SECRETING GLANDS. An ordinary secreting gland consists of an aggregation of follicles (small tubes or sacs), all of which open into a common duct, by which the glandular product is discharged. The follicles are lined with epithelial °pHs, placed upon a hyaline basement membrane, which in turn is surrounded by a network of capillaries. These furnish the blood from which is elaborated the secretion by the cell-substance or protoplasm of the epithelia, according to one theory. Some assert that the secretion is com posed of transformed cell-substance. The secre tion of a gland is either mucous (like saliva), serous (like tears), sebaceous (like the oil of the skin), or albuminous. The secretions of the tes ticle and ovary are notable for containing living cells, the spermatozoids and the ova, respectively. The simplest form of a gland is the inversion of the surface of a secreting membrane into follicles, which discharge their contents upon it by sepa rate mouths. Of this we have examples in the gastric glands and follicles of Lieberkiihn, de scribed in the article DIGESTION. Secreting glands are divided into: (1) Tubular, consisting of cylindrical tubes, single or branching; and (2) saccular, composed of numerous sacs arranged about a short tube which joins other similar tubes. The sacs are called acini, and such glands are also called racemose (Lat. racemus, a bunch
of grapes). To understand the .structure of a complex gland like the liver or kidney, it must be followed from the simplest form in which it is known to occur through its various degrees of evolution. In this way the liver may be traced, from the lowest molluscs (where it exists as simple follicles, lodged in the walls of the stom ach, and pouring their product into its cavity by separate orifices) up to man, in whom it is an organ of extreme intricacy; and similarly in the early foetal state of the higher animals, the liver and other secreting organs more or less resemble the persistent state of those parts in animals lower in the scale. In the same way, the mam mary gland (q.v.), which is a structure of con siderable complexity in the higher animals, pre sents a very simple arrangement in the lowest type of this class, the ornithorhynchus, being merely a cluster of follicles, each of which discharges its contents by its own orifice. Some times a gland has several ducts (as, for example, the lachrymal gland), but as a general rule the most important glands have only a single canal, formed by the union of the individual ducts, which conveys the product of the secreting ac tion of the whole mass.