200 



THE VITAL PROCESSES 



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 them- 

 selves have become smaller (Fig. 86). The granules have 

 no doubt been used up in forming the secretion. These 



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. 



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 sub- 

 stances are prepared and added to the secretion. Accord- 

 ing 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 pro- 

 duced by glands may be considered as belonging to one or 

 the other of two classes, known as the useful and the use- 

 less 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). 



