1911 Encyclopædia Britannica/Physiology

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PHYSIOLOGY (from Gr. φύσις, nature, and λόγος, discourse), the science or theory of the properties, processes and functions of living organisms. Physiology is distinguished from anatomy as dealing specifically with the functions of an organism, rather than its structure. The two main branches of the science are animal and plant (vegetable) physiology, and in animal physiology that of man stands out as primarily associated with the word.

Ever since men began to take a scientific interest in the problems of life two distinct rival explanatory principles of vital History of Theory. phenomena have claimed attention: a natural and a mystical principle. The first outcome of the scientific attempt to explain vital phenomena after the natural method and by a unitary principle was the doctrine of the Pneuma, held by the followers of Hippocrates, which found its clearest expression in Galen's system. According to this doctrine, the origin of all vital phenomena was a very fine substance, the Pneuma, which was supposed to exist in atmospheric air, to be inhaled into the lungs of man, and thus through the blood to reach all the parts of the body, where it produced vital phenomena. This doctrine—an attempt to explain the phenomena of life which was not altogether natural, but even materialistic—was accepted by the middle ages together with Galen's system. With its translation into the Latin spiritus, however, the conception of the Pneuma lost its original force. The spiritus animales of the middle ages developed ere long into mystical powers, the result being the explanation of vital phenomena by a supernatural theory. Not until the scientific renaissance of the 16th and 17th centuries did views again undergo a change. After the establishment of a scientific method in physiology by William Harvey, and the development of Descartes' mechanical system of regarding living bodies, the natural explanation of vital phenomena once more universally found favour. Two schools arose, which endeavoured by dissimilar methods to find a mechanical explanation of vital phenomena: the iatrophysical, originating with the gifted and versatile Borelli, and the iatrochemical, founded by the Dutchman, F. de la Boë (Sylvius). But when both chemical and physical methods of explanation failed at such problems as, for instance, irritability and evolution, another change in opinion took place. By degrees there emerged once more the tendency to explain vital phenomena by mystical means, finding expression in the Animism of Stahl, to quote an example; and in the second half of the 18th century Vitalism, originating in France, began its victorious march throughout the whole scientific world. Again the opinion came to be entertained that the cause of vital phenomena was a mystical power (force hypermécanique)—that “vital force” which, neither physical nor chemical in its nature, was held to be active in living organisms only. Vitalism continued to be the ruling idea in physiology until about the middle of the 19th century, and its supremacy was only gradually overthrown by the great discoveries in natural science of that century. The chemical discoveries resulting from Wohler's synthesis of urea first showed that typical products of the animal body, the production of which had hitherto been supposed to be solely the result of the operation of vital force, could be obtained artificially by purely chemical methods. Then above all came the discovery of the law of the Conservation of Energy by Robert Mayer (1814-1878) and Hermann von Helmholtz (1821-1894), and its application to the living organism by Mayer, Helmholtz, Pierre Louis Dulong (1785-1838), Edward Frankland, Max Rubner and others, to prove that the manifestations of energy by the organism are simply the result of the quantity of potential energy received into the body by means of food. Finally, the stupendous results arrived at by Darwin and the establishment of the fundamental law of “biogenesis” by Ernst Haeckel, prepared the way for a natural explanation of the enigma of evolution and structure of organisms. Thus by the second half of the 19th century the doctrine of vital force was definitely and finally overthrown to make way for the triumph of the natural method of explaining vital phenomena, which down to the present time has continued to spread and flourish with an unparalleled fertility. It would, it is true, appear as if in our day, after the lapse of half a century, mystical tendencies were again disposed to crop up in the investigation of life. Here and there is heard once more the watchword of Vitalism. But all the so-called neo-vitalistic efforts—such as those of Alexander von Bunge (1803-1890), Georg Evon Rindfleisch (b. 1835), Johannes Reinke (b. 1849) and others—have nothing to do with the old vitalism. They originate solely in a widespread confusion with regard to the boundaries of natural science, their principal tendency being to amalgamate psychological and speculative questions with problems of purely natural science. In the face of all these efforts, which by their unfortunate designations of Vitalism and Neo-vitalism give rise to entirely false conceptions, and which by their intermingling of psychological questions and questions of natural science have led to mere confusion in research, it is essential that natural philosophy should be called upon to realize its own limits, and above all clearly to understand that the sole concern of physical science is the investigation of the phenomena of the material world. Physiology, as the doctrine of life, must therefore confine itself to the material vital phenomena of organisms. It is self-evident, however, that only such laws as govern the material world will be found governing material vital phenomena—the laws, that is, which have hitherto been brought to their most exact and most logical development by physics and chemistry, or, more generally speaking, by mechanics. The explanatory principles of vital phenomena must therefore be identical with those of inorganic nature—that is, with the principles of mechanics.

The investigation of vital phenomena in this sense requires, in the first place, an exact knowledge of the substratum in which Ultimate Elements of Life. these phenomena are manifested, just as in chemistry and physics a thorough knowledge of the composition of of the material world is a necessary premise to the investigation of the phenomena of inorganic nature. The knowledge of the composition and structure of organisms has in the course of the scientific development of anatomy attained to an ever-increasing minuteness of detail, without having as yet reached a definite limit. The last important step in this direction was the discovery by Matthias Jakob Schleiden (1804-1881) and Theodor Schwann (1810-1882) that all organisms are built up of elementary living structural components, namely of cells (see Cytology). The details of the anatomical construction of organisms are described under various appropriate headings, and a general guide to these will be found under Anatomy and Zoology. We would here merely point out that a cell is the simplest particle of living substance which appears to be permanently capable of life. Different elements are essential, however, to the existence of the cell—two, at least, so far as has hitherto been discovered—the protoplasm and the nucleus. It must at present be regarded as at least very doubtful whether the centrosome, which in recent times it has been possible to demonstrate as existing in very many cells, and which appears sometimes in the protoplasm, sometimes in the nucleus, is a general and third independent cell-constituent. On the other hand, the number of special constituent parts which appear in various cell-forms is very large. A question which has long been discussed, and which has received special and animated attention, is that with regard to the finer structure of the cells—with regard, that is, to the protoplasm and the nucleus lying in it. Views on this subject have diverged very widely, and several totally diverse theories have been opposed to one another. One theory maintains that the living cell-substance has a reticular structure; another, that it is fibrillous. According to a third theory, the essence of the construction of the cell-substance lies in the granules which it contains; and according to a fourth, it lies in the ground-substance in which these granules are embedded. One view holds this ground substance to be homogeneous, another regards it as possessing a fine foam-structure. It may at present be regarded as incontrovertible that living substance is more or less fluid, and that there does not exist any general structure for all cell-forms. But in some special cases all the theories which have been quoted are to a certain extent correct. In different cells there are reticular, fibrillous and granular differentiations respectively, and differentiations in foam-structure; in many cells, however, the protoplasm appears to be beyond doubt homogeneous and without a distinct structure, and only under certain conditions to assume changing structures. But the fact which is of most importance for the right understanding of vital phenomena is that the cell-substance is always more or less fluid, for only in a fluid substratum can such intense chemical processes be enacted as are to be found in every living cell.

Where the analytical powers of the microscope in anatomy can go no farther, chemical analysis of the composition of the cell steps in. By its means the discovery is made that there is no elementary difference between organic and inorganic nature, for only such chemical elements as are known to exist in the inorganic world are found in the organic. On the other hand, however, the living cell-substance possesses chemical compounds which find analogues nowhere in inorganic nature. The characteristic organic substances which are present in every cell are proteids and proteid-compounds. Besides these there occur, widely disseminated, carbohydrates, fats and other organic substances, which partly originate in the decomposition of proteids and their compounds, and are partly used for their construction. Lastly, there are in addition great quantities of water and some inorganic salts.

Such are the structure and composition of the substratum in which vital phenomena play their part. When we consider General Phenomena of Life. vital phenomena themselves in the various living organisms—in protista, plants, animals, man—there appears an incalculable diversity of phenomena. Here, however, as in the case of the structure of organisms, we have to analyse and to penetrate ever farther and deeper till we reach the fundamental phenomena. We then find that the great variety of vital manifestations may be traced back to a few fundamental general groups, which are precisely the same groups of phenomena as those to be observed in inorganic nature. All the processes that take place in the organic world may be regarded from the three different standpoints of their changes in substance, in energy and in form; for substance, energy and form are all necessary to our conception of matter. Accordingly, the general elementary vital phenomena likewise fall into three groups—metabolism, the mechanism of energy, and the assumption of form. Every cell, so long as it is living, takes in certain substances from its environment, submits them to chemical transformation in its interior, and gives out other substances. This metabolism is manifested in several special functions—in nutrition and digestion, respiration and circulation, secretion and excretion. The essence of the whole process is the fact that while out of these ingested stuffs living substance is always again being formed by the living substance which already exists, it is itself continually undergoing decomposition, and the products of this decomposition are what the cell gives off again to the outside. With metabolism, however, there is inseparably associated a transformation of energy. These substances taken in by the cell contain a large quantity of potential energy, which is transformed into kinetic energy. This has for its result the manifold activities of the organism, more especially motion, heat, electricity and light. Finally, the chemical transformations in living substance may also manifest themselves outwardly in changes of form, as is the case generally in the matter of growth, reproduction and development. The three general elementary groups of vital phenomena are therefore in reality merely the expression of the various aspects of one and the same process—of the actual vital process itself. The ultimate object of all physiology is to discover what this vital process is—that is to say, what is the exact cause of these manifold vital phenomena—a goal from which it is at the present day still very remote.

As every physical and chemical phenomenon of inorganic nature occurs only under distinct conditions, so vital phenomena Conditions of Life. are also dependent upon certain conditions of life. Every living body, every living cell, requires food, water, oxygen, and, further, a certain temperature and a certain pressure in its environment. These are the general conditions of life. But the special conditions on which depends the continued existence of the individual forms of organism are as numerous as the forms of organisms themselves. Now, just as the physicist or chemist varies those conditions under which a phenomenon occurs in order to get at its causes, so does the physiologist try to experiment with vital phenomena, altering the vital conditions; and testing the changes which are thereby produced. The great importance of this method consists in the power it gives the experimenter of analysing vital phenomena systematically from definite points of view. Every change in its normal vital conditions which produces any effect whatsoever upon an organism is termed a stimulus. This is the only general definition we have for a conception which is of such vast importance to physiology. According to it, experimental physiology is entirely a physiology of stimuli. It further follows from this conception of stimulation that there must be an enormous multiplicity of stimuli, since each particular vital condition may be subjected to some change capable of acting upon it as a stimulus. But, besides this, other factors may be brought to bear upon organisms which have absolutely no place among their vital conditions; for instance, many chemical reagents and electric currents. These influences come under the general definition of stimulus, because they likewise imply a change in the conditions under which the organism lives. From their qualitative nature stimuli are distinguished as chemical, thermal, photic, mechanical and electrical. Each of these several varieties may, however, be applied quantitatively in various degrees of intensity, and may in consequence produce quite different results. This opens up to experimental physiology a vast field of research. But the physiology of stimulation is not only of the greatest value as a means of research; its importance is much increased by the fact that in nature itself stimuli are everywhere and constantly acting upon the organism and its parts. Hence the investigation of their action comes to be not merely a means, but a direct end of research.

Although it is not at present possible to define all the laws that govern stimulation, on the one hand because the number of Actions of Stimuli. stimulating effects known to us in the whole organic world is as yet too limited, and on the other because those already known have not yet been thoroughly analysed, yet it is within our power to classify stimulating effects according to their various characteristics, and to ascertain a few facts concerning their general and fundamental conformity to law. The first fact, apparent from a glance at a great many of the various forms of stimulation, is that all their effects are manifested in either a quantitative or a qualitative alteration of the characteristic vital phenomena of each living object. The quantitative is the usual mode of action of stimuli. It is generally found that a stimulus either increases or diminishes the intensity of vital phenomena. In the first case the effect is one of excitation; in the second of depression. It is the more important to bear in mind this twofold operation of stimuli, owing to the fact that in former times physiologists were very apt to conceive of excitation and stimulation as identical. It is now, however, an undisputed fact that depression may also occur as a typical effect of stimulation. This is most apparent in cases where the same stimulus that produces excitation may on being applied for a longer period and with greater intensity, produce depression. Thus narcotics (alcohol, ether, chloroform, morphia, &c.) on certain forms of living substance produce the phenomena of excitation when their action is weak, whereas when it is stronger they produce complete depression. Thus, likewise, temperature stimuli act differently upon vital phenomena according to the degree of temperature: very low temperatures depressing medium temperatures exciting with increasing intensity, and higher temperatures from a certain height upwards again depressing. The effects of stimulation are not, however, always manifested in merely quantitative changes of the normal vital phenomena. Sometimes, especially in the case of long uninterrupted and chronic stimuli, stimulation is found gradually to produce phenomena which are apparently quite foreign to the normal vital phenomena of the cell in question. Such qualitative alterations of normal vital phenomena are perceptible chiefly in chronic maladies in the cells of different organs (the heart, liver, kidneys, spleen, &c.), in which the vital conditions become gradually more and more modified by the cause of the malady. To this category pertain all the so-called chronic processes of degeneration which in pathology are known as fatty degeneration, mucous degeneration, amyloid degeneration, and so forth. The characteristic element in all these processes is that the normal metabolism is diverted into a wrong channel by the altered vital conditions of the cells of the organ affected, so that substances are formed and accumulated in the cell which are entirely foreign to its normal life. But this class of stimulation is still very obscure as regards causes and inner processes, and it is within the range of possibility that the ultimate cause of the qualitative changes in the normal metabolism is to be found simply in the processes of excitation and depression which chronic stimulation produces in separate parts of the metabolism. Thus, at least with regard to fat-metamorphosis (fatty degeneration), it is highly probable that fat is deposited in the protoplasm simply because, owing to an inadequate supply of oxygen, it cannot, when it originates, be oxidized in the same proportion as it is formed, whereas in the normal cell all fat which originates in metabolism is consumed as soon as it is produced. According to this conception, therefore, fatty degeneration is attributable primarily to a depression of the processes of oxidation in the cell. If we may accept this view as correct with regard to the other metamorphic processes also, the qualitative changes in vital phenomena under the influence of stimuli would after all depend simply upon the excitation or depression of the constituent parts of the vital process, and, according to such a view, all stimuli would act primarily only as exciting or as depressing agents upon the normal process of life.

In accordance with the three groups into which general vital phenomena are divided, it follows as a matter of course that the excitation or depression produced by a stimulus can manifest itself in the cell's metabolism, assumption of form, and manifestation of energy. The effects of excitation upon the production of energy are the most striking, and were therefore in former times frequently thought to have a claim par excellence to rank as stimulating effects. These reactions attract most attention in cases where the production of energy is proportionately very great—as with muscle, for instance, which is made to twitch and perform work by a feeble stimulus. Processes of discharge (Auslösungsvorgänge), however, lie at the bottom of cases like these. Potential chemical energy, which is stored up in a considerable quantity in living substance, is converted by the impulse of the stimulus into kinetic energy. Therefore the amount of the effect of stimulation—that is to say, the quantity of work performed—bears no proportion whatever to the amount of energy acting as a stimulus upon the muscle. The amount of energy thus acting may be very small as contrasted with an enormous production of energy on the part of the living substance. It will not do to make generalizations, however, with regard to this proportion, as was frequently done in former times. All processes of stimulation are not processes of discharge. The influence of many stimuli, as has been observed, consists far more in depression than in excitation, so that in certain circumstances a stimulus actually diminishes the normal liberation of energy. There is therefore no general law as to the proportion which the amount of energy acting as a stimulus upon living substance bears to the amount of energy liberated.

Among special varieties of stimulation there is one class of stimuli which has attracted particular attention—namely, those which act unilaterally upon free-moving organisms. It is principally with the lowest forms of life that we have here to do—unicellular protista and free-living cells in the bodies of higher Directive Stimulation. organisms (sperm-cells, leukocytes, &c.). When from one direction a stimulus—be it chemical, thermal, electrical, or of any other kind—acts upon these organisms in their medium, they are impelled to move in a course bearing a definite relation to the source of the stimulus—either directly towards that source or directly away from it, more rarely in a course transverse to it. This directive action of stimulation is under such a fixed conformity to law, that it vividly recalls such purely physical processes as, for instance, the attraction and repulsion of iron particles by the poles of a magnet. For example, if light falls from one side upon a vessel full of water containing unicellular green algae, according to the intensity of the light these organisms swim either towards the illuminated side, where they form a compact mass on the edge of the vessel, or away from it, to cluster on the opposite edge. In the same way infusoria in water are observed to hasten towards or to flee from certain chemical substances, and leukocytes in our bodies act in the same manner towards the metabolic products of pus-forming bacteria which have penetrated into an open wound. The suppuration of wounds is always accompanied by an amazing conglomeration of leukocytes at the seat of the lesion. Perhaps the most striking effects are those of the constant electric current upon unicellular organisms, since in this case the motion follows the cause with absolutely automatic regularity, certainty and rapidity. Thus, for example, after the establishment of the current many Infusoria (Paramaecium) accumulate at the negative pole with great celerity and without deviation, and turn round again with equal celerity as soon as the direction of the current is altered. As such cases of directive stimulation may occur among all varieties of stimuli whenever stimuli act unilaterally, they have been designated, according to the direction in which they occur in relation to the source of the stimulus, as positive or negative chemotaxis, phototaxis, thermotaxis, galvanotaxis, and so forth. The strange and perplexing element in these phenomena becomes clear to us as soon as we know the characteristic method of locomotion for each form of organism, and whether the stimulus in question in the given intensity exercises an effect of excitation or of depression upon the special form. The direction of motion is the essential mechanical result of unilateral stimulation of the organs of locomotion. Seeing that these reactions are exceedingly widely distributed throughout the whole organic world, and possess a deep biological significance for the existence and continuance of life, the interest they have awakened is thoroughly justified.

One of the most important physiological discoveries of the 19th century was that of the “Specific Energy of Sense-substances.” “Specific Energy.” Johannes Müller was the first to establish the fact that very different varieties of stimuli applied to one and the same organ of sense always produce one and the same variety of sensation, and that, conversely, the same stimulus applied to the different organs of sense produces a different sensation in each organ—the one, in fact, which is its specific attribute. Thus, for example, mechanical, electrical and photic stimuli applied to the optic nerve produce no other sensation than that of light; and, conversely, any one variety of stimulus—take the electrical, for example—produces sensations of light, hearing, taste or smell, according as it affects the optic, auditory, gustatory or olfactory nerves. This law of the “Specific Energy of Sense-substances,” as Johannes Müller (1809-1875) called it, has come to have a highly important bearing upon scientific criticism, since it proves experimentally that the things of the outer world are in themselves in no way discernible by us, but that from one and the same outward object—the electric current, or a mechanical pressure, for instance—we receive altogether different sensations and form altogether different conceptions according to the sense-organ affected. But this law does not possess significance for psychology alone; as regards physiology also it has a much more general and more comprehensive force than Müller ever anticipated. It holds good, as demonstrated by Ewald Hering (b. 1834) and others, not of sense-substances only, but of living substance generally,. Each cell has its specific energy in Johannes Müller's sense, and in its extended form there is no more general law for all the operations of stimuli than this law of specific energy. To take examples, whether a muscle be stimulated by a chemical, mechanical, thermal or electrical stimulus the result is in each case the same—namely, a twitching of the muscle. Let a salivary gland be stimulated chemically, mechanically, electrically or in any other way, there always follows the same specific action—a secretion of saliva, no matter what be the kind of stimulus acting upon it, the liver-cell always reacts by producing bile, and so on. On the other hand, one and the same stimulus—the electric current, for example—gives in each form of living substance a specific result: twitching in the muscle, secretion of saliva in the salivary gland, production of bile in the liver-cell, &c. That is, of course, with the proviso that the effect of the stimulus be exciting and not depressing. The following general formulation, however, of the law of specific energy brings the depressing stimuli also within its scope: “Different stimuli produce in each form of living substance an increase or a diminution of its specific activity.” As already observed, it will probably be found that those weak chronic forms of stimulation which produce qualitative changes may also be comprised under this general law.

The knowledge thus far acquired from analysis of vital phenomena and their changes under the influence of stimuli Mechanism of Life. affords but a very indefinite temporary basis for the theory of the actual vital process itself, of which vital phenomena are the outward manifestation. The conceptions to which physiological research has hitherto attained in this matter are of a more or less doubtful nature. The facts contained in them still require to be linked together by hypotheses if we are to obtain even a vague outline of what lies hidden behind the great riddle of life. Such hypotheses, serving as they do to link facts consistently together, are absolutely essential, however, to the further progress of research, and without their aid any systematic investigation would be impracticable. But at the same time it must never be forgotten that these hypotheses are merely provisional, and that whenever they are found to be no longer in harmony with the widening range of new experiences and ideas they must either be proved to be facts or be subjected to modification. This is the point of view from which we must deal with modern ideas concerning the nature of the actual vital process—the mechanism of life

The fundamental fact of life is the metabolism of living substance which is continually and spontaneously undergoing Metabolism. decomposition, and building itself up anew with the help of the food-substances it takes in. These processes of decomposition and of reconstruction may be briefly designated as dissimulation (catabolism) and assimilation (anabolism) respectively. Now the question arises: How are we to understand this process of dissimulation and assimilation from a mechanical standpoint? It is quite evident that we have to do with some chemical occurrence; but how are the chemical transformations brought about? There are obviously two possibilities. It is conceivable that the decomposition of food-stuffs and the formation of excretion-products in the cell-body are caused by the repeated casual encounter of a great series of chemical combinations and by their repeatedly reacting upon one another in the same manner, bringing about transformations and forming waste products which are excreted, while at the same time certain chemical affinities are always taking in from without new chemical combinations (food-stuffs) and uniting them. This theory was in fact occasionally advanced in former times, particularly in its chemical aspect, and the belief was especially entertained that the enzymes in living substance might play an important part in these transformations. This assumption, however, leads to no clear and lucid image of what takes place, and, moreover, draws too largely upon auxiliary hypotheses. It has therefore met with but little acceptance. The other possible explanation of metabolism is that its whole process is confined to one single class of chemical combinations whose tendency it is to be constantly undergoing spontaneous decomposition and regeneration. This latter theory was founded by Ludimar Hermann (b 1838), Eduard Friedrich Pflüger (b. 1829) and others, and has met with universal recognition because of its naturalness, simplicity and clearness.

Starting with this hypothesis, the path of further research lies clear and well defined before us. In the first place, we are Proteids. obviously met by the question: What conception are we to form of these combinations on which hinges the whole vital process? Among the organic matters which compose living substance, proteids perform the most important part. Proteids and proteid-compounds form the only organic matter which is never absent from any cell. They form also the greater part of all the organic compounds of the cell, unless reserve-stuffs are accumulated to a considerable extent, and they are by far the most complicated of the compounds of living substance. While animal life is impossible without proteid food, there are, on the other hand, animals which can continue to subsist on proteid alone. This series of facts proves very conclusively that proteids and their compounds play by far the most important part of all organic matter in the processes of life. The idea thus naturally presents itself that the required hypothetical compound forming the central point of metabolism will be found to bear a very close relation to proteids. But another point must be here considered. The proteids and their compounds known to us are, comparatively speaking, stable compounds, which never undergo spontaneous decomposition so long as they are protected from outward injury, whereas the hypothetical combination which lies at the centre of organic metabolism is extraordinarily liable and continually undergoing spontaneous decomposition. Therefore we have to think not of ordinary proteids in this case, but of still more complicated combinations, the atoms in the molecule of which have a strong tendency to group themselves in new arrangements. Owing to their fundamental importance, these combinations have been termed “biogens.” When we come to inquire how such labile biogen molecules are built up out of the proteids of food, we find our knowledge very much restricted. Doubtless the intramolecular addition of inspired oxygen has much to do with it; for living substance when deprived of oxygen loses its irritability—that is to say, its tendency to decomposition. The fact that the decomposition of living substance is always associated with the formation of carbonic acid—a circumstance obviously necessitating the aid of oxygen—also points to the absolute indispensableness of oxygen in the matter. Pflüger has further suggested that the molecule of living substance owes its lability and its tendency to form carbonic acid when joined by oxygen atoms principally to cyanogen groups which are contained in it. According to this view, the following is supposed to be the process of the formation of biogen molecules: It is assumed that the biogen molecules already present in living substance take out of the proteids of food certain groups of atoms, and dispose them so as to produce cyanogen-like compounds. The addition of oxygen atoms then brings the biogen molecule to the maximum of its power of decomposition, so that—partly spontaneously, but more especially when impelled by a stimulus—it breaks down somewhat explosively, causing the formation of carbonic acid. In this proceeding, according to the hypothesis which is the most widely accepted and the most fruitful in results, would lie the very germ of the vital process.

If we accept these views as far as their general principle is concerned, assimilation is the re-formation of biogen molecules Mechanism of Cell-life. by those already existing, aided by food-stuffs; dissimulation, the decomposition of biogen molecules. To this primary process, however, is attached a whole series of secondary chemical processes, which serve partly to work upon the food so as to fit it for the building up of biogen molecules, and partly to form out of the direct decomposition-products of the biogen molecules the characteristic secretion-products of living substance (excretions and secretions). The various workings of matter in the cell are rendered very much more complex by the circumstance that the living cell exhibits various morphological differentiations—above all, the differentiation in protoplasm and nucleus. Again, a transformation of energy is inseparably connected with metabolism. Along with food and oxygen potential chemical energy is continually being introduced into the cell, to be accumulated in the biogen molecules, and at their decomposition transformed into kinetic energy, which finds an outlet in the various manifestations of energy in the cell—motion, heat, and so forth. In the light of this hypothesis the operations of stimuli also become comprehensible. Seeing that there is an initial tendency to the occurrence of certain definite chemical processes, which are associated with the reconstruction and decomposition of biogen molecules, various stimuli will either further or hinder the course of this metabolic series. A cell which is exposed to no outward disturbance, and which continues always in the unvarying medium provided by an exact sufficiency of food, will be in “metabolic equilibrium”—that is to say, its assimilation and its dissimulation will be equal (A = D). When, however, the influence of external stimuli is brought to bear upon them—that is to say, any change in their environing vital conditions—A and D will either be altered in similar proportion, or their mutual equilibrium will be disturbed. In the former case the vital processes will merely be intensified in their course; in the latter and usual case the result will be determined according to the part of metabolism excited or depressed. When the effect of a stimulus is to excite D continuously in a high degree without correspondingly increasing A, the result is a dying off—an atrophy. In the contrary case, when A remains continuously greater than D, the result is growth, increase and Metabolic Equilibrium. reproduction of the cell. Experience proves, however, that A and D stand in a certain relation of mutual dependence to each other, with the result that when D has been increased by a stimulus, for example, A correspondingly increases during the stimulation, and continues to do so after its cessation, till the loss in living substance produced by the stimulation of D is eventually made good, and metabolic equilibrium is restored. The muscle may be taken as an example of this self-regulation of metabolism common to all living substance (Hering's Selbststeuerung des Stoffwechsels). When a muscle has been fatigued by some stimulation causing an enormous increase of D, there is a corresponding spontaneous increase in A. After some time the muscle is observed to have recovered. It has once more become capable of performing work; its metabolism is again in equilibrium.

The vital phenomena of the cell may be derived mechanically from metabolism and the changes it undergoes under the influence of stimuli. Our ability to do this will increase more rapidly as we become better acquainted with the details of the metabolism of the cell itself. The foregoing outline must be regarded, of course, as embodying only a fragmentary hypothesis, which can serve as a guide for further research only so long as it does not clash with facts, and which must be amplified, specialized and developed with the widening of specific knowledge regarding the cell's metabolism. The relations already known are so exceedingly complex that only by slow degrees can we pursue the investigation of separate fragments of the entire metabolic series. The differentiation of nucleus and protoplasm in the living substance of the cell alone gives rise to an extraordinary complication in the metabolic process, for these two parts of the cell stand in the most complicated correlation with Cell-Processes the Secret of Life. one another as well as with the environing medium—a fact of which the experiments made by vivisection in various free-living cell-forms have furnished abundant evidence. The farther such knowledge advances, the more rounded, clear and free from hypotheses will become our conception of the cell's metabolism. But the cell is the elementary component part of all organisms, and from the life of individual cells is constructed the life of the separate tissues and various organs, and thus of the entire organism. Hence the cell is the only vital element which the organism possesses, and therefore the investigation of the vital processes in its separate cells leads ultimately to a knowledge regarding the mechanism of life in the whole.

Vegetable physiology is dealt with in the article Plants: Physiology. For detaills of different parts of the animal body, see Animal Heat; Respiratory System; Vascular System; Touch; Smell; Taste; Vision; Hearing; Voice; Muscle and Nerve; Sleep; Hypnotism; Brain; Spinal Cord; Sympathetic System; Blood; Lymph; Phagocytosis; Digestive Organs, Nutrition, &c.

The principal modern English textbooks of animal physiology are those of Sir Michael Foster (1885), A. E. Schäfer (1898), Noel Paton (1908), Halliburton (1909), and Starling (1909). See, however, the bibliographical notes to the separate articles.

(M. V.)