SECRETION is the term employed in physiology to designate the process of separation of those matters from the nutritious fluids of the body which are destined not to be directly applied to the nutrition and renovation of its organized fabric, but (1) to be either at once removed as injurious to its welfare, or (2) to be employed for some ulterior purpose in the chemical or physical processes of the economy itself, or to exert some kind of action upon other beings. For this definition of secretion considered as a pro cess we are indebted to Dr. Carpenter; but the reader must bear in mind that the term Is also very commonly used in another sense—namely, to designate the products which are thus secreted. In this latter sense, it is customary to speak of the biliary, urinary, or cutaneous secretion, when the bile, urine, and sweat are indicated.
Although it is impossible to divide with strictness the secreted products (as many physi ,ologists have attempted to do) into the excrementitions and the is to say. into (1) those which have no further function to discharge in the animal body, and which, if not excreted, would act as poisons, and (2) those which are subservient to further uses in the system—yet we may group them according to the preponderance of their excre mentitious or reerc-mentitious character. Dr. Carpenter approves of this mode of arrange ment, and proposes that those secretory processes should be arranged in the first division in which the depuration of the blood is obviously the chief end, while those should be classed under the second in which the ulterior purpose of the separated fluid would seem to be the of its production; and he further suggests a subdivision of this second group, according as this ulterior purpose is connected with the operations of the economy itself, as in the case of the tears, and the saliva, the gastric juice, etc., or is destined to act on some other organism, as is the case with the secretion of the testes, the milk, etc. The organs which yield the various secretions are termed glands (q.v..); but neither the form nor the internal arrangement of the parts of a gland have any essential connection with the nature of its product; the true process of secretion, under whatever form it may present itself, being always performed by the intervention of cells (q.v.). For a notice of the mode in which the cells are arranged in various glandular structures, the reader is referred to the articles GLAND, LIVER, KIDNEY, =MUCOUS 31E31 TIRANE, etc.
We shall now briefly notice the causes which render the due performance of the functions of secretion essential to the well-being of every animal. 1. Nearly all the solids and fluids of the body are liable to continuous decomposition and decay in conse quence of their peculiar chemical composition. There is an obvious necessity that the products of incipient decomposition should be carried off and replaced by newly-orga nized matter. 2. The exercise of the various animal functions is essentially destructive to the structures by which they are accomplished; every operation ofi the muscular or nervous system appearing to require, as a necessary condition, a disintbgration or break ing up of a certain portion of their tissues, probably by an act of oxidation. Hence, for the due preservation of health, the disintegrated or effete matters must be removed and their place supplied. 3. When more food is taken than the wants of the system require, all that is not appropriated to the reparation of the waste, or to the increase in the weight of the body, must be thrown off by the excretory organs without ever hav ing become converted into organic tissue. If this excess were not speedily removed by the excretory organs, the current of the blood would speedily become poisoned.
The following may lie regarded as a tolerably complete list of the substances which are produced within tho organisms of man and the lower animals by the disintegration of its various tissues, and which are met with in one or other of the products of secretion: 1. Products of secreting processes, including a, the biliary acids and the prod ucts of their disintegration; b, the pigments of the bile; c, pigments allied to those of the bile and blood, viz., limmatoitlin and melanin; d, cholesterin and its allies; e, the sugars and allied bodies. 2. Products of the actual regressive metamorphosis of tissues ,—a,nitrogenous amide-like bodies, such as leucine, tyrosine, creatine, creatinine, allantoiu. cystin, guanine, sarcine, xanthin, and urea; b, nitrogenous acids, as hippuric, uric, and cynuric acids; c, indifferent nitrogenous bodies, such as the pigments occurring in the urine; and excretine; and d, non-nitrogenous acids, as acetic, benzoic, butyric, carbonic, formic, lactic, oxalic, succinic, and valerianic acids. Some of these products, however, only occur in the secretions in cases of disease.