What a Gland Does. Enzymes.--In man there are the saliva gland of the mouth, the gastric glands of the stomach, the pancreas and liver, the two latter connected with the small intestine, and the intestinal glands in the walls of the intestine. Besides glands which aid in digestion there are several others of which we will speak later. As we have already learned, a gland is a collection of cells which takes up material from within the body and manufactures from it something which is later poured out as a secretion. An example of a gland in plants is found in the nectar-secreting cells of a flower. Certain substances, called _enzymes_, formed by glands cause the digestion of food. The enzymes secreted by the cells of the glands and poured out into the food tube act upon insoluble foods so as to change them to a soluble form. They are the product of the activity of the cell, although they are not themselves alive. We do not know much about enzymes themselves, but we can observe what they do. Some enzymes render soluble different foods, others work in the blood, still others probably act within any cell of the body as an aid to oxidation, when work is done. Enzymes are very sensitive to changes in temperature and to the degree of _acidity_ or _alkalinity_[42] of the material in which they act. We will find that the enzymes found in glands in the mouth will not act long in the stomach because of the change from an alkaline surrounding in the mouth to that of an acid in the stomach. Enzymes seem to be able to work indefinitely, providing the surroundings are favorable. A small amount of digestive fluid, if it had long enough to work, could therefore digest an indefinite amount of food. Footnote 42: The teacher should explain the meaning of these terms. Gland Structure.--The entire inner surface of the food tube is covered with a soft lining of _mucous membrane_. This is always moist because certain cells, called _mucus cells_, empty out their contents into the food tube, thus lubricating its inner surface. When a large number of cells which have the power to secrete fluids are collected together, the surface of the food tube may become indented at this point to form a pitlike _gland_. Often such depressions are branched, thus giving a greater secreting surface, as is seen in the figure on page 298. The cells of the gland are always supplied with blood vessels and nerves, for the secretions of the glands are under the control of the nervous system. How a Gland Secretes.--We must therefore imagine that as the blood goes to the cells of a gland it there loses some substances which the gland cells take out and make over into the particular enzyme that they are called upon to manufacture. Under certain conditions, such as the sight or smell of food, or even the desire for it, the activity of the gland is stimulated. It then pours out its secretion containing the digestive enzyme. Thus a gland does its work.