*General Structure of Glands.*—While the various glands differ greatly in size, form, and purpose, they present striking similarities in structure. All glands contain the following parts: 1. Gland, or secreting, cells. These are _specialized_ cells for the work of secretion and are the active agents in the work of the gland. They are usually cubical in shape. 2. A basement membrane. This is a thin, connective tissue support upon which the secreting cells rest. 3. A network of capillary and lymph vessels. These penetrate the tissues immediately beneath the secreting cells. 4. A system of nerve fibers which terminate in the secreting cells and in the walls of the blood vessels passing to the glands. These structures—secreting cells, basement membrane, capillary and lymph vessels, and nerve fibers—form the essential parts of all glands. The capillaries and the lymph vessels supply the secreting cells with fluid, and the nerves control their activities. *Kinds of Glands.*—Glands differ from one another chiefly in the arrangement of their essential parts.(73) The most common plan is that of arranging the parts around a central cavity formed by the folding or pitting of an exposed surface. Many such glands are found in the mucous membrane, especially that lining the alimentary canal, and are most numerous in the stomach, where they supply the gastric juice. If these glands have the general form of tubes, they are called _tubular_ glands; if sac-like in shape, they are called _saccular_ glands. Both the tubular and the saccular glands may, by branching, form a great number of similar divisions which are connected with one another, and which communicate by a common opening with the place where the secretion is used. This forms a _compound_ gland which, depending on the structure of the minute parts, may be either a _compound tubular_ or a _compound saccular_ gland. The larger of the compound saccular glands are also called _racemose_ glands, on account of their having the general form of a cluster, or raceme, similar to that of a bunch of grapes. The general structure of the different kinds of glands is shown in Fig. 85. [Fig. 85] Fig. 85—*Diagram illustrating evolution of glands.* _A._ Simple secreting surface. 1. Gland cells. 2. Basement membrane. 3. Blood vessel. 4. Nerve. _B._ Simple tubular gland. _C._ Simple saccular gland. _D._ Compound tubular gland. _E._ Compound saccular gland. _F._ A compound racemose gland with duct passing to a free surface. _G._ Relation of food canal to different forms of glands. The serous coat has a secreting surface.