General Physiology: An Outline of the Science of Life

In all fields of non-living nature, especially in physics and chemistry, investigation has shown that all phenomena thus far known and investigated may be reduced in the last instance to a single common cause, namely, the movement of very small material elements. The whole physical world is conceived as consisting of separate, indivisible, extremely small, elementary particles called atoms, and the various motions of the atoms, which fill universal space, are regarded as producing all phenomena in nature.

If it be the task of physiology to explain the occurrence of vital phenomena, i.e., to investigate their causes, it then becomes a question whether, in living nature likewise, all phenomena can be reduced to the motions of atoms, or whether it is necessary to take refuge in another principle. Next to determining the boundaries of the field of investigation, the chief task of physiology lies in answering this question. Since early times mankind has been conscious of the great gap that exists between two groups of vital phenomena, the physical and the mental. Hence the above question is a double one. If it be possible actually to reduce the physical phenomena of life to the same elementary causes as the phenomena of the lifeless world, the result will not necessarily hold good for psychical phenomena, and the relations between the physical and the psychical must be analysed. If it be impossible to trace psychical phenomena to the same ultimate cause as the events of the physical world, another explanation must be sought, and the important question will then arise whether psychical phenomena can be explained at all. But if it be allowed that they can be brought into causal relations with the phenomena of the physical world, the question will still remain. What are atoms ? The question of the possibility of answering this will then arise. If it can be answered, will our craving for causality then be satisfied ?

The investigation of life is thus confronted with a multitude of questions, which tax to the uttermost the capabilities of the human mind. An examination of the history of physiological research is not only interesting, but important for a correct judgment of the present condition of physiology and the future course which it has to take in order to accomplish its established purpose. The earliest traces of naive physiological ideas are lost in the impenetrable obscurity of prehistoric times. A picture of them has been handed down in the mythology of the early civilised races. This represents .a condition in which all knowledge and all formation of ideas are grouped about the veneration of higher

1 The account of the earlier epochs in the development of physiology is based upon the following works : K. Sprengel, Versuch einer pragmatischen Geschichte der Arzneikunde ; H. Haeser, Lehrbuch der Geschichte der Medicin. In his Elemente der allgemeinen Physiologic, Preyer gives a short sketch of the history of physiology based upon the latter book. beings. The primitive worship of the early races and the knowledge associated with it may be regarded as an indivisible whole, from which, in the course of the hundreds and thousands of years, theological, philosophical, scientific, and medical conceptions gradually and slowly have been crystallised out as independent groups of ideas.

The early notions of life were very naive and crude. All that moved was living and was endowed with mind. The property of motion was the criterion of life. Wind, water, fire and stars were personified. Meteorites which moved through the air, called " bsetyli," were regarded by the Phoenicians as endowed with mind, and were believed to be healing, while Susruta, the author of the Yajurveda, the most ancient Indian work upon the art of healing, represented all motile bodies as living, in distinction from nonmotile, or lifeless, bodies. The art of healing, which was almost wholly a doctrine of drugs, and in primitive ages was developed especially upon the Pontus, where witchcraft flourished and where Hecate was reverenced, was crudely empirical, was in league with magic and mystery, and wholly lacked a physiological basis.

In these earliest times only one class of phenomena received detailed consideration, namely, the higher psychical phenomena, which reveal man's life most directly to himself. A doctrine of the mind was developed even in ancient Egypt, probably under Indian influence, which had for its basis the dualism of body and mind, and reached its culmination in the idea of the passage of the mind after the death of the body into other bodies. Later, this notion was transplanted to Greece by the Greek philosophers, especially Pythagoras. In general, from the earliest times onward, the phenomena of mental life served as a peculiar stimulus for priests and philosophers, the earliest theorisers, and in antiquity, of all fields of investigation, psychology was cultivated the most.

While physiological notions were scarcely influenced by medicine until long after Hippocrates, in Greece they were enriched in a significant manner by the first blooming of philosophy as a distinct discipline independent of the priesthood. The oldest Greek philosophers, the Ionic " physiologists," the Eleatics, as well as the Atomists and the independent thinkers of the same time, whose aim was the development of a cosmology, were forced in the pursuit of this aim to reflect upon the origin of living nature. Whatever judgment may be passed upon the unbridled character of the speculations of these ancient thinkers, the correctness of their notions regarding many of the phenomena of life will always remain a very surprising fact. Among many of these early philosophers it is singular to meet with ideas which, after more than two thousand years, have again become current and are reckoned among the most important foundations of the present science of life. This is particularly true of opinions concerning the origin and development of

the organic world. The notion of the derivation of man from animallike ancestors originally inhabiting the water, is found clearly expressed by Anaximander (b. about 620 B.C.); and Heraclitus (about 500 B.C.) had an idea of the significance of the struggle for existence (epts). But the theory of Empedocles (b. 504 B.C.) upon the origin of living things is the clearest and most surprising. According to him, plants appeared first, then the lower animals, and from them the higher animals and, finally, men were developed by a process of perfection. The effective principle in this perfecting process he perceived in the fact that ill-adapted individuals are destroyed in the struggle for life, while those that are capable of living produce offspring. Almost twenty-five hundred years elapsed before this simple conception of the descent and natural selection of organisms, clearly expressed by Empedocles, was empirically grounded by Darwin and was established as the natural explanation of the otherwise marvellous multiplicity of organic forms.

Many ideas, more or less correct, regarding special physiological phenomena are found also among the early Greek philosophers. But these scattered truths are mingled with so many fantastic and purely arbitrary notions that, from their associations, they lose their real value. No coherent, systematic observations or reflections concerning vital phenomena exist before Aristotle. From the side of practical medicine, likewise, the investigation of life experienced no considerable advance, even when medical art, hitherto without a critic, was placed by Hippocrates (460-377 B.c.) upon a sound basis.

A physiological doctrine appeared first among the followers of Hippocrates, probably under the influence of Plato's philosophy, and it was soon perfected and controlled all the medical ideas of that time. This is the doctrine of the spirits (trvevfia), in the main thought of which can be found the first germ of a fundamental physiological truth. This doctrine asserts that the pneuma, an excessively subtile material agent, is attracted by the human lungs, passes from the lungs into the blood, and is distributed by the latter throughout the body. All vital phenomena depend upon the action of this agent. This conception, which, naturally, was adorned with all sorts of absurd accompaniments, suggests strongly our modern ideas concerning the role of oxygen in the organism.

The first intimation of an attempt to explain vital phenomena appears in the early Hippocratic doctrine of the pneuma. This was expanded, especially in the Alexandrian school, by Herophilus (about 300 B.C.) and Erasistratus (d. 280 B.C.), the latter of whom distinguished a 7rvev/j,a ^COTIKOV (vital spirits) in the heart and a (animal spirits) in the brain. From this it is evident that the problem of physiology, the explanation of vital phenomena, had already begun more or less clearly to be recognised. Hitherto, individual physiological facts had been observed, and physiological questions had been discussed incidentally. But now, the more clearly the problem of physiology began to be formulated, the more the treatment of physiological questions began to assume the character of scientific investigation.

Aristotle (384-322 B.C.), the great polyhistor of antiquity, established the preliminary conditions for this advance by accumulating .a vast mass of material in the form of facts. The significance of Aristotle's relation to physiology does not lie in explaining vital phenomena — very often his explanations are uncritical, and, moreover, they do not appear prominent in his work — but rather in observing and recording a great number of physiological phenomena. In the midst of this material by the side of striking and acute researches, there occurs, as might have been expected, much erroneous observation ; such, for example, is the origin of eels and frogs from mud by spontaneous generation. Nevertheless, his recorded observations form the basis of the new stage of development into which physiology passed after Aristotle, and which is •characterised by the clear recognition of the physiological problem and its vast importance in practical medicine.

After Aristotle, by his systematising work, had laid a broad empirical foundation for natural science, the doctrine of the pneuma received a wider extension among the later pneumatic physicians, especially through the efforts of Athenaeus and Aretaeus (both about 50 A.D.). It is in the nature of this doctrine, that it must endeavour to comprehend and explain the phenomena of life from a single point of view ; and, accordingly, we find now for the first time a clear, conscious recognition of the physiological problem and a systematic comprehension of physiological phenomena. The man who first clearly perceived the nature and significance of physiology was Galen (131 — about 200 A.D.). Galen saw that practical medicine could not thrive unless it were based upon a very detailed knowledge of the normal vital phenomena of the human body. The investigation of the vital functions of the body must be the first pre-requisite of an art of healing. This practical aim was the first incentive to the development of physiology, and controlled the science almost exclusively until the eighteenth century. Galen was also the first to recognise clearly the importance of a knowledge of the anatomy of the body in an understanding of the functions of its parts, and laid great value upon the dissection of animals ; he himself dissected pigs and monkeys especially. Moreover, he perceived the importance of animal experimentation in the investigation of physiological phenomena ; and, although the experimental method did not assume under him that exact form and that fundamental

importance which many centuries later Harvey knew how to give it, Galen himself practised vivisection upon pigs and monkeys. Along with general recognition of his immortal service, Galen has often been reproached with the charge that he was not content with collecting physiological facts, making observations and devising experiments, but that he felt strongly the necessity of arranging his collected material into a complete and comprehensive system of physiology, in which he allowed hypothesis and philosophical speculation a place that exact investigation ought to have filled. Nothing can be more unjust than this reproach. If Galen had been satisfied with ascertaining disconnected physiological facts, physiology and with it all medicine would not have been advanced one step farther than Aristotle had already brought them. Galen's greatest importance lies in the union of scraps of physiological knowledge into a coherent system. Isolated observations obtain value only in connection with other facts, and only a survey of the relations of facts makes possible further systematic progress. It is only natural that, in this first attempt to put together the material of physiological observation into a coherent picture of the life of the human body, recourse must now and then be had to hypothesis, even much bold hypothesis. The single fault from which Galen's system suffers is not its binding cement of philosophical speculation, but the peculiar dualism that misled him, in accordance with which, in explaining vital phenomena, he strove to give at the same time a place both to the rigid idea of necessity, which sprang from his exact scientific investigations, and to teleology, which was derived from the Aristotelian philosophy.

Nevertheless, in a just estimation of his time, when Aristotelian ideas had already begun a universal sway that was to last more than a thousand years, Galen can scarcely be reproached for this, the less when it is recalled that the teleological idea of a final purpose in all things appears here and there in modern natural science even to-day, quite independent of philosophy. Galen's system is based upon the doctrine of the spirits (pneuma). The causes of all the vital phenomena of the human body, which is composed of the four fundamental juices, viz. : the blood, the phlegm, the yellow and the black gall, are the three different forms of spirits, of which the animal spirits (irvevfjia ^V^LKOV) have their seat in the brain and the nerves, the vital spirits (irvevfjua fari/cov) in the heart, and the natural spirits (Trvev/jia (frvo-i/cov) in the liver. These three forms, which must be regenerated continually by the receipt of vital spirits from the air, are the agencies that maintain the functions of the respective organs. The body possesses many functions, but they may be arranged, according to the forms of the spirits, into three classes, and each function is carried on by a faculty (SiW/zt?) corresponding to its respective

pneuma. The psychical (animal) functions comprise thinking, feeling, and voluntary motion ; the sphygmical (vital) functions, the heart-beat, the pulse, and the production of heat ; the physical (natural) functions, nutrition, growth, secretion, and reproduction with its related activities. The blood is formed in the liver, and the veins arise there. Through the veins the blood goes to the right ventricle of the heart, where the useful part is separated from the useless ; the former is carried to the left ventricle, while the latter goes through the pulmonary artery to the lungs. In the lungs, the useless part is regenerated by the spirits and made useful. It is remarkable with what prophetic gift Galen pointed to a constituent of the air as the spirits, the nature of which he could not yet divine. He expresses clearly the supposition that it will be possible at some time to isolate that constituent of the air that forms the spirits. More than fifteen hundred years elapsed before Galen's supposition was confirmed by the discovery of oxygen by Priestley and Lavoisier. The blood, regenerated by the receipt of spirits in the lungs, flows through the pulmonary veins into the left heart, whence, together with the rest of the useful blood, it is carried by the aorta and its branches throughout the whole body. Galen's views upon the nervous system are equally interesting. In the brain and the spinal cord are the origins of the sensory and the motor activities of the nerves. The motor nerves act by pulling like a string upon the motor organs.

In the special physiology of nerves, Galen investigated particularly the action of the vagus and the intercostal nerves upon respiration and the action of the heart, and he cut the spinal cord transversely and longitudinally — experiments which show how deeply he had penetrated into an understanding of the mutual relations of the individual organs of the body. Galen's physiological system was for that time a monumental work, and the fact that Galen's views continued for thirteen hundred years as the unassailable code of medicine is surely not to be ascribed simply to the decay of the ancient culture and to the complete barrenness of the middle ages in scientific matters. The development of physiological investigation took not a single step forward during the middle ages. The Arabians, who had come to possess the ancient culture, were, indeed, prominent as physicians, but Islam forbade them alike independent investigation and philosophical thought. Even Avicenna (Ibn Sina, 980-1037), who was the most prominent of the Arabian physicians and showed philosophical tendencies, performed no original work. With slight changes his system was the system of Galen, whose glory he obscured by his own powerful authority in the civilised world of that time. Moreover, the many famous medical schools which arose at that time in Italy, France, and Spain trained many able physicians, but did not advance beyond Galen's ideas, not-

withstanding the fact that here and there an isolated physiological observation was made. This condition of stagnation continued until into the sixteenth century. An independent advance in physiology is first met with in the •sixteenth century. One of the first to abandon Galen's system was Paracelsus (1493-1541), who developed a complete system of nature. It was permeated with theosophical notions, a tendency that appeared still stronger in his followers and drove them wholly over to mysticism. Nevertheless, it contained many original, although frequently absurd, ideas. Paracelsus opposed the weak echoes of Galen's system and its outgrowths which had appeared during the middle ages, and this was at that time an important advance. The foundation of his system is the unity of nature. Nature is a unit, the macrocosm. In man as the centre of nature all forms of existence are contained. Hence, man is to be regarded as a microcosm. Nature, however, must not be considered as complete but as for ever becoming. The more special aspects of his system are arbitrary and unimportant, and, as is usual in such cases, this first beginning of independent investigation was comparatively crude ; before all other things it lacked a purely empirical and experimental basis.

At the same time, in France and in Italy a freer tendency began to appear in the medical schools. Fernelius (1497-1558) had many new ideas, although they were based wholly upon Galen's system. From the various forms of Galen's pneuma he separated the anima. The former consists of the most subtile material .substance ; the latter is the soul, which is to be recognised only by its effects. He advanced the further idea that the phenomena within the organism depend finally upon certain mysterious causes.

Special physiological investigation received an impulse from the great anatomical discoveries in the schools of France and Italy, where knowledge of the anatomy of the human body was placed upon a wholly new and strictly empirical basis by Vesalius, Eustachio, Faloppio, arid others. Researches upon the structure of the heart and the course of the vessels were the most fruitful for physiology. The doctrine of the circulation of the blood, as founded by Galen, underwent fundamental changes. By proving the imperviousness of the interventricular septum, Serveto •(1511-1553) refuted Galen's idea that the blood goes from the right ventricle of the heart directly into the left ventricle. His followers, Colombo (d. 1559) and Cesalpino (1519-1603), added to this new facts upon the circulation of the blood in the lungs ; and Argentieri (1513-1572), who opposed the doctrine of the animal

spirits, and happily thought to put in its place heat as the cause of vital phenomena, emphasized the fact that the nutrition of the whole body is provided for by the blood alone. By these special investigations into the physiology of the blood, the path was made easy to the greatest discovery of this period, that of the circulation of the blood by Harvey (1578-1657). The importance of Harvey's discovery lies in the fact that he first established the physiological connection of the arteries and veins peripherally, and the passage of the blood from the arteries into the venous trunks and thence to the heart ; he thus laid a basis for the fact that all the blood passes through the heart and moves in a closed circuit through the whole body. He added to this a great number of special facts concerning the mechanism of the circulation, all of which — and herein lies the great significance of his work — rested upon keen observation and an exact experimental basis. Following the exact tendencies of his time — the time which brought forth also a Copernicus, a Galileo, a Bacon, and a Descartes — Harvey, by his brilliant discovery, raised the experimental method again to an honourable position in physiology, after it had remained in complete oblivion for thirteen centuries. The spirit of the conscientious investigator and great logical acuteness characterise Harvey's personality and stamp him as the first real physiologist after the long night of the middle ages. A second doctrine " de generatione animalium" stands with equal honour by the side of his doctrine of the circulation of the blood. In this he put forward the dictum, " omne vivum ex ovo" which has since obtained vast significance in the science of life, and in the various forms in which it has been expressed in recent times controls all modern physiological views of organic reproduction.

Of the adherents of the great theosophical school which Paracelsus instituted, one only is important in the history of physiology, namely, van Helmont (1577-1644), since, in spite of the mysticism that characterised the whole theosophical tendency, he made thoroughly accurate observations. Starting with the Paracelsian doctrine of the unity and the constant development of nature, he conceived all natural bodies to be composed of matter and " archeus " (energy). Things exist and live only in this combination. As a result of it, all things are living. There are, however, different grades of life, and the so-called lifeless bodies exist at the lowest grade. Among van Helmont's special physiological ideas, his chemical doctrine of ferments is especially interesting. He rejects Galen's idea that digestion goes on in the stomach through the action of heat, and puts in its place the correct conception, that digestion is performed by a " ferment " associated with the gastric juice.

The philosophical systems of Francis Bacon (1561-1626) and Descartes (1596-1650) exercised a great influence upon the further development of physiology. Bacon's monistic philosophy, which, because of its vigorous accentuation of the inductive method of investigation, has become the basis of all modern natural science, inaugurated the great series of new and exact physiological observations, founded upon experiment, which has continued from that time to enrich our knowledge of vital phenomena. The philosophy of Descartes, although purely dualistic, was full of importance for the physiology of the senses and the theory of knowledge, because of the theory of sense-perception which formed its starting-point. Descartes was the first to maintain the proposition that the only thing in the universe of which we have certain knowledge is subjective psychical sensation. Mind, sensation, thought, is the only fixed point from which the universe can be surveyed and discovery proceed. " Cogito, ergo sum!' Senseperception, therefore, gives us no information concerning things, for it is deceptive ; and things, i.e., bodies, are in reality wholly different from what they appear through our sense-organs to be. These propositions are capable of the widest application. Further, they are so well grounded and so precisely and clearly expressed, and give so admirable a basis for a philosophical system, that one must wonder how it is possible for Descartes, who was usually a clear and consistent thinker, to be so inconsistent as to arrive finally at the complete dualism of body and mind. It is a temptation to believe that he maintained ultimate consistency secretly ; but that, for practical reasons, he allowed the pressure of the ecclesiastical conditions of his time to give this unexpected turn to his philosophy, while content with the feeling that the unpredjudiced thinker would note and correct the evident incongruity.

Of the greatest physiological importance, however, is his clear discernment in his dualism that the bodies of animals and men act wholly like machines and move in accordance with purely mechanical laws. But here again dualism comes in as a disturbing element, for Descartes ascribes the impulse to all movement to the soul, which, from its seat in the only unpaired organ of the brain, the pituitary gland, controls the individual parts of the body. Nevertheless, the general physiological ideas of Descartes have been of great value to physiology, and the gifted thinker also made many very important special physiological observations, which markedly advanced our knowledge of physiological optics and acoustics.

Descartes' notion that, as regards its vital activities, the human body is to be regarded as a complicated machine, was especially fruitful for physiology in the ingenious application which Borelli (1608-1679) made of it in the science of animal movement. Borelli undertook for the first time to reduce the movements of the organic motor apparatus to purely physical principles, and thus laid the foundation of our present mechanics of animal motion.

The chief result of this undertaking found expression in the inauguration of a peculiar school founded upon Borelli's doctrine, the iatromechanical school (called also iatrophysical and iatromathematical), which played a considerable role in the further development of physiology, since it endeavoured to explain other vital phenomena of the animal body upon purely physical principles. At the same time, some of Borelli's followers, especially Glisson, by regarding contractility as a property residing within musclesubstance itself, became the precursors of the later doctrine of the irritability of muscle.

Almost contemporaneous with the founding of the iatrophysical .school there arose another school, the iatrochemical, which for a time flourished by the side of the former. Its founder was Sylvius (1614-1672). Dissatisfied with the narrowness of the iatrophysicists, but recognising the importance of their principle in •explaining vital phenomena, Sylvius emphasized the chemical side in addition to the physical, and in accordance with this elaborated •chiefly the physiology of digestion and respiration by extending van Helmont's doctrine of the ferments. In the theory of respiration also, Mayow (1645-1679) expressed very pertinent thoughts upon the analogy between respiration and combustion.

At this time physiology derived considerable assistance, the value of wtjich for physiological investigation, however, has not been completely taken advantage of even to the present day, from the invention of the compound microscope and the microscopic discoveries made by means of it by Leeuwenhoek (1632-1723), Malpighi (1628-1694) and Swammerdamm (1637-1685). The knowledge of the physiology of reproduction and development, more than all else, was thus markedly advanced. The first microscopic discoveries in this field naturally led to many excusable •errors. When, for example, aqueous infusions of decomposable substances were made and the appearance of Infusoria in immense numbers was observed in them, spontaneous generation from lifeless substances was believed to have taken place, contrary to Harvey's dictum, " omne vivum ex ovo," but in accordance with Aristotle's •earlier assumption even for higher animals. On the other hand, Harvey's dictum became the starting-point of important discoveries ; for Malpighi followed the development of ova with the microscope, while Leeuwenhoek's pupil, Ludwig van Hammen, discovered spermatozoa, the importance of which Leeuwenhoek immediately recognised.

These and a great number of special physiological discoveries which active investigation brought forth give to the period of the seventeenth and eighteenth centuries after Harvey's appearance the character of the dawn of exact investigation in physiology, just as the influence of exact methods pervaded and animated all science of that period. Yet, as is constantly happening in the history of science, systems appeared at that time as evidence of a reaction against excessive specialisation, which fell into the opposite extreme of lacking all exact foundation and resting upon pure speculation. Boerhaave (1668-1738) skilfully avoided this pitfall in his eclectic system, which was put together from the various dogmas of his time and assumed as the source of all vital phenomena a "principiwni nervosum " in the form of a very subtile fluid. But the systems of Hoffmann (1660-1742) and of Stahl (1660-1734) did not escape it. Hoffmann's "mechanicodynamical " system is purely teleological. and arose under the influence of the philosophy of Leibnitz. Hoffmann regarded the ether as the ultimate cause of all vital phenomena ; its movement follows mechanical principles, but it receives its immediate impulse from the idea of the purpose of its own existence, that resides in every ether-atom. But Stahl's " animistic system," which combated Hoffman's doctrines, rests still more upon a speculative basis. At the foundation of Stahl's system there lies a dualism of body and mind, according to which the body in its activities follows mechanical laws, but is animated and preserved from decay and destruction by the " anima." Upon the nature of the anima Stahl expresses himself in very uncertain and contradictory terms. In spite of its unsupported speculations and its many contradictions,, animism obtained numerous adherents, which may be explained by the facts that the innumerable details acquired by the many special researches of the period were not properly sifted, and there was. no coherent understanding of vital phenomena.

Haller (1708-1777) responded in a genuinely scientific manner to the need of a unitary arrangement of the details, and, as it had once been with Galen and, later, with Harvey, a new epoch in the development of physiological investigation dates from his appearance. As Galen had first recognised the practical significance of physiology and had made the knowledge of vital phenomena the basis of practical medicine, and as Harvey by the introduction of exact experimental investigation had created the fruitful method, the employment of which in the sixteenth and seventeenth centuries called forth the enormous mass of individnal discoveries, so Haller for the first time brought together as a whole all the extensive material of facts and theories in his " Memento, physiologiac corporis humani." He thus made physiology an independent science, which was not simply to serve practical purposes in the interest of the art of healing but also to pursue purely theoretical aims for itself alone.