SECRETION, in physiology, the proc ess 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 liv ing body, and is as necessary to health as nutrition. Where the secreted mate rials 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 sub stances existing ready formed in the blood, and are merely extracted there from. In general, however, the structure of the parts engaged in eliminating ex cretions 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) follit ular secretion; and (3) glandular secre tion. 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 al
most all the points of the skin little open ings 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 pecially ascribed, and the number of them is considerable. The glandular secretions are of seven different sorts, namely, tears, saliva, bile, pancreatic fluid, urine, se men, and milk.
In botany, in consequence of the action of air and light on the watery contents of the green leaves of plants, the mate rials within them are subjected to a very active chemical condition, by which vari ous 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 sub stances, numerous acids, various alka loids, 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 front them as excretions.