General Physiology: An Outline of the Science of Life

mechanical work, and this can be changed by means of a dynamo into electricity and be made to serve finally for the production of the electric light. Thus we perform daily the remarkable experiment of re-transforming, after millions of years, into its original form, the kinetic energy of the sun's rays which the plants of the Carboniferous age employed for storing up carbon, and thus illuminating our nights with the radiance of the sun that shone upon the surface of the earth in immemorial times (Cf. Bunge).

The application of the law of the conservation of energy to the energetics of organisms was attempted by Robert Mayer, and has since been undertaken many times. By the calorimetric researches also of Dulong, Helmholtz, Rosenthal, Rubner, and others, the proof has been afforded experimentally that this law is as true in living nature as in lifeless. But our knowledge is extremely scanty concerning the mode of action of energy in the various performances of the body, concerning the transformations undergone by the energy in its path through the living substance. In this respect plant physiology, which is indebted especially to the striking researches of Pfeffer upon the energetics of the plant-cell for important discoveries and suggestions, is relatively farther advanced than animal physiology. In this subject of the energetics of living substance the future offers a wide field of labour, which is full of reward.

The second of the great discoveries, which also has yielded chiefly to plant physiology its most important results, but has not yet been employed at its full value in the science of the physiology of animals, is the fact that organisms are composed of cells. The beginnings of the cell-theory are to be found in botanical studies. The microscopists of the seventeenth and eighteenth centuries, especially Malpighi, Treviranus,Mohl,and Meyen found that plants are composed of small microscopic chambers, or cells, and elongated tubes which have liquid contents. The elongated tubes soon proved themselves to be structures that arise from series of cells by a dissolution of the transverse walls. Brown found next a more solid nucleus as a wide-spread structure in the liquid cellcontents. But Schleiden first put into general form the idea that all plants are composed of cells, and he distinguished as an essential constituent of the cell-contents, besides the cell-sap and nucleus, the semi-liquid motile plant-slime, which was termed by Mohl protoplasm. In the meantime the wide occurrence of cells in the animal kingdom had become recognised, and, soon after Schleiden, Schwann founded the cell-theory for the animal kingdom by showing that animals are composed of cells or cell-products, and in their development progress from forms that contain only a few similar cells. Later, embryology established the fact that in general all organisms are developed from a single cell, the eggcell, into a cell-community which may become large and powerful,

and in which the various parts, tissues, and organs consist of specific forms of cells. Although this knowledge carried with it the fact that the cell is the element of the living organism and the place where the life-processes occur, nevertheless, the cell, except in botany and embryology, has not yet been made a subject of special physiological study. We shall see presently that precisely in this direction is to be expected an essential advance in the physiology of the future.

The third discovery, which thus far has not been fruitful in physiology, is that of descent in the organic world. The theory of descent, sketched in its outlines by Lamarck, and firmly founded by Darwin upon the principle of selection, has produced a great revolution in all morphological research, and impressed upon modern morphology its characteristic stamp. The theory shows that all the varied forms of organisms stand in genetic relationship to one another by descent, and that ultimately all have been derived from the simplest organisms. The theory of selection ascribes the enormous variety of forms to natural selection conditioned by the struggle for existence ; in this struggle only those individuals of a generation survive that are best adapted to existing external conditions — in other words, those that are best fitted to live. Thus, after an oblivion of more than two thousand years the ancient idea of Empedocles of the descent and gradual change of the organic world by selection has celebrated its resurrection in the present century by the empirical foundation - work of Darwin. Embryology, so far as it relates to the development of form in organisms, has flourished to an unexpected degree from the powerful stimulus given it as the result of Darwin's theory, especially by Haeckel and his pupils, but so far physiology has not availed itself of the evolution idea. The evolution of vital activities, the origin and development of the many functions possessed by the individual parts of the living body, is thus far almost a terra incognita. During the last few decades but one physiological problem of evolution, the problem of heredity, has been very actively discussed, and this almost exclusively from the zoological side. But the point has now been reached where experimental physiology alone is able to bring about further advance.

It has been learned that the problem of physiology lies in the explanation of vital phenomena, and it has been seen, in its main features, how physiological research has developed in the course of history. It is now incumbent upon us to summarise with reference to the development of science what physiology has already accomplished in the direction of its established goal, and to inquire by what path it may reach this goal. The aim of Physiology is to explain vital phenomena, i.e., to discover their elementary causes, to put them into causal relation with one another, to see whether their elementary causes are the same as those of the phenomena of inorganic nature. What has been accomplished in this direction ?

The answer brings little encouragement, for, when the various branches of physiology are carefully reviewed, it is found that thus far practically nothing has been learned beyond the gross mechanical and chemical activities of the vertebrate body. The causes upon which these activities depend are, for the most part, a complete puzzle. We know that respiration depends upon the laws of aerodynamics; by the rhythmic diminution and increase of pressure of the air in the lungs, as a result of the contraction and relaxation of the respiratory muscles, the air streams passively in and out ; oxygen is removed from it by the red corpuscles of the blood and is chemically united with the substance of the corpuscles. But we have scarcely an idea as to how the contraction of the respiratory muscles comes about, or what events call forth the change of form, termed contraction and relaxation, and the performance of work in the individual muscle-cells.

We know that the circulation of Hood in our bodies follows the laws of hydrodynamics, that it is conditioned by the rhythmic variation of differences of pressure within the vascular system, which are brought about by the contraction and relaxation of the heart-muscle. We have here again exactly the same problem as in respiration, for, although Engelrnann has recently proved that the causes of the rhythmic contractions of the cardiac muscle lie in the living substance of the muscle-cells, as to the manner in which the contractions come about physiology has enlightened us very little.

We know that the digestion of the ingested food takes place strictly in accordance with chemical laws ; the chemical substances secreted by the gland-cells of the digestive canal transform the food chemically, exactly as we can imitate the processes by the help of those digestive secretions outside the body in the testtube. But physiological chemistry leaves still unexplained how the gland-cells come to secrete their specific substances, why the cells of the salivary glands produce only ptyalin, and the cells of the gastric glands only pepsin, although the same food is brought to both by the blood.

We know that in resorption the food-stuffs, changed chemically by the digestive juices, are taken up through the cells of the intestinal wall into the body. We know, moreover, that a great part of the ingested fat, after being divided into microscopic globules, is taken into the protoplasmic bodies of the intestinal epithelium-cells by their own activity, while the same cells do not take up other particles of equal microscopic size, such as granules of pigment. But Physiology has not yet learned how this selective faculty of the intestinal epithelium-cells is to be explained mechanically.

We have seen how in the development of the human body the succession of definite morphological stages up to the complete man, which previously was so mysterious, may be understood naturally from the fundamental law of biogenesis. But it is still a much-debated question how in this development of the cells that arise from the segmentation of the egg some become gland-cells, others nerve-cells, and others epidermis-cells. We have learned that the movements of the skeletal bones, the arms, the legs, and the joints, follow purely mechanical and mathematical laws, especially the laws of the action of levers. But the action of the skeletal muscles which causes the movement of the skeletal bones is the same puzzle that is mentioned above, namely, the contraction of the muscle-cells.

From the law of the conservation of energy we know that the heat and the electricity produced by the living body are derived from chemical changes which the ingested food undergoes in the body-tissues. But we do not know at all with what chemical processes the cells of the various tissues are concerned in the production of this heat and electricity. We know, finally, that the higher sense-organs of man are constructed in accordance with the principles of physical apparatus ; the eye, e.g., according to the principle of a camera obscura, so that a reduced inverted image of an object in the external world is formed upon its background according to the laws of the refraction of light. But it is a constant puzzle as to what occurs in this process in the retinal cells and how from them by the mediation of the optic nerves the ganglion-cells in the brain are excited to produce in us the idea of the image.

This enumeration might be long continued, but what has been said suffices for the recognition of a general fact. Everywhere, to whatever branches of physiology we may turn, wherever the gross activities of the body are traced to the activity of the individual cells, we always come upon an unsolved problem. The pessimist, indeed, might be led to maintain with Bunge ('94): " All processes in the organism which may be explained mechanically are no more phenomena of life than are the movements of the leaves and branches of a tree that is shaken by the storm, or the movement of the pollen that the wind wafts from the male poplar to the

female." But, if we despair of a chemico-physical explanation of vital phenomena, nothing remains but to take refuge again in the long-buried doctrine of vital force. In fact, very recently this idea has again appeared in various places, notably in the writings of Hanstein, Kerner, Bunge, Rindfleisch, and other men of science. We might, however, be much more inclined to despair if we should look at the field of psychical phenomena. In the physiology of the brain and the sense-organs, indeed, much has been cleared up concerning the physical relations of certain psychical processes. But the old riddle of the causal relations between body and mind, which occupied so fully the thinking intellect even in earliest times, remains apparently wholly untouched by natural science.

Under such disheartening conditions the investigator is constantly oppressed by the questions : Are there limits to our knowledge of vital phenomena ? If so, where do these limits lie ? Or are we upon a false path ? Was our attitude of inquiry into nature a mistaken one, so that we have not understood her answer ? Are there limits to our knowledge of nature ? And if so, where do they lie ? These questions have repeatedly arisen in the present generation, which is proud of its achievements in natural science, and have been treated in various ways. We can most fittingly consider them in connection with the well-known address of E. du Bois-Reymond ('84), " Ueber die Grenzen des NaturerJcennens," in which the recently deceased author, who was a master of language among German naturalists, discussed this theme in his accomplished style.

With the lack of philosophical methods of thought which unfortunately is so wide-spread in the science of to-day, the most remarkable ideas upon the basis of our knowledge of nature are often met with. This circumstance unfortunately justifies speculative philosophy in looking with contempt upon science, its rival in the recognition of truth. It is, therefore, necessary to examine these questions somewhat carefully, and, first, to inquire concerning the limits of knowledge, not only in organic, but in all nature.

Modern science, especially physics and chemistry, is here the leader, and endeavours to reduce all the phenomena of the physical world to motions of atoms. Accordingly, du Bois- Reymond, in order to obtain a fixed point upon which to base his considerations, defines a knowledge of nature as follows : " A knowledge of nature — more accurately expressed, scientific knowledge or knowledge of the physical world, with the aid and in the sense of theoretical natural science — is the reduction of changes in matter to the motions of atoms, which motions are accomplished by the intrinsic forces of the atoms independently of time ; in other words, it is the resolution of natural events into the mechanics of atoms." Recent science has, in fact, succeeded in showing in gross outline how natural phenomena may be derived from definite motions of atoms. We know that in all bodies the atoms are moving, in gaseous bodies very actively, in liquids more slowly, in solids very little. We know that light, heat and electricity depend upon regular, excessively rapid vibrations of atoms ; that sound is caused by definite modes of atomic vibration : and that chemical changes of bodies are conditioned likewise by characteristic movements and rearrangements of atoms.

Following a fanciful conceit of Laplace, who imagines a human mind perfected to the highest degree and possessing such a knowledge of atomic motions as we have in astronomy of the motions of the stars, du Bois-Reymond continues : " If we were to imagine all changes in the physical world to be resolved, into the motions of atoms, which are due to constant intrinsic atomic forces, the universe would be known in the scientific sense. The condition of the world at any period of time would appear as the immediate result of its condition during the previous period and the immediate cause of its condition during the following period. Law and chance would be merely other names for mechanical necessity. A stage in the knowledge of nature can be conceived in which the whole world-process would be represented by one mathematical formula, by one immeasurable system of simultaneous differential equations, from which could be deduced the place, direction of movement, and velocity of every atom of the universe at every moment."

The human mind is only " a feeble image," it is true, of such a mind fancied by Laplace, but it differs from the latter only in degree, and in the achievements of the latter we can perceive the ideal which the human mind in its development is constantly approaching. Let us imagine for once that we had reached this ideal and were in possession of the " world-formula." What would then be gained ? In order to explain a definite phenomenon of nature, we would need only to introduce into the world-formula certain values resulting from observation, and by computation we would be able to prove the phenomenon in question to be a necessary consequence of our known observations. Our craving for causality would perhaps be captivated for awhile by this play, but soon it would become free again and would call to us with louder and louder voice. So far so good ; we can now understand all phenomena of the physical world in their causal relations to each other ; we can explain them as perfectly definite motions of atoms ; but what is an atom ?'

Here, according to du Bois-Reymond, we stand at one limit of our knowledge of nature. What an atom is, i.e., what matter endowed with energy is, the world-formula does not explain. If we ask how we arrive at the conception of an atom, we find that we conceive it as an excessively small, indivisible, elementary part of a body, derived by continued division of the body ; but if a body be continually divided until its atoms are reached, nothing but body is obtained. Atoms are bodies, and have the general characteristics of bodies. We cannot, therefore, expect to obtain by division something that elucidates the nature of the body. When we explain an unknown phenomenon by the motions of atoms, we merely resolve it into unknown phenomena. What an atom is, we do not learn, for it has only the properties which we attribute to it on the basis of the sense-perception of what large bodies show us, i.e., it is hard, impenetrable, possesses form, and moves. But we obtain not the slightest information regarding the nature of the matter that is endowed with energy, i.e., that of which the physical world consists. Our craving for causality remains, therefore, in this respect unsatisfied, and as the result of our analysis we find ourselves at the first limit of our knowledge.

But this is not the only limit. If, again, we possessed " astronomical knowledge " of the physical world, as du Bois-Reymond expresses it, i.e., the same mathematically exact knowledge of the motions of atoms that we have of the motions of the heavenly bodies, we would then, indeed, understand all phenomena of the physical world, but we would not understand how consciousness arises, how in general a psychical phenomenon, even the very simplest, comes to be. If we had, e.g., astronomical knowledge of our brain, we would know the position and motion of every atom at every moment ; we could also follow definitely the specific physical changes, rearrangements, and motions of atoms inseparably associated with specific psychical phenomena, and " it would be," as du Bois-Reymond says, " of unbounded interest, if with our mental eye turned inward we could observe the cerebral mechanics of an arithmetical problem, like the mechanics of a calculating machine ; or if we could know what dance of the atoms of carbon, hydrogen, nitrogen, oxygen, phosphorus and other elements, corresponds to the delight of musical sensation, what whirl of such atoms to the acme of sense-enjoyment, what molecular storm to the frantic pain resulting from maltreatment of the nervus trigeminus."

We could know all these if we possessed " astronomical knowledge " of the brain. We could thus convince ourselves by selfobservation that consciousness is inseparably associated with atomic motion. But with all this it would remain for ever concealed from us how consciousness arises, how the simplest psychical phenomenon comes to be. However carefully we might follow the motions of individual atoms in the brain, we would see only motions, collisions, and again motion. Thus, it is evident that a mechanical explanation of consciousness, of psychical phenomena, from the motions of atoms is an impossibility for us, and we find ourselves at a second limit of our knowledge of nature, which appears not less impassable than that of a knowledge of matter and energy.

But supposing the first to be passed, and the riddle of matter and energy to be solved, how would it be with the second limit ? Would it be passed at the same time or would it still be impassable ? We can evidently imagine consciousness, or rather the simplest form of mind, to be inherent in the nature of an atom, and, therefore, to be known when the nature of matter is known. In fact, this idea would be the only one that could be adopted by a monistic science, which seeks to explain all phenomena by one principle ; and Haeckel especially, who is an energetic advocate of monism among men of science, has always maintained it. du Bois-Reymond alludes to such a possibility only briefly when he says : " Finally, the question arises, whether the two limits of our knowledge of nature may not perhaps be the same, i.e., whether, if we understand the nature of matter and energy, we may not also understand how under certain conditions matter may have sensa tions, desires, and thoughts. This idea is, of course, the simplest one, and according to the known principles of investigation is preferable to its opposite, according to which, as before said, the world appears doubly inconceivable. But it lies in tfye nature of things that we cannot elucidate this point, and all further words concerning it are idle." Therefore, " as to the riddle of matter and energy and their conceptions," du Bois-Reymond decides upon complete renunciation and proclaims to science not only a temporary " ignoramus," but an eternal and demonstrative " ignorabimus"

We have followed du Bois-Reymond's course of thought thus in detail, in order to show that the knowledge assumed by him as the starting-point of his considerations very soon encounters obstructions, in view of which the world appears incomprehensible. But eternal renunciation falls heavily upon the indefatigable thinker, and he is bound to ask whether this assumed path of knowledge is a right one, whether the definition of a knowledge of nature as a resolution into the mechanics of atoms is correct or justified. We will, therefore, test this basis of our considerations and inquire what knowledge is.

For this purpose we will take the conception " knowledge " in its widest and most general sense. One indispensable condition of the conception is the assumption that something exists. If we make this assumption, if we have something real or actual, a fixed point, then knowledge is simply the causal reduction of all phenomena to this reality. We have a measure for knowledge in the satisfying of our craving for causality ; and the latter will necessarily be satisfied, when once we have placed all phenomena in causal relation to the one reality.

Nevertheless, an objection may here be raised. Let us suppose that we have succeeded in reducing all phenomena to the one reality. (This reality appears in the different philosophical systems under very different names, such as God, thing-in-itself, the unknown, etc. — the terms are equivalent and without material significance.) The question would then arise, whether our craving for causality would be satisfied, or whether it would not force us still farther to ask, What is this thing which exists, this reality, the unknown, the thing-in-itself, God, or whatever it is termed ? In the latter case, here, again, would be a limit to our knowledge. If we understand it rightly, however, this limit would be a logical error, a false conclusion. Our craving for causality arose and became established in the course of evolution by the continual reduction of effects to causes, and it is easily possible that in the present case it would continue for awhile from inertia to hold before us the question, why ? But it is evident that we would thus be guilty of an error of reasoning ; for, if all phenomena were reduced to the one reality, it would be a complete contradiction to wish to know that reality in terms of non-reality. The demand that, after complete knowledge of the world, we must know the world still more involves an evident absurdity. Hence, the above objection is only an apparent one.

We assume, therefore, the desire to reduce all phenomena to that which is real. Then the question arises, What is real ? Here we come in contact with a mistaken view which is especially wide-spread in science and has been faithfully handed down from primitive time as an heirloom from the childhood of the human mind. This is the view that the physical world existing outside of us and independent of our own mind is real, and that, accordingly, we must reduce all phenomena to its laws. The impossibility of such an undertaking is plainly shown in the above argument of du Bois-Reymond. Yet a great many men of science — among those who, like du Bois-Reymond, have reflected upon the limitations of human knowledge, we need mention only the gifted botanist Nageli ('77) — have held it to be possible that even psychical phenomena may be resolved into the processes of matter. Hence it is useful to clarify our ideas as to what matter really is.

At first sight bodies appear to us as actual objects outside of our own minds. Any doubt as to the existence of a physical world outside of mind, will appear absurd to one who has not reflected upon it : a body, e.g., a stone, a tree, a man, which we look upon, really exists, no one will deny this ; we actually see the body, others see it ; and we say it exists. We are right ; without a doubt it exists, but it does not exist outside our mind; for, when we examine carefully the grounds for speaking as we do, we find that what we believe we see or feel as a body outside our mind is actually something quite different.

Let us prove this. We have created our knowledge of a physical world by means of sense-perception. The question as to what can and does give us this knowledge is, therefore, one belonging to the physiology of the senses. Now the physiology of the senses shows that all that comes in through the door of our senses affords us, simply and solely, sensations. The many features that constitute the image of a body, e.g., a piece of gold, are so many different sensations, e.g., a yellow colour, hardness, weight and coldness. Persons with an innate defect in a sense, in whom a certain group of sensations is not mediated, e.g., persons born blind, have, therefore, an idea of the physical world that is wholly different from that of normal persons. This is clearest in those interesting cases in which persons who are born blind and have constructed their physical world solely by means of the senses of touch, hearing, smell, taste, etc., have been made to see by surgical operations. If objects that such persons have often had in their hands be brought for the first time before their eyes without their examining them by the other senses, e.g., by touching, they do not recognise them: a ball appears to them as something wholly new, and only when they touch it do they realise to their surprise its identity. At that moment a new world begins to arise in them. The physical world depends, therefore, wholly upon the development of our sense-organs ; to animals with senseorgans developed differently from ours it must appear very different, in proportion as they receive other sensations. With our death, with the destruction of the senses and the nervous system, the physical world in its previous form disappears.

These facts are of far-reaching significance. They show that what appears to us as matter is in reality our own sensations, or ideas, our own mind. When I see a body or perceive it by means of my other senses, in reality I have not a body outside of myself but only a number of sensations in my mind. Beyond these I know nothing concerning it and can only form hypotheses. It is necessary that we accustom ourselves to this fundamental truth, and that we get rid of the error of the existence of a physical world outside of mind. In order to facilitate this let us consider the consequences of this truth.