Secretion

Collier's New Encyclopedia (1921)

in physiology, the process by which materials are separated from the blood, and from the organs in which they are formed, for the purpose either of serving some ulterior office in the animal economy, or being discharged from the body as excrement. Secretion is one of the natural functions of the living body, and is as necessary to health as nutrition. Where the secreted materials have some ulterior purpose to serve, they are known as secretions; where they are discharged from the body, excretions. Most of the secretions seem to consist of substances not pre-existing in the same form in the blood, but requiring special organs and process of elaboration for their formation. Excretions, on the other hand, commonly or chiefly consist of substances existing ready formed in the blood, and are merely extracted therefrom. In general, however, the structure of the parts engaged in eliminating excretions is as complex as that of the parts concerned in the formation of secretions. The secretions may be arranged into three sorts: (1) exhalations; (2) follieular secretion; and (3) glandular secretion. The exhalations take place as well within the body as at the skin or in the mucous membranes, and are thus divided into external and internal. The follicles are divided into mucous and cutaneous, and into simple and compound. In almost all the points of the skin little openings exist which are the orifices of small hollow organs with membranous sides, generally filled with an albuminous and fatty matter. The small organs are called the follicles of the skin. The glands, however, are the principal organs to which the office of secreting is more especially lly ascribed, and the number of them is considerable. The glandular secretions are of seven different sorts, namely, tears, saliva, bile, pancreatic fluid, urine, semen, and milk. In botany, in consequence of the action of air and light on the watery contents of the green leaves of plants, the materials within them are subjected to a very active chemical condition, by which various substances are formed, -as protein matters, gum, sugar, starch, etc., all of which are essentially necessary to the growth of the plant. Besides these are other matters, such as coloring substances, numerous acids, various alkaloids, etc., which, after their production, perform no further active part in the plant, and are hence removed from the young and vitally active parts to be stored up in the older tissues of the plants as secretions, or removed altogether from them as excretions.