By the distinguished labors of Emil Fischer' even the structure of carbohydrate bodies has been determined, and bodies belonging to this group have been synthetically constructed in the laboratory. Moreover, the work of Schützenberger, Grimaux, and Pickering gives promise that before long proteid bodies may be produced by similar methods. Physiologists have shown, furthermore, that the digestion that takes place in the stomach or intestine may be effected also in test-tubes, and at the present day probably no one doubts that in the act of digestion we have to deal only with a series of chemical reactions which in time will be understood as clearly as it is possible to comprehend any form of chemical activity. Indeed, the whole history of food in the body follows strictly the great physical laws of the conservation of matter and of energy which prevail outside the body. No one disputes. the proposition that the material of growth and of excretion comes entirely from the food. It has been demonstrated that the measureable energy given off from the body is all contained potentially within the food that is eaten." Living things, so far as can be determined, can only transform matter and energy; they cannot create or destroy them, and in this respect they are like inanimate objects. But, in spite of the triumphs that have followed the use of the experimental method in physiology, every one recognizes that our knowledge is as yet very incomplete. Many important manifestations of life cannot be explained by reference to any of the known facts or laws of physics and chemistry, and in some cases these phenomena are seemingly removed from the field of experimental investigations. As long as there is this residuum of mystery connected with any of the processes of life, it is but natural that there should be two points of view. Most physiologists believe that as our knowledge and skill increase these mysteries will be explained, or rather will be referred to the same great final mysteries of the action of matter and energy under definite laws, under which we now classify the phenomena of lifeless matter. Others, however, find the difficulties too great,—they perceive that the laws of physics and chemistry are not completely adequate at present to explain all the phenomena of life, and assume that they never will be. They suppose that there is something in activity in living matter which is not present in dead matter, and which for want of a better term may be designated as vital force or vital energy. However this may be, it seems evident that a doctrine of this kind stifles inquiry. Nothing is more certain than the fact that the great advances made in physiology during the last four decades are mainly owing to the abandonment of this idea of an unknown vital force and the determination on the part of experimenters to make the greatest possible use of the known laws of nature in explaining the phenomena of life. There is no reason to-day to suppose that we have exhausted the results to be obtained by the application of the methods of physics and chemistry to the study of living things, and as a matter of fact the great bulk of physiological research is proceeding along these lines. It is interesting, however, to stop 1 Die Chemie der Kohlenhydrate, Berlin, 1894.

2 Rubner: Zeitschrift für Biologie, Bd. xxx. S. 73, 1894.

for a moment to examine briefly some of the problems which as yet have escaped satisfactory solution by these methods.

The phenomena of secretion and absorption form important parts of the digestive processes in higher animals, and without doubt are exhibited in a minor degree in the unicellular types. In the higher animals the secretions may be collected and analyzed, and their composition may be compared with that of the lymph or blood from which they are derived. It has been found that secretions may contain entirely new substances not found at all in the blood, as for example the mucin of saliva or the ferments and HCl of gastric juice; or, on the other hand, that they may contain substances which, although present in the blood, are found in much greater percentage amounts in the secretion-as, for instance, is the case with the urea eliminated in the urine. In the latter case we have an instance of the peculiar, almost purposeful, elective action of gland-cells of which many other examples might be given. With regard to the new material present in the secretions, it finds a sufficient general explanation in the theory that it arises from a metabolism of the protoplasmic material of the gland-cell. It offers, therefore, a purely chemical problem which may and probably will be worked out satisfactorily for each secretion. The selective power of gland-cells for particular constituents of the blood is a more difficult question. We find no exact parallel for this kind of action. in chemical literature, but there can be no reasonable doubt that the phenomenon is essentially a chemical or physical reaction involving the activity of some of the forms of energy with which the study of inanimate objects has already made us partially familiar. We may indulge the hope that the details of the reaction will be discovered by more complete chemical and microscopical study of the structure of these cells. If in the meantime the act of selection is spoken of as a vital phenomenon, it is not meant thereby that it is referred to the action of an unknown vital force, but only that it is a kind of action dependent upon the living structure of the cell-substance.

The act of absorption of digested products from the alimentary canal was for a time supposed to be explained completely by the laws of imbibition, diffusion, and osmosis. The epithelial lining and its basement membrane form a septum dividing the blood and lymph on the one side from the contents of the alimentary canal on the other. Inasmuch as the two liquids in question are of unequal composition with regard to certain constituents, a diffusion stream should be set up whereby the peptones, sugar, salts, etc. would pass from the liquid in the alimentary canal, where they exist in greater concentration, into the blood, where the concentration is less. Careful work of recent years has shown that the laws of diffusion and osmosis are not adequate to explain fully the absorption that actually occurs; a more detailed account of the difficulties met with may be found in the section on Digestion and Nutrition. It has become customary to speak of absorption as caused in part by the physical laws of diffusion and osmosis, and in part by the vital activity of the epithelial cells. It will be noticed that the vital property in this case is again an elective affinity for certain constituents similar to that which has been

referred to in discussing the act of secretion. The mere fact that the physical theory has proved so far to be insufficient is in itself no reason for abandoning all hope of a satisfactory explanation. Most physiologists probably believe that further experimental work will bring this phenomenon out of its obscurity and show that it is explicable in terms of known physical and chemical forces exerted through the peculiar substance of the absorptive cell.

The facts of heredity and consciousness offer difficulties of a much graver character. The function of reproduction is two-sided. In the first place there is an active multiplication of cells, beginning with the segmentation of the ovum into two blastomeres, and continuing in the larger animals to the formation of an innumerable multitude of cellular units. In the second place there is present in the ovum a form-building power of such a character that the great complex of cells arising from it produces not a heterogeneous mass, but a definite organism of the same structure, organ for organ and tissue for tissue, as the parent form. The ovum of a starfish develops into a starfish, the ovum of a dog into a dog, and the ovum of man into a human being. Herein lies the great problem of heredity. The mere multiplication of cells by direct or indirect division is not beyond the range of a conceivable mechanical explanation. Given the properties of assimilation and contractility it is possible that the act of cell-division may be traced to purely physical and chemical causes, and already cytological work is opening the way to credible hypotheses of this character. But the phenomena of heredity, on the other hand, are too complex and mysterious to justify any immediate expectation that they can be explained in terms of the known properties of matter. The crude theories of earlier times have not stood the test of investigation by modern methods, the microscopic anatomy of both ovum and sperm showing that they are to all appearances simple cells that exhibit no visible signs of the wonderful potentialities contained within them. Histological and experimental investigation has, however, cleared away some of the difficulties formerly surrounding the subject, for it has shown with a high degree of probability that the power of hereditary transmission resides in a particular substance in the nucleus, namely in the so-called chromatin materal that forms the chromosomes. The fascinating observations that have led to this conclusion promise to open up a new field of experimentation and speculation. It seems to be possible to study heredity by accepted scientific methods, and we may therefore hope that in time more light will be thrown upon the conditions of its existence and possibly upon the nature of the forces concerned in its production.

In the facts of consciousness, lastly, we are confronted with a problem seemingly more difficult than heredity. In ourselves we recognize different states of consciousness following upon the physiological activity of certain parts of the central nervous system. We know, or think we know, that these so-called psychical states are correlated with changes in the protoplasmic material of the cortical cells of the cerebral hemispheres. When these cells 1 Wilson: The Cell in Development and Inheritance, 1896.

are stimulated, psychical states result; when they are injured or removed, psychical activity is depressed or destroyed altogether according to the extent of the injury. From the physiological standpoint it would seem to be as justifiable to assert that consciousness is a property of the cortical nerve-cells as it is to define contractility as a property of muscle-tissue. But the shortening of a muscle is a physical phenomenon that can be observed with the senses-be measured and theoretically explained in terms of the known properties of matter. Psychical states are, however, removed from such methods of study; they are subjective, and cannot be measured or weighed or otherwise estimated with sufficient accuracy and completeness in terms of our units of energy or matter. There must be a causative connection between the objective changes in the brain-cells and the corresponding states of consciousness, but the nature of this connection remains hidden from us; and so hopeless does the problem seem that some of our profoundest thinkers have not hesitated to assert that it can never be solved. Whether or not consciousness is possessed by all animals it is impossible to say. In ourselves we know that it exists, and we have convincing evidence, from their actions, that it is possessed by many of the higher animals. But as we descend in the scale of animal forms the evidence becomes less impressive. It is true that even the simplest forms of animal life exhibit reactions of an apparently purposeful character which some have explained upon the simple assumption that these animals are endowed with consciousness or a psychical power of some sort. All such reactions, however, may be explained, as in the case of reflex actions from the spinal cord, upon purely mechanical principles, as the necessary response of a definite physical or chemical mechanism to a definite stimulus. To assume that in all cases of this kind conscious processes are involved amounts to making psychical activity one of the universal and primitive properties of protoplasm whether animal or vegetable, and indeed by the same kind of reasoning there would seem to be no logical objection to extending the property to all matter whether living or dead. All such views are of course purely speculative. As a matter of fact we have no means of proving or disproving, in a scientific sense, the existence of consciousness in lower forms of life. To quote an appropriate remark of Huxley's made in discussing this same point with reference to the crayfish, "Nothing short of being a crayfish would give us positive assurance that such an animal possesses consciousness." The study of psychical states in ourselves, for reasons which have been suggested above, does not usually form a part of the science of physiology. The matter has been referred to here simply because consciousness is a fact that our science cannot as yet explain.

So far, some of the broad principles of physiology have been considered— principles which are applicable with more or less modification to all forms of animal life and which make the basis of what is known as general physiology. It must be borne in mind, however, that each particular organism possesses a special physiology of its own, which consists in part in a study of the properties exhibited by the particular kinds or variations of protoplasm in each individual, and in large part also in a study of the various mechan

isms existing in each animal. In the higher animals, particularly, the combinations of various tissues and organs into complex mechanisms such as those of respiration, circulation, digestion, or vision, differ more or less in each group and to a minor extent in each individual of any one species. It follows, therefore, that each animal has a special physiology of its own, and in this sense we may speak of a special human physiology. It need scarcely be said that the special physiology of man is very imperfectly known. Books like the present one, which profess to treat of human physiology, contain in reality a large amount of general and special physiology that has been derived from the study of lower animal forms upon which exact experimentation is possible. Most of our fundamental knowledge of the physiology. of the heart and of muscles and nerves has been derived from experiments upon frogs and similar animals, and much of our information concerning the mechanisms of circulation, digestion, etc. has been obtained from a study of other mammalian forms. We transfer this knowledge to the human being, and in general without serious error, since the connection between man and related mammalia is as close on the physiological as it is on the morphological side, and the fundamental or general physiology of the tissues seems to be everywhere the same. Gradually, however, the material for a genuine special human physiology is being acquired. In many directions special investigation upon man is possible; for instance, in the study of the localization of function in the cerebral cortex, or the details of body metabolism as obtained by examination of the excreta, or the peculiarities of vaso-motor regulation as revealed by the use of plethysmographic methods, or the physiological optics of the human eye. This special information, as rapidly as it is obtained, is incorporated into the text-books of human physiology, but the fact remains that the greater part of our so-called human physiology is founded upon experiments upon the lower aninals.

Physiology as a science is confessedly very imperfect; it cannot compare in exactness with the sciences of physics and chemistry. This condition of affairs need excite no surprise when we remember the very wide field that physiology attempts to cover, a field co-ordinate in extent with the physics as well as the chemistry of dead matter, and the enormous complexity and instability of the form of matter that it seeks to investigate. The progress of physiology is therefore comparatively slow. The present era seems to be one mainly of accumulation of reliable data derived from laborious experiments and observations. The synthesis of these facts into great laws or generalizations is a task for the future. Corresponding with the diversity of the problems to be solved we find that the methods employed in physiological research are manifold in character. Inasmuch as animal organisms are composed either of single cells or aggregates of cells, it follows that every anatomical detail with regard to the organization of the cell itself or the connection between different cells, and every advance in our knowledge of the arrangement of the tissues and organs that form the more complicated mechanisms, is of immediate value to physiology. The microscopic anatomy of the cell (a branch of