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 particulary 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. 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.

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. 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.

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.

Excretory Work of Glands. - The process of removing wastes from the body is called excretion. While in theory excretion may be regarded as a distinct physiological act, it is, in fact, leaving out the work of the lungs, but a phase of the work of glands. From the cells where they are formed, the waste materials pass into the lymph and from the lymph they find their way into the blood. They are removed from the blood by glands and then passed to the exterior of the body.

The Necessity for Excretion is found in the results attending oxidation and other chemical changes at the cells. Through these changes large quantities of materials are produced that can no longer take any part in the vital processes. They correspond to the ashes and gases of ordinary combustion and form wastes that must be removed. The most important of these substances, as already noted, are carbon dioxide, water, and urea. A number of mineral salts are also to be included with the waste materials. Some of these are formed in the body, while others, like common salt, enter as a part of the food. They are solids, but, like the urea, leave the body dissolved in water.

Waste products, if left in the body, interfere with its work (some of, them being poisons), and if allowed to accumulate, cause death. Their removal, therefore, is as important as the introduction of food and oxygen into the body.

D.C. Heath and Co. - Publishers
Original copyright 1909

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