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Rh publications quoted above. See also Lauchert, Geschichte des Physiologus (Strassburg, 1889) and E. Peters, Der griechische Physiologus und seine orientalischen Übersetzungen (Berlin, 1898).  PHYSIOLOGY (from Gr., nature, and , discourse), the science or theory of the properties, processes and functions of living organisms. Physiology is distinguished from anatomy as dealing specifically with the functions of an organism, rather than its structure. The two main branches of the science are animal and plant (vegetable) physiology, and in animal physiology that of man stands out as primarily associated with the word.

Ever since men began to take a scientific interest in the problems of life two distinct rival explanatory principles of vital

phenomena have claimed attention: a natural and a mystical principle. The first outcome of the scientific attempt to explain vital phenomena after the natural method and by a unitary principle was the doctrine of the Pneuma, held by the followers of Hippocrates, which found its clearest expression in Galen's system. According to this doctrine, the origin of all vital phenomena was a very fine substance, the Pneuma, which was supposed to exist in atmospheric air, to be inhaled into the lungs of man, and thus through the blood to reach all the parts of the body, where it produced vital phenomena. This doctrine—an attempt to explain the phenomena of life which was not altogether natural, but even materialistic—was accepted by the middle ages together with Galen's system. With its translation into the Latin spiritus, however, the conception of the Pneuma lost its original force. The spiritus animales of the middle ages developed ere long into mystical powers, the result being the explanation of vital phenomena by a supernatural theory. Not until the scientific renaissance of the 16th and 17th centuries did views again undergo a change. After the establishment of a scientific method in physiology by William Harvey, and the development of Descartes' mechanical system of regarding living bodies, the natural explanation of vital phenomena once more universally found favour. Two schools arose, which endeavoured by dissimilar methods to find a mechanical explanation of vital phenomena: the iatrophysical, originating with the gifted and versatile Borelli, and the iatrochemical, founded by the Dutchman, F. de la Boë (Sylvius). But when both chemical and physical methods of explanation failed at such problems as, for instance, irritability and evolution, another change in opinion took place. By degrees there emerged once more the tendency to explain vital phenomena by mystical means, finding expression in the Animism of Stahl, to quote an example; and in the second half of the 18th century Vitalism, originating in France, began its victorious march throughout the whole scientific world. Again the opinion came to be entertained that the cause of vital phenomena was a mystical power (force hypermécanique)—that “vital force” which, neither physical nor chemical in its nature, was held to be active in living organisms only. Vitalism continued to be the ruling idea in physiology until about the middle of the 19th century, and its supremacy was only gradually overthrown by the great discoveries in natural science of that century. The chemical discoveries resulting from Wohler's synthesis of urea first showed that typical products of the animal body, the production of which had hitherto been supposed to be solely the result of the operation of vital force, could be obtained artificially by purely chemical methods. Then above all came the discovery of the law of the Conservation of Energy by Robert Mayer (1814-1878) and Hermann von Helmholtz (1821-1894), and its application to the living organism by Mayer, Helmholtz, Pierre Louis Dulong (1785-1838), Edward Frankland, Max Rubner and others, to prove that the manifestations of energy by the organism are simply the result of the quantity of potential energy received into the body by means of food. Finally, the stupendous results arrived at by Darwin and the establishment of the fundamental law of “biogenesis” by Ernst Haeckel, prepared the way for a natural explanation of the enigma of evolution and structure of organisms. Thus by the second half of the 19th century the doctrine of vital force was definitely

and finally overthrown to make way for the triumph of the natural method of explaining vital phenomena, which down to the present time has continued to spread and flourish with an unparalleled fertility. It would, it is true, appear as if in our day, after the lapse of half a century, mystical tendencies were again disposed to crop up in the investigation of life. Here and there is heard once more the watchword of Vitalism. But all the so-called neo-vitalistic efforts—such as those of Alexander von Bunge (1803-1890), Georg Evon Rindfleisch (b. 1835), Johannes Reinke (b. 1849) and others—have nothing to do with the old vitalism. They originate solely in a widespread confusion with regard to the boundaries of natural science, their principal tendency being to amalgamate psychological and speculative questions with problems of purely natural science. In the face of all these efforts, which by their unfortunate designations of Vitalism and Neo-vitalism give rise to entirely false conceptions, and which by their intermingling of psychological questions and questions of natural science have led to mere confusion in research, it is essential that natural philosophy should be called upon to realize its own limits, and above all clearly to understand that the sole concern of physical science is the investigation of the phenomena of the material world. Physiology, as the doctrine of life, must therefore confine itself to the material vital phenomena of organisms. It is self-evident, however, that only such laws as govern the material world will be found governing material vital phenomena—the laws, that is, which have hitherto been brought to their most exact and most logical development by physics and chemistry, or, more generally speaking, by mechanics. The explanatory principles of vital phenomena must therefore be identical with those of inorganic nature—that is, with the principles of mechanics.

The investigation of vital phenomena in this sense requires, in the first place, an exact knowledge of the substratum in which

these phenomena are manifested, just as in chemistry and physics a thorough knowledge of the composition of of the material world is a necessary premise to the investigation of the phenomena of inorganic nature. The knowledge of the composition and structure of organisms has in the course of the scientific development of anatomy attained to an ever-increasing minuteness of detail, without having as yet reached a definite limit. The last important step in this direction was the discovery by Matthias Jakob Schleiden (1804-1881) and Theodor Schwann (1810-1882) that all organisms are built up of elementary living structural components, namely of cells (see ). The details of the anatomical construction of organisms are described under various appropriate headings, and a general guide to these will be found under and. We would here merely point out that a cell is the simplest particle of living substance which appears to be permanently capable of life. Different elements are essential, however, to the existence of the cell—two, at least, so far as has hitherto been discovered—the protoplasm and the nucleus. It must at present be regarded as at least very doubtful whether the centrosome, which in recent times it has been possible to demonstrate as existing in very many cells, and which appears sometimes in the protoplasm, sometimes in the nucleus, is a general and third independent cell-constituent. On the other hand, the number of special constituent parts which appear in various cell-forms is very large. A question which has long been discussed, and which has received special and animated attention, is that with regard to the finer structure of the cells—with regard, that is, to the protoplasm and the nucleus lying in it. Views on this subject have diverged very widely, and several totally diverse theories have been opposed to one another. One theory maintains that the living cell-substance has a reticular structure; another, that it is fibrillous. According to a third theory, the essence of the construction of the cell-substance lies in the granules which it contains; and according to a fourth, it lies in the ground-substance in which these granules are embedded. One view holds this ground substance to be homogeneous, another regards it as possessing a fine foam-structure. It may at present be regarded as