Physiology (Gr. physis and logos, 'disconrse upon nature') is the science which treats of the behaviour of living beings, and of the functions of their parts. It is thus the sister-science to Morphology (q.v.), in which the outer form of living creatures and the structure and arrangement of their parts are considered. Both are included under the more general term Biology (q.v.). A peculiar use of the term physis is due to Hippocrates, who applied it to a spiritual entity which he supposed to be everywhere present, and to keep the processes of the body in order. This use of the word is still kept alive in oft-repeated phrases, as when in speaking of a sick person it is recommended that the cure be left to nature. There is an Animal Physiology, of which this article will mainly treat, and a Vegetable Physiology (q.v.); also a Comparative Physiology, which, however, is still very imperfect, for the details of the life-processes have been investigated in not more than a dozen animals. Indeed, comparative physiology consists chiefly of a series of inferences as to function from comparative morphology, and these must be often erroneous. There is a still wider science, which might be called Universal Physiology. For as all the organs of the body are mutually related, so that if one be deranged all the others will be more or less affected, so are there close relationships between the various creatures of the globe. Thus, to quote Semper, Animal Life (1881): 'If the American prairies were to cease to produce grass, the first result would be the utter extinction of the now numerous herds of buffaloes, and on their existence depends that of the surviving remnant of the ancient Indian population of America. If the various insectivorous birds of North America were exterminated, within a very few years beyond a doubt all the produce of the rich agricultural districts of that continent would be destroyed. If we change the mode of life of any single animal, the change will instantly have an influence on all the other animals whose healthy existence was in any way dependent on its normal function before it was altered.' The most obvious relation of this sort is that which exists between plants and animals; similar ones hold good for human beings in their relationship to other living things, and to each other. Thus we see in Political Economy, the science which treats of the laws of human activities, a department of the science of physiology. A still wider significance might be given to the science; for in view of the fact that the intimate relations between chemical, physical, and living processes are becoming daily more evident, it would be quite consistent that morphology should deal not only with the forms of plants and animals, but also with those which the dust assumes in the crystal, pyramid, and star, while physiology would treat of the forces and chemical processes concerned.
Knowledge of the bodily functions has been gained in three ways: (1) by observing the normal states of living things; (2) by experiments upon these; (3) by studying the processes of disease. No science can advance rapidly or with certainty without experiment, and most of our precise knowledge of physiology has been gained in this way, from the time when Galen proved that the arteries during life contain blood, or when Harvey demonstrated the circulation of that blood. As an example of how we may learn from disease, we may note the discovery that the spleen produces white blood-corpuscles, following from the observation that in morbid enlargement of that organ the blood contains an increased number of these cells.
The functions of the body consist of (1) Movement, (2) Nutrition, (3) the activities associated with the Nervous System, (4) Growth and Reproduction—the latter being considered as continued growth. Movement is performed by the contraction of muscles, definitely arranged, especially with relation to the skeleton or supporting structure. Nutrition is a general term including all those processes concerned in the supply of matter and energy to the body, and the removal of waste matter. It may be considered under three headings: (1) the introduction of food into the body and its carriage to the tissues; (2) the changes of this matter within the tissues; (3) the removal of waste matters from the tissues and from the body—Excretion (q.v.). The first includes (a) the eating and drinking of solid and liquid food, and the intake of oxygen, a part of Respiration (q.v.); (b) the Digestion (q.v.) of the food; (c) its absorption into the Blood (q.v.); (d) the circulation of the blood and its associate the Lymph (q.v.), by means of which the tissues are bathed in a stream of food, and the waste matters removed from them. The nervous system is the co-ordinator of all the processes of the body; it consists of the Brain, Spinal cord, Sympathetic system, and the associated Nerves and smaller Ganglia; in close connection with it are the sense-organs, the eyes, ears, nose, tongue, and general nerves of touch and temperature; the brain is the seat, or at all events the chief seat, of consciousness, and the 'organ' of thought and other mental processes. The functions of the body are dealt with in separate articles; here we shall give a short account of their relations to each other.
Let us first consider the life of the simplest animals. Almost invisible to unaided sight, flourishing in the stagnant water of ponds, without separate organs, they are little more than tiny masses of jelly-like Protoplasm (q.v.). Their life seems to consist in movement, nutrition, growth, and reproduction; possibly they possess the elements of consciousness. For movement a source of energy is required; this is found in their food—minute organisms, and organic particles dissolved in the water in which they live. These consist of substances of high potential energy. They are either plants which are able to utilise the energy of the sun for their growth, or remains of plants or animals which have fed upon plants (see VEGETABLE PHYSIOLOGY). Thus we see in animal protoplasm a machine for the transformation of potential energy into energy of motion. This machinery is constantly breaking down and being repaired, the protoplasmic matter is continually being replaced by new matter similarly combined. But, as the protoplasm is extremely complex, the simpler substances of the food have to be combined and recombined in a series of stuffs of increasing complexity until the complex living matter itself is formed. These combinations are supposed to be due to a ferment-like power of the protoplasm. This power it is which makes growth possible—i.e. the actual increase in amount of protoplasm. The growth of a crystal out of its solution is probably a process not utterly unlike, though much simpler. Growth of a crystal may seemingly be endless, but growth of a cell never proceeds beyond a certain point, when the process known as Cell-division occurs. The mass of protoplasm divides into two halves, and each half goes on to live as before. The necessity for cell-division arises partly from the conditions of the food-supply. Food is absorbed through the surface of the cell, but with growth the mass to be fed increases faster than surface; therefore starvation must occur at a certain stage of growth unless the cell divides. The higher animals are built up of numberless cells which have all arisen, by division, from a single cell, the ovum; but instead of becoming separated they have all kept together, joined probably by strands of protoplasm. The cells are massed into tissues and the tissues into organs, the organs having special functions. This difference in the behaviour of the cells of different parts of the body is known as Division of Labour (q.v.). We can form some idea of its origin. Imagine a cell to divide many times, but the daughter-cells to remain loosely joined together; the outer and inner cells would live under different conditions and would assume different functions. The whole story of the evolution of life, both in the origin of individual forms and in the growth of nations, is simply the process of the division and organisation of labour. For just as an organism is a collection of cells, each having its own life, yet all bound together for mutual service, so is a nation a collection of individual men and women. And as the perfection of an animal is measured by the completeness of the division of labour among its cells, so is the civilisation of a nation measured by the harmony of organisation of its labour. Further, just as there have been many species of animals which have appeared, lived for a time, and then given place to higher species, so there have been civilisations which have flourished for a time and then died away. Any fairly complex civilisation will serve as a type of the division of labour in the body of one of the higher animals. First there are the persons concerned in the getting of food, like the limbs and mouth of an animal. Then the food is prepared for use by other labourers; such are the digestive organs of the animal. The food has to be distributed to all members of the community by merchants and carriers; the blood and the blood-vessels perform this function. The whole community has to be warned of dangers, directed and governed, and made to act harmoniously by the statesmen of a nation; the same things are done by the sense-organs, brain, and nervous system of an animal.
We have already noted that the source of all the energy of an animal lies in its food. We know that this is either burned as it were within the tissues, used as fuel for the protoplasmic machinery, or used to keep that machinery in repair; in either case the food-stuffs have to be prepared before they can be used. Such preparation is called digestion, which consists in making the solid food-stuffs soluble. The digested food is absorbed into the blood, and all of it, except the fat, is carried direct to the liver. This organ, amongst other functions, regulates the composition of the blood; thus, it stores the sugar in its cells, and gives it out as the other tissues require. Muscular tissue is the great consumer of sugar, which is to the cells what coal is to the steam-engine. But there is another and most important food-stuff that requires no digestion. This is oxygen, which is needed by the protoplasm for its life, and also for the burning of fuel within the living machinery to get heat and energy of motion. The oxygen is held in the Blood (q.v.) by means of a special substance which greedily absorbs it from the air in the lungs, and yet gives it up readily to the protoplasm of the tissues. The blood as is well known circulates round and round the body, pumped by the heart. It is a stream of food material by which each cell of the tissues is fed. For each cell is close to a capillary, which is a very thin walled blood-vessel, through which the fluid food oozes, and thus bathes the tissues. The matter which has thus passed out of the blood-vessels is collected into another system of vessels, the lymphatics, and eventually emptied into one of the great veins. The lymph stream is also the drain into which is thrown by each cell the waste products of its activity. The carbonic acid that is formed in the tissues is carried away by the blood, and escapes out of the system from the lungs. Some of the useless water is also got rid of in the same way, and some more of it is sweated out by the glands in the skin; the rest is filtered out of the blood by the kidneys. There are many other waste matters besides carbonic acid and water. These are to a large extent prepared for excretion in the liver, and to some extent actually taken out of the blood by that organ, being poured into the intestine, mixed with other matters, dissolved in a fluid called Bile (q.v.). They are all taken out of the blood by the kidneys, and cast out of the body along with the water filtered out by the same organs, as urine.
This finishes our sketch of the labours of the inferior members of the cell community. The more skilled workmen are the cells of the sense-organs and the nervous system; these are described in other articles. As has been noted, their function is to inform the community of what is going on in the outside world, and to keep in harmony all the diverse labours of the various organs.
The function of Reproduction is treated in that article. There remains only the duration of life to consider and the fact of death. The general theory of the length of life is set forth in the article on longevity. The usual view of death is that it is inherent in living matter; that there is some cause which renders the cells of the body, after a certain period of life, and after a certain number of divisions, less and less able to nourish themselves, to continue dividing, and to keep the body in repair. Recently it has been suggested by Weismann that death has been evolved by natural selection as a preventive against the continuance in life of maimed individuals (for no one can escape slight injuries) that would be only a burden to the species.
For Comparative Physiology, see the articles on the various functions and groups of animals.
The History of Physiology, in its limited sense as the study of the life-processes of individual organisms, is the history of an ever-deepening analysis. The science begins with the study of the general habits of animals; the life-processes are then resolved into the functions of the various organs, the organs are analysed into their component tissues, the tissues into cells (see CELL), and lastly, the essential constituent of the cell is discovered in Protoplasm (q.v.). The last three stages, beginning with the analysis of organs into tissues, have been developed within the last hundred years. The history looked at from this point of view is enlarged upon in the article BIOLOGY; here we shall give a history of a more detailed nature. Preyer divides it into five periods—(1) the speculative period; (2) that associated with the name of Aristotle; (3) headed by Galen, (4) by Harvey and Haller, and (5) by Müller. The first period opens with the beginning of medical science in India, China, and Egypt. The Jews were acquainted with many laws of practical hygiene and dietetics. Then came the philosophers of Greece. Matter was supposed to consist of four elements, fire, air, earth, and water. The essence of life was referred first to one and then to another of these elements by various philosophers: by Thales to water, by Anaximenes to the air, by Xenophon to the earth, by Pythagoras to fire or heat. Hippocrates, the father of medicine, about 450 B.C., was the first to proceed in a purely rational spirit. Observing carefully the facts of disease, he strove to found the art of medicine upon the results of experience. He attributed diseases to natural causes, and not to special visitations of the gods; and as already noted, he postulated a spiritual essence universally diffused; this he called Nature, Physis, and to this he ascribed the maintenance of things in their normal state, and their restoration if disturbed. The second period is headed by Aristotle, the father of natural history, about 350 B.C. He dissected many animals, and attempted to discover the uses of the various parts. It is difficult to estimate correctly the exact value of Aristotle's work in physiology; it must be measured more by the methods of research which he initiated than by the actual results achieved. Thus, to give an example of his ideas on the subject, the heart he imagined as the seat of the 'rational soul'; the nerves he supposed to arise in the heart; of their function he was ignorant. What is perhaps more surprising is that he described the brain as an inert viscus, cold and bloodless, whose only function was to cool the heart, and not comparable in importance to the other organs of the body. Erasistratus, the grandson of Aristotle, about 300 B.C., was perhaps the first to carefully dissect the human brain. He traced the connection of nerves with it, and even noticed that the complexity of the convolutions of the gray matter was greatest in man, and that they were to some extent a measure of the intelligence. The next 400 years were barren of any useful advance; the practice of medicine reached perhaps its lowest point. The literature is occupied with discussions as to the 'animal and vital spirits,' terms used before Aristotle to express the powers of living things. The animal spirits were those that ruled over those actions of living things that were supposed to be quite different from anything that takes place in things not living, while the vital spirits were those that were concerned in those processes going on in the body which were the result of purely chemical and physical laws. We no longer discuss whether the vital spirits live in the heart and the animal in the brain, but we have not yet settled the exact relationship between the processes of the living world and those of inorganic matter.
About 150 A.D. Galen, a Roman, revived the sounder method of experimental inquiry; he is the leader of the third period. He perceived that mere dissection of dead animals gives no infallible information as to the functions of the living, and accordingly performed many experiments upon living animals. He proved that during life the arteries contain blood and not air, as was thought to be the case up to that time, by simply opening a vessel of a living animal. He also directed much of his study to the brain and nervous system. He was the first to state definitely that the brain, spinal cord, and nerves are the organs of sensation, intelligence, and the originators and guides of properly ordered voluntary movements; and he finally refuted the doctrine of Aristotle by showing that the brain was hot and not cold, and by arguing also that if it were a mere cooler of the blood it need not be elaborately organised. He pointed out that the brain was of the same substance as the nerves, but softer, 'as it should necessarily be, inasmuch as it receives all the sensations, perceives all the imaginations, and then has to comprehend all the objects of the understanding, for what is soft is more easily changed than what is hard.' He discovered also that the nerves of sensation and of motion are distinct, and thus explained the double supply of nerves to the tongue and eyes. For centuries Galen exercised an undisputed sway over the practitioners of medicine and the students of allied philosophy.
Some centuries afterwards the so-called Arabian physiology arose. Avicenna, about the year 1000, was its chief exponent. Once more, however, the discussions were about the nature and residence of the animal and vital spirits. Albertus Magnus, in the 13th century, and Paracelsus, in the 15th century, are representatives of mediæval mysticism. About the same time, during the revival of learning, the mathematicians and chemists were busy seeking to explain bodily functions in terms of mechanical, chemical, and physical laws. In the 16th century Villanovanus described correctly the action of the lungs as purifiers of the venous blood. The study of human anatomy was revived by Vesalius in Italy, and continued by Fabricius; and in the beginning of the 17th century Harvey, who had studied in Italy, made perhaps the most important of all physiological discoveries, that of the circulation of the blood.
This discovery inaugurates the fourth period of the history of physiological research; by it a sound foundation for the whole science was laid, and the development of surgery and medicine made possible. Then, after the invention of the microscope, came many active investigators; among them may be mentioned Malpighi and Leeuwenhoek; and thus the foundations of Histology (q.v.) were laid. Haller, near the end of the 18th century, gave to physiology the form that it now possesses. He attempted to discard from the science all statements of a vague and mystical character, he added many minor discoveries to the store of facts, and ranged the whole in a logical sequence.
The great leader of the fifth period, Johannes Müller, during the first half of the 19th century, gave to the science a greater width. He connected as one philosophy the truths of chemical physics, comparative anatomy and physiology, and embryology. Embryology was founded as a science by Von Baer. Cuvier developed comparative anatomy, and thus gave a foundation to the study of comparative physiology. Lamarck enunciated the laws of evolution. Berzelins placed animal chemistry upon a sound basis. The discovery of the mechanical equivalent of heat by Joule, the enunciation of the cell-theory by Schleiden and Schwann, and the discovery of protoplasm as the essential constituent of the cells by Von Moil and Du Jardin are the great steps which have placed us in our present position. The discovery of reflex action by Marshall Hall, of inhibitory nerve action by Weber, and of the glycogenic function of the liver by Claude Bernard mark important advances. With the work of this last mentioned the physiology of Protoplasm (q.v.) begins. The conception of evolution, rendered acceptable by Darwin's work, is the great harmoniser of all science.
This history of physiology may be shortly summarised as follows. Even to early inquirers it was obvious that many of the life-processes of animals are the result of the action of a set of machines, which, as we know, were supposed to be kept in action by the 'vital spirits.' These machines were called organs, and the work performed was spoken of as their functions. The whole body was conceived of as made up of various organs, and the labours of physiologists were directed towards discovering their functions, a work which to this day is incomplete. This may be called the first phase of physiological philosophy; it lasted until the promulgation of the cell theory and the rapidly following discovery of protoplasm. The idea of protoplasm is to natural science of nearly as much importance as the doctrines of the conservation of matter and energy are in chemistry and physics. The chief labours of physiologists for a very long time will be directed towards attaining exact conceptions of the nature of this protoplasm in terms of chemistry and physics. The old question of animal and vital spirits is still unsolved; we are not able to say whether there is any abrupt distinction between ordinary matter and that which is called living matter, and which forms 'the physical basis of life.' Is it merely that living matter is more complex and unstable than ordinary matter, and therefore far more sensitive to external impulses in the form of ethereal and molecular vibrations; or is there some special vital force at work? If we fully understand the first theory we shall probably believe that there is no such vital force. At any rate the surest path to its discovery lies in determining how far the objective phenomena of life are explicable in terms of ordinary chemical and physical laws. When we find any activity of living matter which we can be certain cannot be so explained, then, and not till then, may we postulate a vital force. Supposing such a discovery ever to be made, it is necessary to observe that it will merely widen our chemistry and physics. The discussion of the subjective consciousness of life is an entirely separate one. Ordinary philosophy postulates two entities, matter and spirit; Materialism holds that matter when it reaches a certain stage of complexity becomes conscious; Monism, which is becoming the fashionable scientific creed, teaches that matter in motion and consciousness are the two sides—one seen from without, the other felt from within—of a single entity.
We may fitly close by quoting Foster's statement of the present problems of physiology. He speaks of them as threefold. (1) On the one hand, we have to search the laws according to which the complex unstable food is transmuted into the still more complex and still more unstable living flesh, and the laws according to which the living substance breaks down into the simple, stable, waste products, void, or nearly void, of energy. (2) On the other hand, we have to determine the laws according to which the vibrations of the nervous substance originate from extrinsic and intrinsic causes, the laws according to which these vibrations pass to and fro in the body, acting and reacting upon each other, and the laws according to which they finally break up and are lost, either in those larger swings of muscular contraction or in some other way. (3) And lastly, we have to attack the abstruser problems of how these neural vibrations, often mysteriously attended with changes of consciousness, as well as the less subtle vibrations of the contracting muscles, are wrought out of the explosive chemical decompositions of the nervous and muscular substances—i.e. how the energy of chemical action is transmuted into, and serves as the supply of that vital energy which appears as movement, feeling, thought.'
See, besides the articles named above and at ANATOMY, those on ANIMAL, ANIMAL CHEMISTRY, ANIMAL HEAT, DIET, FOOD, DEATH, LIFE, &c.; the elementary primer of physiology by Michael Foster; the elementary textbook by Huxley; text-books by Foster (5th ed.), Landois and Stirling, M'Kendrick; Physiological and Pathological Chemistry, by Bunge, trans. by Wooldridge (1890); Chemical Physiology and Pathology, by Halliburton (1891); Comparative Physiology and Anatomy, by Jeffrey Bell (1887); Ency. Brit. article 'Physiology,' by Foster.