CHAPTER XIII

- GLANDS AND THE WORK OF EXCRETION

In our study so far we have been concerned mainly with the introduction of materials into the body. We are now to consider the removal of materials from the body. The structures most directly concerned in this work are known as

*Glands.*--As generally understood, glands are organs that prepare special liquids in the body and pour them out upon free surfaces. These liquids, known as _secretions_, are used for protecting exposed parts, lubricating surfaces that rub against each other, digesting food, and for other purposes. They differ widely in properties as well as in function, but are all alike in being composed chiefly of water. The water, in addition to being necessary to the work of particular fluids, serves in all cases as a carrier of solid substances which are dissolved in it.

*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.

*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 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 role of a _filtering apparatus_ and of a _manufacturing organ_.

[Fig. 86]

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 (