CHAPTER III

OUR LIFE

Development of Physiology in Antiquity and the Middle Ages: Galen--Experiment and Vivisection--Discovery of the Circulation of the Blood by Harvey--Vitalism: Haller--Teleological and Vitalistic Conception of Life--Mechanical and Monistic View of the Physiological Processes--Comparative Physiology in the Nineteenth Century: Johannes Müller--Cellular Physiology: Max Verworn--Cellular Pathology: Virchow--Mammal Physiology--Similarity of all Vital Activity in Man and the Ape

It is only in the nineteenth century that our knowledge of human life has attained the dignity of a genuine, independent science; during the course of the century it has developed into one of the highest, most interesting, and most important branches of knowledge. This "science of the vital functions," physiology, had, it is true, been regarded at a much earlier date as a desirable, if not a necessary, condition of success in medical treatment, and had been constantly associated with anatomy, the science of the structure of the body. But it was only much later, and much more slowly, than the latter that it could be thoroughly studied, as it had to contend with much more serious difficulties.

The idea of life, as the opposite of death, naturally became the subject of speculation at a very early age. In the living man, just as in other living animals, there were certain peculiar changes, especially movements, which were wanting in lifeless nature: spontaneous locomotion, the beat of the heart, the drawing of the breath, speech, and so forth. But the discrimination of such "organic movements" from similar phenomena in inorganic bodies was by no means easy, and was frequently impossible; the flowing stream, the flickering flame, the rushing wind, the falling rock, seemed to man to exhibit the same movements. It was quite natural that primitive man should attribute an independent life to these "dead" bodies. He knew no more of the real sources of movement in the one case than in the other.

We find the earliest scientific observations on the nature of man's vital functions (as well as on his structure) in the Greek natural philosophers and physicians of the sixth and fifth centuries before Christ. The best collection of the physiological facts which were known at that time is to be found in the _Natural History_ of Aristotle; a great number of his assertions were probably taken from Democritus and Hippocrates. The school of the latter had already made attempts to explain the mystery; it postulated as the ultimate source of life in man and the beasts a volatile "spirit of life" (Pneuma); and Erasistratus (280 B.C.) already drew a distinction between the lower and the higher "spirit of life," the _pneuma zoticon_ in the heart and the _pneuma psychicon_ in the brain.

The credit of gathering these scattered truths into unity, and of making the first attempt at a systematic physiology, belongs to the great Greek physician Galen; we have already recognized in him the first great anatomist of antiquity (cf. p. 23). In his researches into the organs of the body he never lost sight of the question of their vital activity, their functions; and even in this direction he proceeded by the same comparative method, taking for his principal study the animals which approach nearest to man. Whatever he learned from these he applied directly to man. He recognized the value of physiological experiment; in his vivisection of apes, dogs, and swine he made a number of interesting experiments. Vivisection has been made the object of a violent attack in recent years, not only by the ignorant and narrow-minded, but by theological enemies of knowledge and by perfervid sentimentalists; it is, however, one of the _indispensable_ methods of research into the nature of life, and has given us invaluable information on the most important questions. This was recognized by Galen seventeen hundred years ago.

Galen reduces all the different functions of the body to three groups, which correspond to the three forms of the _pneuma_, or vital spirit. The _pneuma psychicon_--the soul--which resides in the brain and nerves, is the cause of thought, sensation, and will (voluntary movement); the _pneuma zoticon_--the heart--is responsible for the beat of the heart, the pulse, and the temperature; the _pneuma physicon_, seated in the liver, is the source of the so-called vegetative functions, digestion and assimilation, growth and reproduction. He especially emphasized the renewal of the blood in the lungs, and expressed a hope that we should some day succeed in isolating the permanent element in the atmosphere--the _pneuma_, as he calls it--which is taken into the blood in respiration. More than fifteen centuries elapsed before this _pneuma_--oxygen--was discovered by Lavoisier.

In human physiology, as well as in anatomy, the great system of Galen was for thirteen centuries the _Codex aureus_, the inviolable source of all knowledge. The influence of Christianity, so fatal to scientific culture, raised the same insuperable obstacles in this as in every other branch of secular knowledge. Not a single scientist appeared from the third to the sixteenth century who dared to make independent research into man's vital activity, and transcend the limits of the Galenic system. It was not until the sixteenth century that experiments were made in that direction by a number of distinguished physicians and anatomists (Paracelsus, Servetus, Vesalius, and others). In 1628 Harvey published his great discovery of the circulation of the blood, and showed that the heart is a pump, which drives the red stream unceasingly through the connected system of arteries and veins by a rhythmic, unconscious contraction of its muscles. Not less important were Harvey's researches into the procreation of animals, as a result of which he formulated the well-known law: "Every living thing comes from an egg" (_omne vivum ex ovo_).

The powerful impetus which Harvey gave to physiological observation and experiment led to a great number of discoveries in the sixteenth and seventeenth centuries. These were co-ordinated for the first time by the learned Albrecht Haller about the middle of the last century; in his great work, _Elementa Physiologiae_, he established the inherent importance of the science, independently of its relation to practical medicine. In postulating, however, a special "sensitive force or sensibility" for neural action, and a special "irritability" for muscular movement, Haller gave strong support to the erroneous idea of a specific "vital force" (_vis vitalis_).

For more than a century afterwards, from the middle of the eighteenth until the middle of the nineteenth century, medicine and (especially) physiology were dominated by the old idea that a certain number of the vital processes may be traced to physical and chemical causes, but that others are the outcome of a special vital force which is independent of physical agencies. However much scientists differed in their conceptions of its nature and its relation to the "soul," they were all agreed as to its independence of, and essential distinction from, the chemico-physical forces of ordinary "matter"; it was a self-contained force (_archaeus_), unknown in inorganic nature, which compelled ordinary forces into its service. Not only the distinctly psychical activity, the sensibility of the nerves and the irritability of the muscles, but even the phenomena of sense activity, of reproduction, and of development seemed so wonderful and so mysterious in their sources that it was impossible to attribute them to simple physical and chemical processes. As the free activity of the vital force was purposive and conscious, it led, in philosophy, to a complete _teleology_; especially did this seem indisputable when even the "critical" philosopher Kant had acknowledged, in his famous critique of the teleological position, that, though the mind's authority to give a mechanical interpretation of all phenomena is theoretically unlimited, yet its actual capacity for such interpretation does not extend to the phenomena of organic life; here we are compelled to have recourse to a _purposive_--therefore _supernatural_--principle. This divergence of the _vital_ phenomena from the _mechanical_ processes of life became, naturally, more conspicuous as science advanced in the chemical and physical explanation of the latter. The circulation of the blood and a number of other phenomena could be traced to mechanical agencies; respiration and digestion were attributable to chemical processes like those we find in inorganic nature. On the other hand, it seemed impossible to do this with the wonderful performances of the nerves and muscles, and with the characteristic life of the mind; the co-ordination of all the different forces in the life of the individual seemed also beyond such a mechanical interpretation. Hence there arose a complete physiological dualism--an essential distinction was drawn between inorganic and organic nature, between mechanical and vital processes, between material force and life force, between the body and the soul. At the beginning of the nineteenth century this vitalism was firmly established in France by Louis Dumas, and in Germany by Reil. Alexander Humboldt had already published a poetical presentation of it in 1795, in his narrative of the _Legend of Rhodes_; it is repeated, with critical notes, in his _Views of Nature_.

In the first half of the seventeenth century the famous philosopher Descartes, starting from Harvey's discovery of the circulation of the blood, put forward the idea that the body of man, like that of other animals, is merely an intricate machine, and that its movements take place under the same mechanical laws as the movements of an automaton of human construction. It is true that Descartes, at the same time, claimed for man the exclusive possession of a perfectly independent, immaterial soul, and held that its subjective experience, thought, was the only thing in the world of which we have direct and certain cognizance ("_Cogito, ergo sum_"). Yet this dualism did not prevent him from doing much to advance our knowledge of the mechanical life processes in detail. Borelli followed (1660) with a reduction of the movements of the animal body to purely physical laws, and Sylvius endeavored, about the same time, to give a purely chemical explanation of the phenomena of digestion and respiration; the former founded the _iatromechanical_, the latter the _iatrochemical_, school of medicine. However, these rational tendencies towards a natural, mechanical explanation of the phenomena of life did not attain to a universal acceptance and application; in the course of the eighteenth century they fell entirely away before the advance of teleological vitalism. The final disproof of the latter and a return to mechanism only became possible with the happy growth of the new science of comparative physiology in the forties of the present century.

Our knowledge of the vital functions, like our knowledge of the structure of the human body, was originally obtained, for the most part, not by direct observation of the human organism itself, but by a study of the more closely related animals among the vertebrates, especially the mammals. In this sense the very earliest beginning of human anatomy and physiology was "comparative." But the distinct science of "comparative physiology," which embraces the whole sphere of life phenomena, from the lowest animal up to man, is a triumph of the nineteenth century. Its famous creator was Johannes Müller, of Berlin (born, the son of a shoemaker, at Coblentz, in 1801). For fully twenty-five years--from 1833 to 1858--this most versatile and most comprehensive biologist of our age evinced an activity at the Berlin University, as professor and investigator, which is only comparable with the associated work of Haller and Cuvier. Nearly every one of the great biologists who have taught and worked in Germany for the last sixty years was, directly or indirectly, a pupil of Johannes Müller. Starting from the anatomy and physiology of man, he soon gathered all the chief groups of the higher and lower animals within his sphere of comparison. As, moreover, he compared the structure of extinct animals with the living, and the healthy organism with the diseased, endeavoring to bring together all the phenomena of life in a truly philosophic fashion, he attained a biological knowledge far in advance of his predecessors.

The most valuable fruit of these comprehensive studies of Johannes Müller was his _Manual of Human Physiology_. This classical work contains much more than the title indicates; it is the sketch of a comprehensive "comparative biology." It is still unsurpassed in respect of its contents and range of investigation. In particular, we find the methods of observation and experiment applied in it as masterfully as the philosophic processes of induction and deduction. Müller was originally a vitalist, like all the physiologists of his time. Nevertheless, the current idea of a vital force took a novel form in his speculations, and gradually transformed itself into the very opposite. For he attempted to explain the phenomena of life mechanically in every department of physiology. His "transfigured" vital force was not _above_ the physical and chemical laws of the rest of nature but entirely bound up with them. It was, in a word, nothing more than life itself--that is, the sum of all the movements which we perceive in the living organism. He sought especially to give them the same mechanical interpretation in the life of the senses and of the mind as in the working of the muscles; the same in the phenomena of circulation, respiration, and digestion as in generation and development. Müller's success was chiefly due to the fact that he always began with the simplest life phenomena of the lowest animals, and followed them step by step in their gradual development up to the very highest, to man. In this his method of _critical comparison_ proved its value both from the physiological and from the anatomical point of view. Johannes Müller is, moreover, the only great scientist who has equally cultivated these two branches of research, and combined them with equal brilliancy. Immediately after his death his vast scientific kingdom fell into four distinct provinces, which are now nearly always represented by four or more chairs--human and comparative anatomy, pathological anatomy, physiology, and the history of evolution. This sudden division of Müller's immense realm of learning in 1858 has been compared to the dissolution of the empire which Alexander the Great had consolidated and ruled.

Among the many pupils of Johannes Müller who, either during his lifetime or after his death, labored hard for the advancement of the various branches of biology, one of the most fortunate--if not the most important--was Theodor Schwann. When the able botanist Schleiden, in 1838, indicated the cell as the common elementary organ of all plants, and proved that all the different tissues of the plant are merely combinations of cells, Johannes Müller recognized at once the extraordinary possibilities of this important discovery. He himself sought to point out the same composition in various tissues of the animal body--for instance, in the spinal cord of vertebrates--and thus led his pupil, Schwann, to extend the discovery to all the animal tissues. This difficult task was accomplished by Schwann in his _Microscopic Researches into the Accordance in the Structure and Growth of Plants and Animals_ (1839). Thus was the foundation laid of the "cellular theory," the profound importance of which, both in physiology and anatomy, has become clearer and more widely recognized in each subsequent year. Moreover, it was shown by two other pupils of Johannes Müller that the activity of all organisms is, in the ultimate analysis, the activity of the components of their tissues, the microscopic cells--these were the able physiologist Ernst Brücke, of Vienna, and the distinguished histologist Albert Kölliker, of Würzburg. Brücke correctly denominated the cells the "elementary organisms," and showed that, in the body of man and of all other animals, they are the only actual, independent factors of the life process. Kölliker earned special distinction, not only in the construction of the whole science of histology, but particularly by showing that the animal ovum and its products are simple cells.

Still, however widely the immense importance of the cellular theory for all biological research was acknowledged, the "cellular physiology" which is based on it only began an independent development very recently. In this Max Verworn (of Jena) earned a twofold distinction. In his _Psycho-physiological Studies of the Protistae_ (1889) he showed, as a result of an ingenious series of experimental researches, that the "theory of a cell-soul" which I put forward in 1866[11] is completely established by an accurate study of the unicellular protozoa, and that "the psychic phenomena of the protistæ form the bridge which unites the chemical processes of inorganic nature with the mental life of the highest animals." Verworn has further developed these views, and based them on the modern theory of evolution, in his _General Physiology_. This distinguished work returns to the comprehensive point of view of Johannes Müller, in opposition to the one-sided and narrow methods of those modern physiologists who think to discover the nature of the vital phenomena by the exclusive aid of chemical and physical experiments. Verworn showed that it is only by Müller's comparative method and by a profound study of the physiology of the cell that we can reach the higher stand-point which will give us a comprehensive survey of the wonderful realm of the phenomena of life. Only thus do we become convinced that the vital processes in man are subject to the same physical and chemical laws as those of all other animals.

The fundamental importance of the cellular theory for all branches of biology was made clear in the second half of the nineteenth century, not only by the rapid progress of morphology and physiology, but also by the entire reform of that biological science which has always been deemed most important on account of its relation to practical medicine--pathology, or the science of disease. Many even of the older physicians were convinced that human diseases were natural phenomena, like all other manifestations of life, and should be studied scientifically, like other vital functions. Particular schools of medicine--the Iatrophysical and the Iatrochemical--had already, in the seventeenth century, attempted to trace the sources of disease to certain physical and chemical changes. However, the imperfect condition of science at that period precluded any lasting results of these efforts. Many of the older theories, which sought the nature of disease in supernatural and mystical causes, were almost universally accepted down to the middle of the nineteenth century.

It was then that Rudolf Virchow, another pupil of Müller, conceived the happy idea of transferring the cellular theory from the healthy to the diseased organism; he sought in the more minute metamorphoses of the diseased cells and the tissues they composed the true source of those larger changes which, in the form of disease, threaten the living organism with peril and death. Especially during the seven years of his professorship at Würzburg (1849-56) Virchow pursued his great task with such brilliant results that his _Cellular Pathology_ (published in 1858) turned, at one stroke, the whole of pathology and the dependent science of practical medicine into new and eminently fruitful paths. This reform of medicine is significant for our present purpose in that it led us to a monistic and purely scientific conception of disease. In sickness, no less than in health, man is subject to the same eternal "iron laws" of physics and chemistry as all the rest of the organic world.

Among the numerous classes of animals which modern zoology distinguishes the mammals occupy a pre-eminent position, not only on morphological grounds, but also for physiological reasons. As man belongs to the class of mammals (see p. 27) by every portion of his frame, we must expect him to share his characteristic functions with the rest of the mammals. Such we find to be the case. The circulation of the blood and respiration are accomplished in man under precisely the same laws and in the same manner as in all the other mammals--_and in these alone_; they are determined by the peculiar structure of their heart and lungs. In mammals only is all the arterial blood conducted from the left ventricle of the heart to the body by one, the _left_, branch of the aorta, while in birds it passes along the _right_ branch, and in reptiles along both branches. The blood of mammals is distinguished from that of any other vertebrate by the circumstance that its red cells have lost their nucleus (by reversion). The respiratory movements are effected largely by the diaphragm in this class of animals alone, because only in them does it form a complete partition between the pectoral and abdominal cavities. Special importance, however, in this highest class of animals, attaches to the production of milk in the breasts (_mammae_), and to the peculiar method of the rearing of the young, which entails the supplying of the offspring with the mother's milk. As this nutritive process reacts most powerfully on the other vital functions, and the maternal affection of mammals must have arisen from this intimate form of rearing, the name of the class justly reminds us of its great importance. In millions of pictures, most of them produced by painters of the highest rank, the "madonna with the child" is revered as the purest and noblest type of maternal love--the instinct which is found in its extreme form in the exaggerated tenderness of the mother-ape.

As the apes approach nearest to man of all the mammals in point of structure, we shall expect to hear the same of their vital functions; and that we find to be the case. Everybody knows how closely the habits, the movements, the sense activity, the mental life, and the parental customs of apes resemble those of man. Scientific physiology proves the same significant resemblance in other less familiar processes, particularly in the working of the heart, the division of the breasts, and the sexual life. In the latter connection it is especially noteworthy that the mature females of many kinds of apes suffer a periodical discharge of blood from the womb, which corresponds to the menstruation of the human female. The secretion of the milk in the glands and the suctorial process also take place in the female ape in precisely the same fashion as in women.

Finally, it is of especial interest that the speech of apes seems on physiological comparison to be a stage in the formation of articulate human speech. Among living apes there is an Indian species which is musical; the _hylobates syndactylus_ sings a full octave in perfectly pure, harmonious half-tones. No impartial philologist can hesitate any longer to admit that our elaborate rational language has been slowly and gradually developed out of the imperfect speech of our Pliocene simian ancestors.