3. A network of capillary and lymph vessels. These[pg 198] 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[pg 199] to that of a bunch of grapes. The general structure of the different kinds of glands is shown in 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.
Nature of the Secretory Process.—At one time the gland was regarded merely as a kind of filter which separated from the blood the ingredients found in its secretions. Recent study, however, of several facts relating to secretion has led to important modifications of this view. The secretions of many glands are known to contain substances that are not found in the blood, or, if present, are there in exceedingly small amounts. Then again the cells of certain glands have been found to undergo marked changes during the process of secretion. If, for example, the[pg 200] cells of the pancreas be examined after a period of rest, they are found to contain small granular bodies. On the other hand, if they are examined after a period of activity, the granules have disappeared and the cells themselves have become smaller (Fig. 86). The granules have no doubt been used up in forming the secretion. These and other facts have led to the conclusion that secretion is, in part, the separation of materials without change from the blood, and, in part, a process by which special substances are prepared and added to the secretion. According to this view the gland plays the double rôle of a filtering apparatus and of a manufacturing organ.
Fig. 86—Secreting cells from the pancreas (after Langley). A. After a period of rest. B. After a short period of activity. C. After a period of prolonged activity. In A and B the nuclei are concealed by the granules that accumulate during the resting period.
Kinds of Secretion.—In a general way all the liquids produced by glands may be considered as belonging to one or the other of two classes, known as the useful and the useless secretions. To the first class belong all the secretions that serve some purpose in the body, while the second includes all those liquids that are separated as waste from the blood. The first are usually called true secretions, or secretions proper, while the second are called excretions. The most important glands producing liquids of the first class are those of digestion (Chapter X).