*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 rôle 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 (Chapter X). *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.