Physiology (Gr. Physiology 1300485 , nature, and Physiology 1300486 discourse), strictly speaking, the doctrine of nature, embracing a knowledge of all the physical and natural sciences, but now restricted to the science which treats of the vital phenomena of organized bodies, whether animal or vegetable. It does not include, as a rule, the phenomena of diseased action, which belong to the separate department of pathology. General physiology treats of the vital phenomena as manifested in the organic kingdom as a whole, not as restricted to particular species; as, for instance, the influence of light on animal and vegetable growth, the question of spontaneous and sexual generation, and the duration of life, as compared with the organization and fecundity of the individual. Comparative physiology shows the points of resemblance and diversity in the vital actions of various groups and species more or less related to each other. Special physiology gives the details of the vital phenomena in particular species, such as the kind and quantity of food consumed, the number of young produced at a birth, the frequency and volume of the respirations, and the daily quantity and composition of the secreted and excreted fluids.

Human physiology, as a special branch, is of course confined to the vital operations in the human species; but it is evident that the facts relating to a particular species cannot be fully appreciated, nor even understood, without an acquaintance with general physiology; for it is only in this way that we can determine whether a given phenomenon is dependent on the special organization of that particular species, or one common to all animal and vegetable forms alike. A complete knowledge of anatomy, down to the minutest structures of microscopic forms, is also a necessary preliminary to the successful study of physiology; and the same thing may be said of organic chemistry, so far as it relates to the immediate composition of the animal solids and fluids. Since many of the phenomena of living beings are physical or chemical, a sufficient knowledge of physics and inorganic chemistry is also indispensable in physiological pursuits. - Living beings may be distinguished from inorganic matter by the peculiar arrangement of their heterogeneous parts, solid and fluid, mutually acting upon each other, by their definite form, and determinate bulk; by their origin from parents in the form of germs; by their powers of drawing sustenance from the external world, of excretion, and nutrition; and by the fact that they exhibit a definite term of existence, through which they pass by successive periods or phases of growth and decay.

In eggs and seeds the vital properties exist, though in a dormant state. Even presupposing the existence of organized structure, it is impossible to give a precise definition of life. The ancients held to the opinion that there is an independent entity or vital principle, whose union with the body causes life and its separation from it death. The moderns, having found that the study of this intangible vital principle does not lead to any definite result, have abandoned its pursuit and even the discussion as to its existence, and have devoted themselves to the investigation of the natural phenomena of living bodies, so far as they are appreciable by the human senses and intelligence. With them, therefore, the study of life is simply the study of its phenomena, without any attempt to determine its actual nature. Various definitions of life, however, have been attempted. According to Bichat, " life is the sum total of the functions which resist death;" Treviranus makes it " the constant uniformity of phenomena with diversity of external influences;" and Beclard calls it "organization in action." The theories of the principle of life may be divided into three groups: 1, those which consider the body an inert mass, into which an animating principle, called by various names, has been introduced; 2, in which life is explained by physical laws; 3, which recognize special vital properties or a vital force.

In the first belongs the ancient theory of animism, according to which the world is vivified by a soul or spirit everywhere diffused, a portion of which gives life to man, animals, and plants. The ancient philosophers compared the human microcosm to the macrocosm of the universe, and recognized the same motor forces for organic and inorganic matter. Hippocrates considered unintelligent nature as the mysterious agent in the vital processes. Plato and Aristotle admitted three animating spirits, the vegetative in the plant, the vegetative and sensitive in the animal, and in man an additional intelligent and reasoning spirit, nobler and purer than the others. Paracelsus, in the 16th century, pretended to explain the functions of life by chemical and cabalistic arts, attributing to sidereal spirits and the planets a direct action upon the body, the sun upon the heart, the moon upon the brain, etc. Van Helmont afterward personified the vital principle under the name of archceus, a name previously employed by Paracelsus; this power was situated at the cardiac orifice of the stomach, and presided directly over digestion by the agency of the gastric juice; the pylorus, another dignitary of the organism, the doorkeeper of the stomach, opened or shut the passage into the intestine under its control; this duumvirate had its subordinates in each organ, which executed the special orders-; health reigned during the peaceful and orderly state of the archceus, but its anger, fright, or irregularity produced diseases - a notion under which we perceive the dimly shadowed idea of the sympathy and mutual dependence of organs now universally recognized.

By the aid of a chemical ferment the archceus could organize matter directly, without the intervention of an egg. Stahl, early in the 18th century, though educated in the chemical school of physiology, found so many vital phenomena inexplicable by physical laws, that he sought for a new basis for the physiological edifice, insisting on the inertia of matter. Organization to him was nothing without the rational soul, at the same time that the latter could do nothing without the body which was created for it; all physiological acts were established and directed by the soul in order to preserve the integrity of the body, by which it is brought into relation with the external world; most of the functions were destined to prevent the decomposition of the soft solids and liquids of the body, and all the movements were voluntary. Descartes, early in the 17th century, put an end to the theory of Van Helmont's archcei; notwithstanding the immense power he attributed to the soul, this philosopher's theories led to the establishment of the chemical and mechanical schools of physiology; he favored the former by introducing ferments, acidity, alkalinity, and effervescence of the humors, among the nutritive functions; and he influenced the latter by explaining the secretions by the round, cubic, or pyramidal forms of the molecules, and the functions of relation by a vibratory movement excited in the nerves by external impressions, propagated to the pineal gland in the brain, and terminating in the cerebral fibres on which it left material traces.

Sylvius of Ley den, in the last half of the 17th century, explained all the functions of the body by the effervescence and fermentation of the fluids, entirely excluding the solids from his physiological scheme; food fermented in the stomach under the influence of the gastric fluids, and digestion was perfected hy the actions established by the addition of the bile and the pancreatic secretion; the movement of the blood in the heart was due to the effervescence arising from the meeting of an oily volatile salt of the bile with a saccharine acid of the lymph, producing at the same time the animal heat; the vital spirits, entirely material, were prepared in the brain by distillation, having much of the properties and nature of alcohol; all diseases were caused by the predominance of this or that chemical element in the fluids, and to counteract a supposed acidity very powerful chemical preparations were rashly administered. "While Sylvius taught these doctrines upon the continent, Willis promulgated similar ones in Great Britain; he also made the chyle effervesce in the heart under the influence of salt and sulphur, which took fire together and produced the vital flame. According to Haller, even the great mind of Newton was led astray by such vagaries as these.

Many of the popular ideas of peccant humors, for which a multitude of empirical remedies are continually extolled and exhibited, date back to the chemical theories of the 17th century. Boerhaave and his school, early in the 18th century, substituted mechanical for chemical forces in physiology, explaining the phenomena of life on the principles of mechanics and mathematics, according to the idea of Descartes. The then admitted doctrine of Harvey of the circulation of the blood and the discoveries of Galileo favored the progress of this school. Food was reduced in the stomach to minute particles by trituration; the circulation was a complete hydraulic machine, and the heart a perfect sucking and forcing pump; the weight of the blood and the loss of its motive power from friction in the vessels were exactly calculated, and the force of the heart's contraction estimated at 180,000 lbs.; the differences in the secretions were explained by the diameter, foldings, and number of the divisions of the vessels in the secreting organs, and by the diverse forms of the molecules, some of which were admitted and others excluded by these kinds of organic sieves; animal heat was the result of the friction of the blood globules against each other and against the walls of the minute vessels.

Electricity was at one time considered the active agent of the vital functions, and certain analogies indeed favored this view; electricity dethroned the vital principle, and the barrier erected with so much labor between living and inorganic bodies was again thrown down. Glisson, in England, toward the middle of the 17th century, while the physical theories of life were in vogue, maintained the activity of matter, and that all the functions of life depend on a property of living animal substance which he calls irritability, entirely independent of physical or mechanical forces - all parts of the body, even the bones and the fluids, possessing this property. This theory was completely forgotten until toward the middle of the 18th century, when various authors made use of the terms contractile force and tonicity. Haller (1747) admitted two properties, irritability and sensibility (vis insita and vis nervosa). This irritability is the property of contracting under stimuli (the will for the ordinary muscles and their contents for the hollow ones), now styled contractility, distinct from and more powerful than elasticity, independent of the nervous force, and improperly called vital inasmuch as it is manifested after death.

Sensibility is the power of perceiving the impressions derived from contact. This theory gave a great impulse to physiological science, which before this was in a very confused state. Barthez, in the last half of the 18th century, adopted the phrase vital principle, which he regarded as distinct from the soul, and as having its own proper existence and its motor and sensitive forces, the former residing in the muscles, the latter in the fluids and especially in the blood; this system met with great favor. Bichat, a quarter of a century later, reduced the vital properties to two, contractility and sensibility, each divided into the animal or voluntary and the organic or involuntary. Brown, a few years before this, had elevated the property of excitability in the tissues into the vital principle; according to him, all diseases are either sthenic or asthenic, the vital force being increased in the former and diminished in the latter; the doctrine gave rise to the contra-stimulant practice of Rasori and others.

Blumenbach, toward the end of the 18th century, attributed all the formative actions to a force which he called nisus forma-tivus. 'Broussais, early in the 19th century, made pathology a branch of physiology, and gave to his system the name of physiological doctrine of disease; his celebrated theory placed essential fevers among the inflammations of the digestive tube, as forms of gastroenteritis. Gerdy admitted 17 vital principles, or so many distinct series of phenomena inexplicable by physical laws, a list which on his principles might be very greatly and inconveniently extended. - Modern physiology recognizes the fact that many of the phenomena presented by living bodies are purely physical or chemical, and are to be studied by precisely the same methods as any other physical or chemical phenomena. Such are the mechanism of the joints and the movements of the limbs upon the trunk; the extent, force, and rapidity of muscular contraction in general; the changes which take place in the food during digestion and in the air. during respiration; the exhalation and imbibition of various matters by the blood vessels in the course of the circulation; the pressure and velocity of movement of the blood itself, and its changes of color and constitution.

Not that these phenomena are exactly the same with those which are to be seen in the inorganic world; on the contrary, they are peculiar in their details, and can only be met with in living bodies, because there only are the necessary conditions for their production. But this peculiarity is merely one of detail, not of their essential character; consequently they are to be studied in the same way as similar phenomena elsewhere. The temperature of the blood is to be ascertained by the thermometer, like that of any other fluid; the gases absorbed and exhaled in respiration are to be analyzed and measured in the same way as if they came from any other source; but all these experiments and investigations, to be successful, must be performed upon the living body, since it is in the living body alone that the necessary conditions of the vital phenomena exist, even the simplest. On the other hand, there is a second division of the vital actions, in which the phenomena are apparently different from those of the inorganic world, and accordingly require to be studied in a peculiar manner; these are the phenomena of the nervous system, and consist of the various forms of sensibility, the motor stimulus, reflex action, and the sympathetic relation between various organs and parts of the body.

It is evident, of course, that these phenomena also must be studied by observations made upon the body during life, or at least before the vitality of the nerves and nervous centres has disappeared. - The third and last division of the vital functions is perhaps the most peculiar of all, and the most widely removed from the phenomena presented by inorganic bodies; these are the actions which relate to reproduction or the generation of young individuals. These phenomena, unlike those of the first two divisions, do not relate to the preservation of the individual, but to that of the species. They consist of the appearance of a succession of different forms by which the embryo is developed into a perfect organization, and in which life is transmitted by an unbroken series of parents and progeny. Nothing similar to .this is ever seen in the inorganic world, and accordingly the phenomena of this kind form the most striking characteristic of beings endowed with life. - The vital phenomena are also sometimes divided in a different manner, into those of vegetative and those of animal life. The vegetative functions are those which are common to both the animal and vegetable kingdoms, and include those relating to the internal nutrition and growth of the body and the reproduction of the species.

All the physical and chemical changes going on in the blood and the tissues, secretion, the formation and elimination of excrementitious material, the production and growth of the egg and the embryo, and the changes in the body which show themselves at different periods of life, belong to the phenomena of this class. The animal functions consist in the phenomena of sensation, consciousness, intelligence, of voluntary or excited motion; all those in fact which bring the animal into relation with the external world through the operation of the nervous system. - The functions concerned in the vital processes are treated under Absorption, Abstinence, Adipose, Age, Aliment, Animal, Animal Electricity, Animal Heat, Bile, Blood, Bone, Brain, Capillary Vessels, Cartilage, Cell, Chyle, Chyme, Circulation, Comparative Anatomy, Dentition, Dietetics, Digestion, Embryology, Gland, Heart, Histology, Hunger, Kidney, Liver, Lungs, Lymph, Muscle, Nerve, Nervous System, Nutrition, Perspiration, Respiration, Secretion, Skin, Voice, etc.

Besides the authors above named, reference may be made to the writings of Sir Charles Bell, Todd and Bowman, Carpenter, Magendie, Tie-demann and Gmelin, Burdach, Muller, Wagner, Virchow, Berard, Flourens, Longet, Bernard, Kobin, Brown-Sequard, and Milne-Edwards in Europe, and in America to those of Dunglison, Draper, Dalton, and Flint.