Protoplasmic Action and Nervous Action

The problem of the conditions of specific formdetermination 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." 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 formcharacters. Specificity of crystalline form has been demonstrated most clearly in the case of the haemoglobins; 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 vertebrates 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 configuration 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].) protoplasmic structures; in such a case a specific protoplasmic and ultimately a specific cellular structure would be produced, corresponding to the specific constitution of the structure-forming compounds.

The general nature of the relation between stereochemical 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 constituted 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, fermentability 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 properties 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 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 compounds, anti-bodies, formspecific chemical unions with the antigens, and hence may serve as a means of identifying 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 resemblance 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.

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 condition 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 complementary configuration. A general condition comparable with autocatalysis^ thus determines the specific character 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 representative 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 corresponding 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 equilibrium 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 paralleling the general morphological correspondence which Conklin's work^ has demonstrated between the eggs 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.

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.

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 development. In this manner it has been shown that constancy of development in any particular species requires constancy in the external conditions. For example, the developing sea-urchin larva forms a skeleton of a characteristic 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 formcharacter; 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 addition of special growth-modifying substances; a wellknown 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

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 hormones illustrates such^ cases; metamorphosis (in tadpoles), 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 physiology, to the susceptibility of growth processes to such modifying influences; in such cases the organism ''responds" by changing its rate or manner of growth. Any such response implies a corresponding modification in constructive metabolism; hence such facts show that re-

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

cited general rule, and indicate clearly that the physicochemical 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 stimulation 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 represents 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 facts illustrate the unitary character and control of the metabolic processes underlying the various vital manifestations; they show that growth and development are controlled by the same conditions as the other forms of protoplasmic activity. Hence stimulation is a conception 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.

In general, the term "irritability," as used in physiology, 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 nonvital factors. For example, within the usual

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 hydrolysis 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 accelerations 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

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 movement, 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 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

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 depressant 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.

In considering the general peculiarities of living protoplasm, it is essential to recognize that its characteristic properties and activities depend upon features of composition 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 physiology. 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.

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 compensates 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 surroundings. A further general resemblance is that they frequently 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 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 processes, and the other finer products of the formative activity of protoplasm depends on this continual automatic synthesis, which compensates the tendency to physical breakdown.