484 PHYSIOLOGY with general physiology ; for it is only in this way that we can determine whether a given phenomenon is dependent on the special or- ganization 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 deter- minate 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 exis- tence of organized structure, it is impossible to give a precise definition of life. The ancients held to the opinion that there is an indepen- dent 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 prin- ciple does not lead to any definite result, have abandoned its pursuit and even the dis- cussion 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 in- telligence. 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 con- stant uniformity of phenomena with diversi- ty 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 prop- erties 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 recog- nized the same motor forces for organic and inorganic matter. Hippocrates considered un- intelligent nature as the mysterious agent in the vital processes. Plato and Aristotle ad- mitted three animating spirits, the vegetative in the plant, the vegetative and sensitive in the animal, and in man an additional intelli- gent and reasoning spirit, nobler and purer than the others. Paracelsus, in the 16th cen- tury, 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, &c. Van Helmont af- .terward personified the vital principle under the name of archceus, a name previously em- ployed by Paracelsus ; this power was situated at the cardiac orifice of the stomach, and pre- sided directly over digestion by the agency of the gastric juice ; the pylorus, another digni- tary 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 exe- cuted the special orders ; health reigned during the peaceful and orderly state of the arcJioeus, 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 arckceus 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 phys- ical laws, that he sought for a new basis for the physiological edifice, insisting on the iner- tia of matter. Organization to him was noth- ing without the rational soul, at the same time that the latter could do nothing without the body which was created for it ; all physio- logical 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 func- tions were destined to prevent the decompo- sition of the soft solids and liquids of the body, and all the movements were voluntary. Des- cartes, early in the 17th century, put an end to the theory of Van Helmont's archcei ; not- withstanding the immense power he attributed to the soul, this philosopher's theories led to the establishment of the chemical and mechan- ical schools of physiology; he favored the former by introducing ferments, acidity, alka- linity, 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 mole- cules, and the functions of relation by a vibra- tory movement excited in the nerves by exter- nal 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
