I9 8 THE VITAL PROCESSES 



penetrate the tissues immediately beneath the secreting 

 cells. 



4. A system of nerve fibers which terminate in the se- 

 creting 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. 1 The most 

 common plan is that of arranging the parts around a cen- 

 tral 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 com- 

 pound saccular gland. The larger of the compound saccu- 

 lar glands are also called racemose glands, on account of 

 their having the general form of a cluster, or raceme, sim- 



1 The simplest arrangement of the parts of a gland is that where they are spread 

 over a plain surface. This arrangement is found in serous membranes, such as 

 the pleura and peritoneum. These membranes, however, are not called glands, 

 but secreting surfaces. 



