228 UNCONSCIOUS NERVOUS OPERATIONS 



ing cells, and all uniting in a single tube, or duct, through 

 which the secretions pass. Frequently a large number of 

 branches from a secreting tube are grouped together in 

 clusters to form an organ of considerable size, as the liver 

 or the pancreas. 



The secreting surface is always composed of living 

 cells, and the processes carried on in them are similar to 

 those occurring in other cells. That is, secretion involves 

 building up, or growth, and breaking down, or waste, of 

 the cell substance, along with other changes. Under the 

 microscope the cells of the glands are seen to contain a 

 nucleus and many granules. These granules are products 

 of the cell itself. When the secreting process is going on, 

 water and other substances pass through the cells from 

 the blood, and at the same time the granules are dissolved 

 in the water and pass out along the secretory duct. Secre- 

 tion is a manufacturing process, and not merely a filter- 

 ing out of certain substances from the blood. In each 

 gland the chief or specific constituents of its peculiar juice 

 are formed in the cell and not simply extracted from the 

 blood. 



The mucous membrane of the whole of the alimentary 

 canal is largely made up of glands. 



322. Salivary Glands (Fig. 115). There are three pairs 

 of salivary glands. Those lying in front of each ear are 

 the parotid glands; those under the lower jaw on each 

 side, the submaxillary glands; and those under the tongue, 

 the sublingual glands. They are large glands whose ducts 

 pour their watery secretions into the mouth. 



323. Nervous Action upon the Salivary Glands. Ordina- 

 rily the nervous action affecting the salivary glands is 

 reflex. The organs of taste are stimulated by food, or 

 the sight or odor of food stimulates the optic or olfactory 



