACTORS 251

compounds which interact with the metal are not
infrequent;^ e.g., when the metal is undergoing solution
in a strongly oxidizing acid Hke HNO3. Iron in particular
often illustrates this phenomenon with great beauty and
regularity; in this case the essential condition- of the
rhythm is an alternation between active and passive
states, due, as in the mercury catalysis, to the alternate
formation and dissolution of a protective surface-film of
oxidation-product. All of these inorganic rhythms are
highly susceptible to variations in external conditions,
and especially to electrical influences.

The rhythmical processes so frequent in living
organisms (rhythms of cilia, muscle, nerve cells, vacuoles,
and cell-division) show many close parallels with these
inorganic "surface-reaction" rhythms. As in metals,
they are associated with rhythmical variations of electri-
cal potential and with rhythms of chemical or metaboHc
alteration and surface- change (clearly demonstrable, e.g.,
in cell-division), and are similarly susceptible to changes
in the surrounding conditions (temperature, H-ion
concentration, presence of salts and surface-active
compounds, electrical polarization, etc.) . These parallels
imply a similarity in the essential determining conditions
in the living and the non-living systems. Since rhythmi-
cal catalysis in metals is dependent on the polyphasic
character of the system — this being the condition which
makes possible rapid local variations of potential,
resulting from changes in the composition and structure
of surface-films — the hypothesis that the organic rhythms
are similarly conditioned naturally suggests itself,
particularly when the film-pervaded or emulsion-like

^ For earlier observations cf. Bredig and Wienmayr, loc. cit.

252 PROTOPLASMIC ACTION AND NERVOUS ACTION

structure of living matter and the various facts showing
the dependence of stimulation on membrane processes
are taken into consideration. The general susceptibility
of living matter to electrical influence suggests that in
protoplasm there may be a similar dependence of the
chemical reactions upon processes of electrolysis occurring
at the boundaries between the protoplasmic phases.
This general interpretation is also consistent with the
readiness and rapidity with which chemical influence is
transmitted from region to region in Hving matter;
the many close resemblances between such transmissions
and the transmission of the waves of electro-chemical
alteration over the surface of mercury or passive iron will
be considered later in detail. In these inorganic systems
the chemical reactions are directly determined by the
potential-differences existing between different portions
of the metalhc surface; these potential-differences arise
as the result of local alterations of the surface-films, and
the local circuits thus arising effect the chemical change
by electrolysis. Similarly in living matter the waves of
chemical and physiological alteration accompanying the
transmission of stimulation (i.e., excitation-waves, nerve-
impulses, etc.) are always associated with waves of
electromotor variation. Bernstein first showed for
motor nerves (in 1866) that the physiological effect and
the bioelectric variation have the same velocity of propa-
gation;^ and all of the more recent evidence confirms
the view that the electric variation is the essential
component of the transmitted process.

Recently I have discussed in some detail the parallels
between the transmission of chemical effects in systems

Cf. Bernstein's Elektrobiologie, chap, iii, for references.

ELECTRICAL AND OTHER FACTORS 253

consisting of metals immersed in electrolyte solutions
and the transmission of physiological influence in living
protoplasm.^ In all such phenomena in metals the
essential condition is the presence of a thin film of electro-
chemically alterable material formed or deposited at
the interface between the metal and the electrolyte
solution. Local circuits between adjoining regions of the
film-covered surface, differing in composition or physical
condition in such a manner as to give rise to an E.M.F.
sufficient for electrolysis, are in all cases the essential
factor. By the action of these local currents the film
is locally altered or removed or rendered permeable

,^^^ — -^ J ^ _ ^ ^

^ • ^mmw^ '' :wMWi^'m^Mm"hk^^^ -^ :* ^ ^

^; ■."■.■■■ •■■■ ■■■ \ <r ^'' ,=^-- ^-^

active passiue-

Fig. 3. — Indicating the conditions of the local circuit at the boundary between the
active and the passive areas of an iron wire in nitric acid; the direction of the current
(positive stream) is indicated by the arrows, the active region (shaded) being anodal, the
passive cathodal. The local intensity of the current in the passive region (and hence the
reducing or activating effectiveness) decreases in the order A<B<C; beyond a certain
distance from the boundary, e.g., XY , it will be insufficient to activate.

over a certain area (usually cathodal, e.g., XY , Fig. 3),
adjoining the boundary between the two regions, and
the similar circuit which is then formed at the boundary
between the newly altered area and the unaltered area
beyond repeats the effect; hence the alteration auto-
matically spreads over the whole surface. A wave of
such transmission is necessarily associated with a wave
of electromotor variation.

A brief account of the phenomena of activation and
transmission in passive iron will indicate more clearly

^ American Journal of Physiology, XLI (191 6), 126; Science, XL VIII
(1918), 51; L (1919), 259, 416; Journal of Physical Chemistry, XXIV
(1920), 165; Jour. Gen. Physiol., Ill (1920), 107, 129.

254 PROTOPLASMIC ACTION AND NERVOUS ACTION

the general nature of such processes.' Passivity is
readily induced in an iron wire by immersion in strong
nitric acid (sp. gr. 1.42); the metal then remains un-
altered or chemically inactive when transferred to weak
acid (sp. gr. 1.2), unless it is artificially "activated."
Activation may be induced by various means, chemical,
mechanical, and electrical. The following simple and
readily performed experiments will bring out clearly
the chief resemblances between the processes of activation
and transmission in the metallic systems and in Kving
protoplasm.

When the passive iron wire is immersed in a dish of
nitric acid of about 60 volumes per cent concentration
(i.e., of commercial HNO3 of sp. gr. 1.42), no change
occurs, and the surface of the metal remains bright and
unaltered. If, however, it is then touched with a piece
of ordinary ''active" iron, or with a base metal Uke
zinc, a local reaction, accompanied by effervescence and
a darkening of the bright metaUic surface, is at once
initiated and sweeps rapidly over the whole wire from
end to end. In acid of the foregoing concentration, the
local reaction ceases in one or two seconds (at 20°), and
the metal reverts automatically to the passive state.
Immediately after this repassivation it is resistant to
activation and transmits the reaction imperfectly; on
standing, transmissi\dty gradually returns and within
a minute is usually again complete; the metal can then
be activated as before and the same phenomenon is
repeated. The passive wire may be activated mechani-

^ For a general review of the phenomena of passivity in metals
cf. Bennett and Burnham, Journal oj Physical Cheviistry, XXI (1917),
107.

ELECTRICAL AND OTHER FACTORS 255

cally by jarring or bending or by scraping with a piece
of glass; summation effects are a conspicuous future of
this form of activation; a single scrape or blow, or a suc-
cession of these at infrequent intervals, being usually
ineffective, while several scrapes in rapid succession cause
typical activation. Chemical activation may be shown
by the appucation of a reducing agent Uke sugar. The
same kind of effect is produced in the wire, whatever the
method of activation, the local change simply initiating
a propagated effect whose nature and extent depend
on the special conditions existing in the metal-electrolyte
system. There is here an evident analogy with explo-
sions or other kinds of ^'trigger effects.'' Hence the
system, when in a fully transmissive state, behaves in
the " all-or-none " manner.

In reality electrical activation is illustrated in all
these cases, even when the local alteration initiating the
reaction is mechanical or chemical; i.e., the electrical
factor is the essential one in the transmission of the
effect. The special conditions of electrical activation
are, however, best shown by a somewhat different kind
of experimental arrangement. Two passive wires, placed
parallel one or two centimeters apart, are immersed in
a vessel containing dilute HNO3 and are connected by
wires and an open key to a battery (e.g., of about 2
volts E.M.F.). When the key is closed, the cathodal
wire (that connected with the negative pole or zinc of
the battery) is at once activated, while the anodal wire
remains unchanged. This experiment shows that activa-
tion is a polar effect and dependent on cathodal reduction.
Activation also requires a certain minimal E.M.F. in the
battery, usually exceeding one volt, and a certain minimal

256 PROTOPLASMIC ACTION AND NERVOUS ACTION

duration of flow of the current. Electrical summation-
effects similar to those of mechanical activation can also
be demonstrated under appropriate conditions.^

Another influence of the current is especially interest-
ing from its resemblance to the physiological phenomenon
of electro tonus; this consists in a modification of the
susceptibiHty of the wire to mechanical or other activa-
tion. During the flow of the current the automatic
return of passivity in the active (cathodal) wire is
delayed, or with sufficient strength of current prevented,
and the anodal wire becomes more resistant to mechani-
cal or other activation. If with two passive wires in a
circuit, as above, a constant current too weak to cause
activation under these conditions (e.g., the current from
one Edison cell) be passed, the cathodal wire, although
remaining passive, is rendered temporarily more sus-
ceptible than before to activation by other means,
e.g., mechanical treatment, while the anodal wire
becomes less susceptible.^

All of the foregoing phenomena have their parallels in
the behavior of Hving irritable tissues under the influence
of the electric current; the corresponding physiological
phenomena are summation, polar stimulation, chronaxie,
enhancement of irritabihty near cathode, and its decrease
near anode (''electro tonus").

Phenomena of a similar kind are seen in the mercury-
peroxide system; here also the inactive mercury may
be activated by making it the cathode in a circuit, or

^ E.g., by using the brief contact of a copper wire as an activating
agent. The local current thus produced may be too brief for activation
by a single contact, while several contacts in close succession will pro-
duce the effect.

" For a somewhat fuller description cf. Jour. Gen. Physiol.^ Ill
(1920), 136.

ELECTRICAL AND OTHER FACTORS 257

the rhythm of an automatic pulsation may be altered.
During the flow of the current the catalytic effect at
the cathode is heightened, while at the anode it is
decreased or the rhythm may be abolished/ The paral-
lelism betwxen these effects and those produced by the
constant current on the action of rhythmical tissues
like the heart is evident.

A more detailed account of the phenomena in film-
covered metallic systems of this kind is not possible
within the limits of space, but attention should be called
to another interesting feature of the electrical initiation
of these reactions. A pecuHarity of the electrical
activation of passive iron is that it is not readily produced
by currents which rise slowly from a minimal strength to
a strength sufficient to activate with sudden closure.^ In
other words, the rate of change of the activating current
is an essential factor in the effect produced. This is a
well-known and highly characteristic feature in the
response of living tissues to electrical activation. Each
tissue has its characteristic time-factor of electrical excita-
tion or so-called "chronaxie," and this is closely related
to the rate of change required of a stimulating current
(cf. p. 288). The time-relations of the activating current
in the metalHc system resemble those in Hving tissues
in the further respect that in the case of alternating
currents the relation between the intensity required
for activation and the rate of alternation follows the
same law, as shown by Bredig and Kerb for the mercury
H2O2 system;^ that is, there is an inverse relation be-

^ Bredig and Wilke, Biochem. Zeitschrift, XI (1908), 67.
^ Science, XLVIII (19 18), 57.

3 Bredig and Kerb, Verh. naturhistorisch-med. Vereins zu Heidel-
berg, X (1909), N.F., 23.

258 PROTOPLASMIC ACTION AND NERVOUS ACTION

tween the intensity required to activate and the root of
the number of alterations (i/i/n = const.). This relation
has been shown by Nernst^ and others to be generally
characteristic of electrical excitation in living tissues;
it indicates that a current of a given intensity must flow
for a certain minimal time in one direction through the
irritable system in order to cause activation. I.e., the
change in the electrical polarization of the surface con-
cerned in activation must last for more than a certain
critical time, presumably the time necessary to produce
a certain critical degree of chemical change. The
physical conditions of response to electrical influence
thus appear to be of the same kind in the living system
and in the inorganic model.

^ Nernst, Arch. ges. Physiol., CXII (1908), 275; cf. chap. xii.

CHAPTER XII

STIMULATION AND TRANSMISSION OF EXCITATION

IN PROTOPLASM

Responsiveness to stimulation is a universal character-
istic of living matter. Typically the reaction to a
stimulus involves a performance of work (i.e., trans-
formation of energy) which has no definable proportion
to the work done by the stimulating agent upon the
living system. The stimulus usually acts locally, yet
the whole living system — cell, tissue, or even entire
organism — may be thrown into activity. Transmission
of physiological influence from the immediate site of
stimulation to other regions of the living system is
thus a constant feature of stimulation. Hence the
subject of the essential conditions determining this
transmission is one of fundamental biological interest;
evidently the living system can react as a whole, i.e., in
a unified or correlated manner, only in so far as the
physiological processes in any single region occur in
correlation with those in other regions. This trans-
missive property of protoplasm is the primary integrative
factor in organisms; its highest development has been
attained in the nervous system of higher animals.^

In general, local variations in protoplasmic activity,
implying variations in the rate or character of the
underlying chemical or metabolic processes, influence
other processes occurring at a distance from the active

^ Cf. my review of the subject of protoplasmic transmission in
Physiological Reviews, II (1922), i.

259

26o PROTOPLASMIC ACTION AND NERVOUS ACTION

region. The physical constitution of the living substance
is evidently of such a nature as to permit rapid trans-
mission of chemical influence to a distance; in other
words, some form of '^ chemical distance-action" is a
constant feature of protoplasmic action. It is natural
to connect this feature with the essential or fundamental
features of the physical structure of protoplasm. We
have already seen that this structure is polyphasic and
film-pervaded, or emulsion-like. It is therefore highly
interesting to note that the inorganic transmissive
processes just considered, which bear such a striking
resemblance to the transmissive processes of protoplasm,
are in fact determined by chemical and structural
alterations in thin surface-films, and that these alterations
occur under the influence of local electric circuits. In
such a system as passive iron in nitric acid the chemically
reactive material whose alteration determines the
transmission is spread out in a thin layer or film at an
interface (metal-electrolyte) which is the seat of a
potential difference. The surface of contact of this
material with the adjacent layer of electrolyte solution
is a large one, relatively to the total mass of reacting
substance. This arrangement makes for a rapidly
acting and sensitive type of reaction-system, since the
removal or alteration of a very small quantity of material
may, by altering electromotor conditions at the surface,
form the condition for a spread of chemical effect,
electrically conditioned, which may be very extensive
and rapid.^ Transmission depends on the instantaneous

^ It may be pointed out here that the importance of extremely
small quantities of certain special substances, e.g., vitamines in animals,
is probably a correlative of the control of chemical reactions in protoplasm

STIMULATION AND TRANSMISSION 261

passage of an electric current through the circuit con-
stituted by the two chemically or structurally different
portions of this thin interfacial film, together with the
electrically conducting phases (in this case metal and
nitric acid) between which it is interposed. In living
protoplasm, with its film-partitioned constitution, it
seems probable that the structural arrangement or
disposition of the chemically reactive material which
determines the response to stimulation is of a similar
kind; i.e., that this material is disposed in the form of a
thin film between two electrically conducting phases,
a type of arrangement allowing transmissions to occur
under conditions of essentially the same physical kind
as in the foregoing inorganic type of system.

It has already been pointed out that stimulation
processes cannot be considered separately from the
processes of transmission or conduction. In general the
effects of local alteration in protoplasm tend to spread;
i.e., to produce chemical and physiological effects in
other regions than those immediately acted upon by the
stimulating agent. In some cases this spread is limited
in extent; but in others, especially nerve, there appears
to be no limit to the distance through which the change
of activity may be transmitted. Hence the total effect
of any stimulation has no fixed relation, quantitative or
qualitative, to the direct physical effect produced by
the stimulus at its point of application. In many

by film-structure. When material is in a film, small quantities may
determine large chemical effects, because under these conditions what
is important is not so much the quantity of material as the area which
it covers. Surface relations rather than mass or volume relations then
become the controlling factor.

262 PROTOPLASMIC ACTION AND NERVOUS ACTION

irritable systems with highly developed transmissive
properties, e.g., the nerve fibers and muscle cells of
higher organisms, the character and intensity of the
response are quite independent of those of the stimulus,
provided the latter attains the threshold value. A full
response, involving the whole irritable element, results
from either a ''weak" or a "strong" stimulus; this is
the ''all or none" type of behavior, which is found also
in many physical systems in unstable equilibrium, and
also in explosive systems or others in which chemical
change is rapidly transmitted; e.g., the passive iron
system. In all such cases there is a "release" of stored
energy, and the work performed by the releasing agent
has no definite relation to the energy transformed in
the resulting process.'

The phenomena of stimulation in living organisms
are so various that one hesitates to regard them all as
determined by conditions of the same physico-chemical
kind. Nevertheless, it is a striking fact that whatever
the special peculiarities of the organic activity or response
in different living systems may be, the conditions of
initiation and control are remarkably uniform. The
universal susceptibility to the electric current, to mechani-
cal disturbance, and to certain kinds of chemical influ-
ence, especially the influence of inorganic salts and the
lipoid-solvent or surface-active group of organic com-
pounds, indicates that the fundamental structural and
chemical conditions underlying the response to stimula-
tion are the same in all forms of protoplasm. It is

* The typical case is one of "trigger action," which is a characteristic
feature of all modes of organic response (cf. the interesting discussion of
Lotka: "Natural Selection as a Physical Principle," Proceedings of the
National Academy of Science, VIII [1922], 151).

STIMULATION AND TRANSMISSION 263

especially to be noted that the two of the most general
features of stimulation-processes, viz., the susceptibility
to the electric current, and the reversible modification or
suppression of irritability by the surface-active groups
of compounds (anaesthesia or narcosis), indicate definitely
a dependence of protoplasmic activity on the polyphasic
structure of the system. The inference from such facts
is that the chemical reactions of protoplasm are controlled
by the peculiar conditions resident at the protoplasmic
interfaces or phase-boundaries; and the resemblance
between the conditions of activity of irritable proto-
plasmic systems and of the inorganic models just
described confirms this inference.

Some of the more general features of the phenomena
of stimulation in living organisms have already been
discussed briefly. Since continued life depends on a
regulated interchange of material and energy with the
environment, it is to be assumed that all fundamental
vital activities are capable of varying in correlation
with, or "in response to," environmental change; the
character and rate of the interaction of the living system
with its environment are thus controlled. Normally
the responses of any organism to stimulation are of such
a kind as to favor its continued or stable existence in
this environment. A certain difficulty in defining the
conception of stimulation arises here, since many cases
exist where physiological activities, which in themselves
are injurious or destructive to the living system as a
whole, may be induced by environmental change;^ such

^ The oxidation rate of sea-urchin eggs may be increased by pure
NaCl solution to a degree which apparently is directly destructive.
The case of fatigue carried to an injurious extreme is analogous.

264 PROTOPLASMIC ACTION AND NERVOUS ACTION

instances would scarcely be classed as responses to
stimulation. In fact many opportunities for verbal
mystification arise in attempting to ''define" the concept
^'stimulation." In order to limit the following discus-
sion, we shall regard as a "stimulus" any influence
acting from without upon the living system which changes
the rate or the character of the normal vital activities;
the resulting change of physiological activity is the
"response." Even with this simplified conception, the
range of phenomena is still too great to be readily
included under any strictly drawn definition; but such
a definition need not be insisted upon, provided the
general nature of the relations between organism and
environment is clearly understood. Variation of vital
activity, within the physiological range, occurring as a
constant correlative or sequence of environmental
change of some kind, is the essential phenomenon
whose conditions we are considering.

In multicellular organisms, "internal" and "external"
(proprioceptive and exteroceptive) stimuli are often
distinguished,^ since in many cases the environment
which furnishes the normal stimuli for an irritable cell
or cell-system may be not the external world but some
other part of the same organism. Responses of special
organs or organ-systems to stimuli originating else-
where within the same organism form, in fact, a
regular part of many normal physiological cycles in
higher animals; thus the pancreas is stimulated by
secretin in the blood stream, and the respiratory
center by increased H-ion concentration of the blood;
the central nervous system is continually adjusting its

* Cf. Sherrington, Integrative Action of the Nervous System.

STIMULATION AND TRANSMISSION 265

activity to changes in bodily conditions, and so on.
Evidently the effects of external stimuli upon the sense
organs cannot be regarded as forming a significantly
different class from these phenomena, so that from the
standpoint of general physiology the foregoing distinc-
tion is a purely formal one and has little objective
importance.

Various terms are applied to special processes which
may be included under the general conception of stimula-
tion, as just defined. The term ''activation" is used
with reference to the initiation of development in a
resting egg cell by a spermatozoon or a parthenogenetic
agent; acceleration, or simple increase in the rate of an
already existing process, is a frequent form of response
(e.g., secretion, the heart-beat, or other regular muscular
movement, growth, etc.); retardation or inhibition is
perhaps equally frequent. In cases of automatism, like
that of the heart, the rhythm may be regarded as deter-
mined by periodic stimuli furnished by processes within
the cell. Since all of these phenomena may occur, or
undergo modification, in response to changes of environ-
mental condition, all are to be considered under the
general conception of stimulation.

The most general features of the stimulation-process
are best studied in those irritable tissues or cells which
give a prompt and definite response to electrical or
mechanical stimulation, such as the nerves and muscles
of higher animals; and the majority of investigations,
on stimulation, especially those of a quantitativ^e kind,
have been carried out with these tissues, usually after
isolation. The results gained have, however, a general
applicability to other irritable living systems.

266 PROTOPLASMIC ACTION AND NERVOUS ACTION
GENEEAL CONDITIONS OF STIMULATION

It is well known that different living systems may
vary widely in their sensitivity to the same stimuli,
and also that irritability is often specialized with reference
to particular physical agents. Sensory elements wdth
special sensitivity to light, contact, slight changes of
temperature, or chemical substances, are found in all
higher animals. These differences are referred to special
features of chemical and structural organization.
Chemical sensitivity in particular is often minutely
specialized; and such instances as the special sensitivity
exhibited by the sensitized smooth muscle of guinea-pigs
in anaphylaxis indicate clearly that many forms of
specific chemical irritability are dependent on the
presence of specific chemical compounds (apparently
in this case proteins) in the irritable cell, probably in
the protoplasmic surface layer. Similarly, photo-sensi-
tive elements like the retinal rods and cones contain
compounds of definite photochemical properties (visual
purple and related substances) upon which the special
responsiveness undoubtedly depends.^ We must recog-
nize, therefore, in addition to the general susceptibility
to mechanical or electrical stimuli possessed by all forms
of protoplasm, a variety of specific or selective forms of
irritability depending on special features of structure or
organization. Selective irritability is shown especially
by the sensory nerve-termini or receptors of higher
animals; these are classified, according to the agents to

* Thus Hecht and Williams have recently shown that the curve
of absorption of visual purple is almost identical with tlie curve of
visual sensitivity for different wave-lengths {Jour. Gen. Physiol., IV
[1922], i).

STIMULATION AND TRANSMISSION 267

which they are specially responsive, as chemo-receptors,
thermo-receptors, photo-receptors, etc.

Such special sensitivity may be described as consisting
in a lowering of the threshold of stimulation for a particu-
lar agent, and need not affect the general sensitivity to
mechanical and electrical stimulation. Some specific
irritability is superposed upon the general or non-
specific irritability. Thus a nerve or muscle may be
stimulated by mechanical, thermal, chemical, osmotic,
and electrical stimuli; similarly, a highly specialized
receptor such as a retinal element may be stimulated
by these agents as well as by light of a definite wave-
length. In all cases, however, the response following
stimulation has a specific character which is dependent
on the special structure or organization of the irritable
system or on its relations with other systems. In the
field of sensory stimulation this generalization is known
as the ''law of specific energies."

LOCAL CHANGE AND PROPAGATED EFFECT

We have seen that an irritable system with a highly
developed general sensitivity, e.g., a muscle or a nerve,
may be excited by a variety of stimulating agents, and
the question arises why such physically dissimilar agents
produce the same physiological effect. The general
sequence of events when such a tissue is stimulated may
be briefly described as follows. Some local change,
whose precise nature is determined by the nature of the
stimulating agent, occurs at the site of stimulation; a
state of ''excitation" is there initiated which, however,
does not remain confined to this region, but spreads or
is propagated to a distance, often at a high velocity.

268 PROTOPLASMIC ACTION AND NERVOUS ACTION

This propagated effect, or '' propagated disturbance"
(Keith Lucas' term), as it appears at a distance from
the point of stimulation, has its own definite peculiarities,
which are independent of the nature of the initiatory
local change or stimulus. For example, any single nerve-
impulse in a normal frog's nerve, by whatever means it
is initiated, travels at a constant velocity (assuming the
state of the tissue to be normal and the temperature and
surrounding conditions constant), and its most readily
observed physical accompaniment, the bioelectric varia-
tion, has a definite range of potential change and definite
time-relations. In other words, the propagated dis-
turbance differs from the local change in exhibiting
constant and specific features, qualitative and quantita-
tive, whose nature is determined by the special or
inherited constitution of the tissue.

Almost any kind of sufficiently rapid local alteration
may initiate such a wave of physical and chemical
disturbance. The distinction between the local change
and the propagated effect is a fundamental one in any
theory of stimulation. The former is the ''releasing"
event and follows upon some simple physical change
produced in the tissue by the stimulating agent; the
latter is the distinctively physiological process, and as
such has specific characters of a complex kind, dependent
on the nature of the irritable system and as yet
imperfectly analyzed.

Especially significant is the fact that all irritable
elements, apparently without exception, respond to
electrical stimuli, or are influenced in their already
existing activity by the electric current. The electrical
sensitivity of highly irritable tissues, such as vertebrate

STIMULATION AND TRANSMISSION 269

motor nerve, is extreme; the frog's sciatic nerve may
be stimulated by a current of .000001 ampere or less;
it is well known that the neuro-muscular apparatus of
the frog was used by Galvani as the most sensitive
means known to him by detecting variations in the
electrical state of bodies; in fact it is to this property of
living tissues that we owe the discovery of current
electricity by Volta. Galvani's experiments also showed
• — although their significance was disputed at the time —
that electric currents are produced in the activity of
living tissues. Thus two of the most fundamental
properties of living matter, its electrical sensitivity, and
its production of electrical currents during activity,
were early observed; and many of the chief problems
of general physiology at the present time relate to the
physico-chemical conditions and physiological signifi-
cance of these properties. That they are among the
chief factors controlling normal cell-processes seems
certain.

NATURE OF THE LOCAL CHANGE

It is remarkable that complete stimulation, involving
a change in the activity of the entire cell, is produced
in many if not all irritable elements by agents which
affect directly only the surface-layer of protoplasm.
Some local modification of surface conditions seems to
be all that is required to set in motion the whole complex
process of stimulation. This is best shown in the
mechanical stimulation of single cells; thus in a ciliated
protozoon like Paramcecium a slight touch is sufficient
to call forth the characteristic motor reaction, involving
a reversal of the direction of ciliary activity over the
whole surface of the organism. The extraordinary

270 PROTOPLASMIC ACTION AND NERVOUS ACTION

sensitivity of many blood cells to mechanical contact
illustrates the same phenomenon; a slight touch with a
capillary needle is often sufficient to cause a rapid and
complete disintegration of a leucocyte or a red blood
corpuscle;^ i.e., a wave of alteration involving the "
breakdown of the semi-permeable surface-film is propa-
gated over the entire protoplasmic surface. The break-
down of the explosive corpuscles in Crustacea^ and of
nematocytes in coelenterates are other examples of the
same kind of process. Such facts suggest that in normal
cases of stimulation, as in nerve, where a local stimulus
initiates a temporary disturbance, which passes like a
wave over the entire irritable element, a similar disinte-
gration of the surface-protoplasm occurs, with the differ-
ence that this change is immediately and rapidly reversed
by the formation of a new surface layer.

That some such process occurs during the transmis-
sion of the excitation-wave in a nerve or other irritable
tissue is indicated by the character of the local bio-
electric variation. A reversible surface-change is
known to accompany the transmission of the activation-
wave along a passive iron wire immersed in nitric acid;
the passivating surface-film is removed by electrolytic
reduction in the neighborhood of each active area, and
is then immediately reformed by the oxidizing action of
the acid and of the local electric current (at the anodal
areas); and the destruction and re-formation of the
film are associated with definite and rapid variations
of potential. In the stimulated living system a similar

^ Cf. Chambers, Anatomical Record, X (19 16), 190.

2 Cf. Hardy, Journal of Physiology, XIII (1892), 165; Tait, Quarterly
Journal of Experimental Physiology^ XII (1918), 42.

STIMULATION AND TRANSMISSION 271

reversible variation of potential accompanies the passage
of the excitation-wave. The resemblance of the trans-
mission phenomenon in passive iron to the protoplasmic
type of transmission is in fact so detailed as to confirm
strongly the hypothesis that in the latter case also the
essential feature of the transmission-process is the
breakdown and reconstruction of the thin protoplasmic
surface-film under the influence of the local bioelectric
circuits.

If the processes in the living system and in the simple
inorganic model are in fact similar in their dependence
on surface-changes of this kind, it becomes evident at
once why protoplasm is so readily excited by conditions
acting upon its surface layer. A mechanical agent,
by interrupting the continuity of the surface-film, or by
otherwise altering it so as to give rise to a local circuit,
may initiate a wave of electromotor variation and
disintegration which travels automatically over, the whole
surface, and in so doing alters the physiological activity
of the whole system. Any other sufficient local altera-
tion (chemical, thermal, etc.) may produce the same
effect. The critical factor in the local process of excita-
tion thus appears to be the alteration of the protoplasmic
surface-film in such a manner as to change locally the
potential difference between the protoplasm and the
external medium, to a sufficient degree and at a sufficient
rate. The local circuit arising between this altered
region and the as yet unaltered regions adjoining forms
the next link in the chain of events; and if this local
current has sufficient intensity and local density to
break down electrolytically the film over a certain area
of the adjoining region, an indefinite wave of propagation

272 PROTOPLASMIC ACTION AND NERVOUS ACTION

may be initiated, since the same effect will be repeated
at each newly formed boundary between the active and
the inactive areas.

According to this conception the primary action of a
local stimulating agent is to change rapidly the electrical
condition of the cell-surface at its point of application.
The parallel between the irritable protoplasmic system
and the passive iron model is obvious, since in the latter
case also a local alteration of the surface-film involving
a local change of potential is the initiating condition for
the propagated wave of chemical and electromotor
disturbance.

The present view, therefore, regards stimulation as
conditioned by surface processes of the foregoing def-
inite kind. If this is true, the structural arrangements
providing for the transfer of excitation from one irritable
element to another should exhibit features of a character
to correspond. In fact, many peculiarities of the
structure of the central nervous system — especially of
the synaptic junctions — and of the structure and arrange-
ment of nerve-endings, such as the myoneural junctions,
are in harmony with this conception. Nerve end-plates
spread out over the surface of the muscle cell, effecting
intimate contact but not penetrating; the junctions
between neurones are eft'ected by brushlike interlacing
terminals, or by end-feet and similar structures which
are applied to the cell surface with a closeness that
apparently admits of variation. That transmission by
contact, through the influence which the cell-process
exerts upon adjoining processes or upon the cell body,
is the chief mode of transmission in the nervous system
is one of the corollaries of the neurone theory of the

STIMULATION AND TRANSMISSION 273

structure of this system. The well-known experiments
("rheoscopic frog") in which one active muscle or nerve
stimulates another which is in close contact with it,
by means of the bioelectric currents accompanying
activity, show that excitation can be transmitted from
one irritable element to another without direct proto-
plasmic continuity, through a purely electrical influence.
Similarly in the passive iron nitric acid system, activation
is readily transmitted from one wire to another by
contact, and the basis of this transmission is also
electrical.

According to these conceptions, all forms of stimula-
tion are electrical; or, more exactly expressed, electric
currents resulting from local alterations of the cell
surface form a necessary part of the sequence of processes
constituting stimulation. Such a view implies further,
since the activity of the whole cell is altered by stimula-
tion — e.g., all of the fibrils in a muscle cell contract
when the excitation-wave travels over its surface — that
the intracellular processes, including the chemical or
metabolic processes which furnish the energy for the
activity, are largely controlled by processes having their
origin at the cell surface.

This inference is confirmed by a study of the condi-
tions of electrical stimulation. The work of Nernst
and his successors^ has shown that the electric current
does not act by penetrating the living cell (which in
fact is a poor conductor) but by changing its surface-

^ Cf. Nernst, Gottingen Nachrichten, math.-physik. Klasse (1899),
p. 104; Arch. ges. Physiol., CXXII (1908), 275; Lapicque, Jour, de
Physiol, IX (1907), 565, 620; X (1908), 601; XI (1909), 1009, 1035;
Lucas, Journal of Physiology, XL (1910), 225; A. V. Hill, ibid., XL
(1910), 190.

274 PROTOPLASMIC ACTION AND NERVOUS ACTION

polarization. And the semi-permeability of the plasma
membranes of irritable cells (e.g., muscle-cells) with
reference to the inorganic salts of the medium — which,
without penetrating the cell, nevertheless influence
profoundly its properties and activity — shows further
how closely cellular activities are dependent on surface-
conditions. Among these surface-conditions the state
of electrical polarization appears to be of primary
importance.

The facts of electrical stimulation, now to be con-
sidered, show that variations in electrical surface-
polarization have a far-reaching control over the meta-
bolic and other processes occurring in the cell interior.
The means by which this polarization may be altered
are of three chief kinds: (i) changes in the structure or
composition or permeability of the surface-film; (2)
external electrical influences, especially the influence of
electric currents traversing the cell or its medium; and
(3) changes in the composition of the medium or internal
protoplasm. Since, according to the present view, an
inseparable feature of stimulation, the transmission of
the excitation-state, is a direct result of electrical activa-
tion by the currents of local bioelectric circuits, it is
clear that the problem of stimulation resolves itself
largely into the problem of the general conditions under
which the electric current stimulates living matter.
Electrical stimulation is in fact the primary form of
stimulation.

CONDITIONS OF ELECTRICAL STIMULATION

In general the physiological studies of the last two
decades have shown that the stimulation of an irritable

STIMULATION AND TRANSMISSION 275

living system by the constant electric current is subject
to definite quantitative laws; they indicate also that
the current acts primarily through its polarizing action.
Upon this polarizing action follows chemical action as a
secondary consequence.

The essential conditions under which the current
causes stimulation may be briefly summarized as
follows :

1. The current must exceed a certain minimal
intensity; this ''threshold" intensity varies widely for
different irritable tissues and for the same tissue under
different conditions; e.g., during states of fatigue,
narcosis, sensitization, etc.

2. A current of threshold or greater intensity must
traverse the tissue for a certain minimal time; its
stimulating action thus depends not only upon its
intensity but also upon the duration of its flow. The
rule that for equal stimulating action the product of the
intensity into the root of the duration is constant
{il/t = K) appears to hold for most tissues within a
considerable range of intensities.' For the current
of threshold intensity this critical duration varies widely
in different tissues (from a few thousandths to several
seconds) ; it is the expression of a time-factor (chronaxie),
which is specific for the tissue in question.''

These general statements apply to the case of currents
which reach their full intensity rapidly or instantane-
ously; e.g., when the stimulating circuit is suddenly
closed. Such a condition, however, is apparently not
a normal or ''physiological" one, since the bioelectric
current — which, according to our present conception, is

^ Cf. Nernst, loc. cit. ' Cf. Lapicque, loc. cit. (1909).

276 PROTOPLASMIC ACTION AND NERVOUS ACTION

the factor arousing the local excitation at each successive
region in a transmitting element (nerve, etc.) — attains
its full intensity not instantaneously but in a rising
curve of more or less gradual slope, varying from tissue
to tissue, and subsides in a similar manner. Stimulation
by currents of varying intensity is thus the typical
condition prevailing in the organism, and to which,
therefore, especial attention must be directed. The
general empirical rule governing the action of such
currents is as follows:

3. In order to stimulate, a current rising continuously
and uniformly from zero to full intensity must change
its intensity at a certain minimal rate which is charac-
teristic for the tissue; i.e., —= const, (assuming

temperature and other conditions normal). Hence we
find that a slowly increasing current may fail to stimulate,
while one rising to the same intensity at a more rapid
rate stimulates. The same rule applies to stimulation
by the decrease of a current already flowing through
a tissue. The rate of change, in either direction, must
exceed a minimal value which is specific for the tissue.
A time-factor enters, closely related to that already
referred to above (under 2) as "chronaxie."

4. The stimulating action of the current is charac-
teristically polar; i.e., the current produces its primary
physiological effects chiefly at its regions of entrance and
exit, and the effects at the two regions are typically
opposite or antagonistic. Typically, when the current
is made, it initiates excitation at the cathode (i.e., where
the positive stream of the stimulating circuit passes
from the tissue to the appHed electrode), and inhibits

STIMULATION AND TRANSMISSION 277

activity (at the same time depressing irritability) at
the anode. When a current already flowing through
the tissue is broken, stimulation also results, but the
polar relations are reversed; i.e., stimulation is then at
the anode, inhibition at the cathode. This summarized
statement is the usual form of the ''law of polar stimula-
tion." It is important to note, however, that a polar
action is seen in many other physiological processes
occurring under the influence of the current; e.g.,
electrotonus, polar disintegration of cells, galvanotropic
growth, and galvanotaxis. All of these phenomena show
that the direction of the current, relatively to the cell
surface, determines the nature of its physiological action.
The parallel to electrolysis, at the surface of any electrode,
is especially clear in phenomena of this class; it is well
known that where the positive stream passes from the
metallic electrode to the solution (at the anode) it
produces chemical effects (in general of an oxidative
kind) of the reverse nature to those produced where it
passes from solution to electrode (cathode); here the
general chemical action is reducing.

5. That a variation in the electrical state of the
irritable elements, sufficient in degree and rate, is the
determining factor in the physiological action of the
current is seen in the fact that a change in either direction,
i.e., make or break, increase or decrease, may stimulate
or produce other characteristic physiological effects.

6. Finally, summation effects are highly character-
istic; i.e., two or more electric stimuli (induction shocks)
which, acting singly, are ineffective, may cause stimula-
tion if sent in sufflciently rapid succession into the tissue.
The interval between the successive single stimuli must

278 PROTOPLASMIC ACTION AND NERVOUS ACTION

be less than a certain critical time, or no summation
results. This ''summation time" is closely related to
the characteristic time-factor of the tissue, being brief
in tissue with brief chronaxie and vice versa/ Since the
stimuli normally acting in the intact organism are largely
repetitive or rhythmical, summation processes are of
special physiological interest.

An exhaustive discussion of the effects of electricity
on living organisms is not possible within the limits of
space; but those features of electrical stimulation which
indicate its dependence on surface-alteration are of
fundamental theoretical significance and will be con-
sidered in some detail.

Chief among these features are the time-relations of
electrical stimulation. A stimulating current of a given
intensity must flow uninterruptedly for more than a
certain time through the tissue or it produces no apparent
efl'ect. When the relations between the intensity of a
stimulating current and its minimal duration are investi-
gated, a highly characteristic relation appears, indicating
that the action of the current depends upon the transport
of ions to or from the semi-permeable surfaces of the
irritable tissue. The resulting change of electrical
surface-polarization forms the primary condition of
stimulation. This was first clearly shown by Nernst,^
in a paper on the relation between the stimulating
action of alternating currents and the rate of alternation.
In a living tissue, wliich, considered from a simplified
physico-chemical point of view, represents an electrolyte
solution partitioned by membranes not readily permeable

^ Cf. K. Lucas, Journal oj Physiology, XXXIX (1910), 461.
^'Nernst, loc. cit. (1899).

STIMULATION AND TRANSMISSION 279

to ions, there is during the flow of the current a movement
of cations with the positive stream and of anions with
the negative stream; at the semi-permeable membranes
interposed in their path the movement of ions is impeded ;
the cations then undergo an increase of concentration at
those surfaces of the membranes which face toward the
anode, simultaneously w^ith a decrease of concentration
at the opposite faces; the reverse relations hold with
the anions. A gradient of ionic concentration is thus
set up between the layer of solution in immediate contact
with the membrane and the layer at some distance.
This process of concentration at the semi-permeable
surface continues until a condition of equihbrium is
reached at which the rate of diffusion back from the
membrane into the interior of the solution is equal to
the rate at which the ions are transported to the mem-
brane. Assuming the existence of these two opposed
processes, transport to the surface by current and back-
diffusion, it can be shown that to produce a definite
change in concentration at the surface, the product of
the current-intensity into the root of its time of flow
should be constant {i\/t=K).

This result was reached by Nernst from the considera-
tion of the case of a single membrane interposed in the
path of a current. It is evident ,that such a system
offers conditions much simpler than those of an irritable
tissue, which typically consists of a bundle of cells or
fibrils. HilP and Keith Lucas^ have pointed out that
in considering the case of the living cell or nerve-fiber,
with its small linear dimensions, it is necessary to take

^Journal of Physiology, XL (1910). 190.
' Ibid., p. 225.

28o PROTOPLASMIC ACTION AND NERVOUS ACTION

into account the processes at the two opposite surfaces
of each element, since if these surfaces are close enough
together, there is mutual interference of the two processes,
and more complex conditions have to be assumed. Yet
the fundamental condition assumed by Nernst's theory
— a membrane partitioning an electrolyte solution —
exists in the living tissue, hence polarization effects must
result when a current is passed; and it has been found
that the foregoing law relating the current-intensity to
the duration required for a constant polarizing effect holds
true also for the stimulating effect of the current within
a considerable range of intensities and durations, and
especially for higher intensities. This general result,
that polarizing effect and stimulation run closely parallel,
indicates that stimulation is a consequence of the
polarizing action of the current.

Hermann' and others had previously referred the
stimulating action of the current to its polarizing action,
since in any tissue the presence of a reverse current
(polarization current) can always be demonstrated
immediately after the passage of a brief constant current.
A high degree of polarizability is characteristic of living
tissues, and this peculiarity is undoubtedly dependent
on the semi-permeable properties of the cell membranes,
since the polarization current is greatly diminished at
death, at which time the membranes lose semi-
permeability, as already pointed out. Both polariza-
bility and semi-permeability are thus manifestations of
the same condition, hindrance to diffusion of ions.
Lapicque has shown that the polarization currents
obtained from dead partitions (parchment or bladder

^ 'L.'HtnQZ.nn,Handhiich der Physiologie,'Leipzig,lJ (1879), Part I> i93-

STIMULATION AND TRANSMISSION 281

membranes) are related to the intensity and duration of
the polarizing current in the same manner as the stimula-
ting effect of a current traversing a living irritable tissue;
i.e., to produce a constant polarization current, the
product of the intensity of the polarizing current into
the root of its duration must be constant.' It thus
appears certain that the primary or initiatory process in
electrical stimulation is the production of a certain
critical degree of polarization at the semi-permeable
membranes of the irritable tissue. In other words, the
current stimulates by means of its polarizing action,
i.e., by producing a potential difference (or by altering
an already existing potential difference) between the
external and the internal faces of the semi-permeable
plasma membranes.

It should be noted that in itself this result throws
little light upon the special physiological nature of the
stimulation-process; it merely defines the physical con-
ditions under which this process is initiated. The process
itself, as just pointed out, has its specific peculiarities
which are independent of the nature of the exciting agent.
It is, however, an important theoretical advance to
recognize that polarization changes are involved in all
forms of stimulation. That this is the case is further
shown by the invariable participation of bioelectric
currents in stimulation processes^; these currents, like
any others traversing the tissue, must cause changes of
polarization at the cell surfaces. According to the
present view, the spread of excitation is due to the
secondary stimulation-effects resulting from the polariz-
ing action of such currents.

' Lapicque, Compt. rend. soc. hioL, LXIII (1907), 37.

282 PROTOPLASMIC ACTION AND NERVOUS ACTION

We are thus led to consider the kinds of effect which
changes in the electrical polarization across the cell
surface may have upon chemical processes occurring in
this region.

It should first be noted that various phenomena
occurring at metallic surfaces and involving electrolysis
have been shown to follow the same '' square root law"
as the stimulation process. Bredig and Kerb found this
to be true for the influence of alternating currents in
initiating the characteristic rhythmical action in the
mercury hydrogen peroxide system, which, as we have
seen, resembles closely the passive iron model in its
mode of activity; the same was found by Wilke and
Meyerhof in the electrolytic oxidation and reduction of
chromic salts and chromates at platinum electrodes.'

Whenever a sufficient uncompensated potential
difference is established between an electrode and a
solution, as in any battery with closed circuit, the
conditions for chemical change are present; there is a
transfer of electricity associated with a chemical decom-
position or other reaction (oxidation, synthesis, etc.) at
the interface. It is well known that a certain critical
decomposition-voltage must be exceeded in order to
carry out any definite electrolysis, e.g., of a metallic
salt; and if the cell surface possesses the general proper-
ties of an electrode, the chemical reactions there occurring
must be subject to similar conditions. The need for
a certain minimal or ''threshold" current-intensity in
stimulation is thus explained ; it is evident that if the fore-
going theory of transmission is well founded the potential

* Bredig and Kerb, loc. cit.; Wilke and Meyerhof, Arch. ges. Physiol.,
CXXXVII (1910), I.

STIMULATION AND TRANSMISSION 283

difference of the local bioelectric circuit in a conducting
tissue like a nerve must exceed the critical value re-
quired for the electrochemical process which initiates the
chemical reaction of stimulation. The general nature of
the conditions will be considered more fully later when
the phenomena of transmission are discussed in detail.
For the present we may conclude that the significance
of the polarization change involved in electrical stimula-
tion is simply to furnish the condition required for some
critical chemical decomposition at the cell surface.
Presumably this chemical change alters locally the
physical properties of the surface-film in such a way as
to involve local breakdown or increase of permeability;
and then, just as in the passive iron model, an auto-
matically self-propagating wave of chemical decomposi-
tion is initiated. In this process the altered and the
unaltered portions of the cell surface act as two electrode
areas, in a manner analogous to that observed in the
passive wire and similar systems during transmission.

In living tissues the conditions are more complex
than in the simple model considered by Nernst, which
takes account of only one of the conditions of electrical
stimulation. Two chief conditions which this simple
theory disregards are: (i) the existence of a critical
threshold current-intensity, independent of duration;
and (2) the character of the response to currents of
changing intensity. Nernst's theory, however, explains
the essential fact of polar stimulation, in addition to
assigning a definite condition, viz., change of polarization,
for the initiation of the stimulation-process. On the basis
of the law of polar stimulation we may now say further
that a change of polarization in a definite direction^ such as

284 PROTOPLASMIC ACTION AND NERVOUS ACTION

to render the external layer of the solution in contact with
the cell surface less positive than before, i.e., a depolari-
zation, is the critical or initiatory event in stimulation/
But a current of too weak intensity, or one rising to
its maximum too slowly, will not stimulate, whatever
its duration. These discrepancies from the simple
polarization theory must be referred to the special
properties which the irritable tissue possesses by virtue
of being a living structure. Apparently the irritable
element is able to compensate slight or gradual changes
of polarization as a part of its general regulatory capacity.
Thus, if a current be led gradually into a nerve or muscle,
a considerable intensity may be reached without stimula-
tion. But if then the current be suddenly broken,
stimulation results. This behavior seems to imply
that while the current is gradually increasing, the cell by
some regulatory process maintains its normal or resting
physiological polarization essentially unaltered. During
the flow of the external current, part of the polarization
at the cell surface must depend on the presence of this
current, which steadily conveys ions to (or from) the
surface. When this influence is suddenly withdrawn,
by breaking the current, the effect is to alter the polariza-
tion more rapidly than can be compensated by the
activity of the cell, and stimulation results. The fact
that the rate of change to which the irritable element
can thus adjust itself without undergoing stimulation is
rapid for rapidly reacting tissues (i.e., those with brief
chronaxie) and gradual for ''slow" tissues indicates that

* Cf. Briinings, Arch. ges. Physiol., C (1903), 367. Hermann also
recognized that the polarization change of stimulation is in the direction
of rendering the external surface of the irritable element less positive
than before {loc. cit.).

STIMULATION AND TRANSMISSION 285

some specific chemical process, whose rate is determined
by the characteristic metabolic properties of the tissue,
is what preserves the normal resting state of the irritable
elements. Thus we may imagine the material of the
surface-film as being constructed and replaced as rapidly
as it is removed by the chemical (reducing) action of the
current; under these conditions the film (with its resting
polarization) remains unaltered and no stimulation
results. But if the rate of removal exceeds the rate of
replacement, the consequence is alteration of the film
and stimulation. Conditions of essentially this kind
exist in the passive iron model, which shows a similar
type of behavior.

STIMULATION BY CONSTANT CURRENTS

A typical case of stimulation by the constant current
will illustrate how the stimulating effect varies with the
duration of the stimulus. The following table from

,0

Temperature ca. 15^

Duration in t * v /-v . /~

Seconds U) Intensity (i) {-y/t

0.024 0.18 279

0.021 0.18 261

0.017 o. 18 234

0.014 O. 18 212

O.OT 0.2 200

0.007 0.23 " 193

0.0052 0.26 187

0.0035 0.31 184

0.0017 0.44 181

0.00087 0.66 195

Keith Lucas^ gives results obtained with the sartorius
muscle of the frog. Constant currents of known intensity

'J Physiol., XXXVII (1908), 475.

286 PROTOPLASMIC ACTION AND NERVOUS ACTION

were passed through the tissue by non-polarizable
electrodes, and the minimal duration required for
stimulation was determined by varying the distance
between two contacts (one making, the other breaking
the current) in a swinging pendulum.

The essential feature of these results is that below
a certain definite intensity of current (0.18 units),
increasing the duration has no effect in lessening the
intensity required to stimulate; i.e., weaker currents
will not stimulate, whatever their duration. This
intensity represents the critical or threshold value for
any current. But with stronger currents, the duration
required for stimulation becomes less as the intensity
increases, and a close approximation to Nernst's square
root law is found. This signifies that a certain minimal
change of polarization is required to initiate the stimula-
tion process; with currents above a certain critical in-
tensity this polarization is attained with briefer and briefer
durations as the intensity is progressively increased.
Lapicque and other observers have obtained similar
results. The current of threshold value, i.e., of the least
intensity that will stimulate with any duration, must
traverse the tissue for a certain minimal time in order to
stimulate. This time is characteristic for the tissue in
question, and apparently is a direct function of the rate of
certain specific metabolic processes; probably those con-
cerned in the alteration of the surface-film, as indicated by
the duration and other features of the refractory period
(see below). According to Keith Lucas and Mines, the
length of this minimal time varies with the temperature
of the tissue in accordance with a somewhat low
temperature-coefhcient, similar to that of diffusion

STIMULATION AND TRANSMISSION 287

(Qio = i.3).^ For the irritable tissues of the frog, Lucas^
gives the following determinations (at ca. 13°): sub-
stance/3 (nerve end-plate) of the sartorius, .001 second;
motor nerve- trunk, .003 second; muscle fiber (sartorius),
.02 second; ventricle, 2 seconds; for smooth muscle the
period is much longer (several seconds) . Lapicque finds
the least effective duration of the minimal stimulating
current to vary widely for the muscles of different
animals, and gives the following data:^

Tv«-,„^i„ Least Effective Duration

^"^^^^ of Threshold Current

Gastrocnemius (rana esculenta) 003 sec.

Gastrocnemius (r. temporaria) 007 sec.

Rectus abdominis (r. esculenta) 009 sec.

Gastrocnemius (bufo vulgaris) 013 sec.

Foot of snail (helix pomatia) 048 sec.

Foot of snail (solen marginatus) 075 sec.

Ventricle of tortoise (testudo grasca) 082 sec.

Claw muscle of crab (carcinus moenas) 30 sec.

Mantle muscle of mollusc (aplysia punctata) . . .80 sec.

These determinations illustrate the specificity of this
time-factor for different animals. It is interesting to
note that the velocities of the motor nerve impulses in
different animals vary in a closely parallel manner.
To designate this characteristic time-factor in the
electrical stimulation of different irritable systems,
Lapicque has introduced the term ''chronaxie." As
now defined, the term has reference to the least duration
required by a current of exactly twice the threshold
intensity (or so-called "rheobase").

^ Lucas and Mines, Journal of Physiology, XXXVI (1907), 334;
Lucas, ihid., XXXIX (1910), 461; of. p. 472.

2 Lucas, Journal of Physiology, XL (1910), 225; of. p. 245.

3 Lapicque, Compt. rend. soc. bioL, LVII (1905), 503.

288 PROTOPLASMIC ACTION AND NERVOUS ACTION

The characteristic time-factor or chronaxie of a tissue
also expresses itself in the rate of variation of intensity
required by the stimulating current; this rate is greater
the briefer the chronaxie; it is also greater the more
rapidly the stimulation-process develops in the tissue,
as indicated 'by the rate at which the accompanying
bioelectric variation rises to its maximum. The chron-
axie also varies directly with the duration of the
summation-interval for subminimal stimuli.^

STIMULATION BY CURRENTS OF CHANGING

INTENSITY

The stimulating effect of a current of continuously
changing intensity, or of a change in the intensity of a
current already traversing the irritable tissue, varies,
in a manner which is characteristic for the tissue, with
the rate of change, and is largely independent of the
actual intensity. It is significant that this rule, relating
stimulating effect to rate of change, applies also to
mechanical, chemical, and other forms of stimulation,
in aU of which a sudden change is more effective than a
gradual one. A general property of living matter is
apparently here involved. In the activation of the fore-
going metalHc model (passive iron wire in nitric acid)
the same rule holds; e.g., in order to activate the metal
mechanically by scraping with glass, the movement
must be rapid; a slow movement is ineffective. Similarly
in electric activation a current which is gradually
increased up to a sufficient intensity has no effect,
while one of the same intensity, attained suddenly,
causes instant activation.

^ Cf. Lucas, Journal oj Physiology, XXXIX (1910), 463; cf. p. 470.

STIMULATION AND TRANSMISSION 289

The rate of change which a current requires in order
to stimulate a tissue varies with the nature of the tissue,
and is a function of the characteristic chronaxie. When
the chronaxie is brief, the rate of change must be rapid.
When the rate of change of an increasing current is
gradual, a greater final intensity of current is needed
for stimulation than when this rate is rapid. The
following observations of Lucas, on the stimulation of the
frog's sartorius, illustrate the conditions for a single
typical tissue. The rate of change of the exciting current
was controlled by varying the rate of movement of a
shutter which opened and closed a slot in a partition set
across a zinc sulphate solution, forming part of the
stimulating circuit.^ Comparison was made between the
current-strength required: (i) when the circuit was
closed instantaneously; and (2) when the intensity
was increased from subminimal to a stimulating value
at varying rates. The muscle was also stimulated by
currents of the same linear gradient or rate of change
under two conditions, (A) while immersed in pure
0.7 per cent NaCl solution and (B) in a mixture of 0.65
per cent NaCl plus 0.05 per cent CaCla. The following
results are typical:^

Strength of Current Required
Time Required to Reach Full for Stimulation

Intensity (Seconds) ^ (NaCl^ B (NaCl+CaCh)

o (instantaneous) ... i i

0.1 sec I I

0.27 1.05 1.07

0.50 I.I 1.27

0.97 1. 15 1-5

^ Lucas, Journal of Physiology , XXXVI (1907), 253.
2 Lucas, iUd., XXXVII (1908), 459; cf. p. 473.

290 PROTOPLASMIC ACTION AND NERVOUS ACTION

The more slowly the current changes its intensity the
less effective it is as a stimulus. The sensitivity to
rate of change varies with temperature and the composi-
tion of the medium; the necessary rate of change is
greater at higher temperatures; it is also greater when
calcium is present (B) than in the pure NaCl solution
(A). According to Lucas, ''an increase in the concentra-
tion of the calcium would appear to necessitate a more
rapid concentration of the ions concerned in excitation."'
Observations by Mines, on the minimal duration of the
threshold current of constant intensity,^ have shown
that in this case also the duration is briefer when Ca
is present. Such facts indicate that the chronaxie of a
tissue is determined not only by its specific constitution
but also by the external conditions to which it is exposed.
This is well shown in certain studies, by Adrian, on the
effects of peripheral nerve injury.^

The chronaxie of a tissue appears to be closely
related both to the rate of response and to the rate of
recovery of the irritable elements. Thus it shows a
close correlation with the characteristic duration of both
the bioelectric variation of the tissue and the refractory
period. The more slowly a tissue responds to a constant
current, i.e., the longer the minimal duration of the
current of threshold intensity, the more gradual is the
rate of change required for excitation by a current of
changing intensity. The length of the summation-

^ Op. cit. (1908), p. 480.

' Cited in Lucas' paper, op. cit. (1908), p. 472.

3 The chronaxie of a muscle with nerve supply interrupted increases
progressively until innervation is re-established {Archives oj Radiology
and Electrotherapy, May, 191 7).

STIMULATION AND TRANSMISSION 291

interval also appears to be determined by the same
conditions.

SUMMATION

The phenomenon of summation is of great importance
in the analysis of the stimulation process. It shows
clearly that a single subminimal stimulus produces an
effect on the tissue, but that this effect is transient;
within a certain brief time the tissue resumes the same
condition as before the stimulus. But if before this
time has elapsed a second similar stimulus is applied,
its effect is added to that of the first, and the critical
level of disturbance required to initiate an excitation-
wave may be reached. The second stimulus, in order
to be effective, must be sent in before the effect of the
first has subsided; and the more rapid the rate of this
subsidence the shorter is the summation-interval. The
summation-interval is therefore defined as the longest
interval separating the successive subminimal stimuli
of an effective series of two or more such stimuli.^

This interval is shorter than the least duration of the

exciting current of threshold intensity; and its precise

duration varies with the intensity of the subminimal

stimuli employed. Lucas gives the following intervals

for different frog's tissues at 13°, using two subminimal

electric stimuli (induction shocks) which were 5 per cent

below the strength required for stimulation by single

stimuli.^

Motor nerve (sciatic) 0004-.0005 sec.

Muscle (sartorius) 0011-.0019 sec.

Ventricle 008 sec.

Cf. Lucas, Journal of Physiology, XXXIX (1910), 462.
Op. cit. (1910), pp. 466 £f.

292 PROTOPLASMIC ACTION AND NERVOUS ACTION

When the shocks were lo per cent below the threshold,
the interval was much shorter.

The summation-interval is thus longer the more
gradual the excitation-process (the longer the chronaxie)
of the tissue. It varies with temperature and with the
state of the tissue. Lucas finds the temperature-
coefficient to be low {Qzo = ca. 1.3), a fact suggesting that
purely physical changes, e.g., dift'usion-processes (which
have a similar temperature-coefficient), are chiefly con-
cerned in the return of the tissue to the normal after
a slight disturbance.' The influence of the inorganic
salts is again highly interesting. The presence of Ca
shortens the summation-interv^al, just as it shortens the
minimal duration of the threshold constant current and
increases the rate of change required for stimulation by a
changing current.^

According to Lucas and Mines, the effect of temper-
ature on the minimal duration of the threshold current
is the same as on the summation-interval. Such facts
again emphasize the distinction between the local change
produced by the stimulating agent and the propagated

^ Cf. Lucas' discussion, op. cit., p. 473. It is noteworthy that the
time required for the return to the normal properties after complete
stimulation, as measured by the l