26 



HUMAN PHYSIOLOGY. 



are transmitted through motor nerves to the muscles involved in the move- 

 ments of the lower jaw. The medulla not only generates motor impulses, 

 but coordinates them in such a manner that the movements of mastication 

 may be directed toward the accomplishment of a definite purpose. 



Insalivation is the incorporation of the food with the saliva secreted 

 by the parotid, sub-ma xillary and sub-lingttal glands ; the pa rotid saliva, 

 thin and watery, is poured into the mouth through Steno's duct ; the sub- 

 maxillary and sub- lingual salivas, thick and viscid, are poured into the 

 mouth through Wharton's and Bartholini's ducts. 



In their minute structure the salivary glands resemble each other. They 

 belong to the racemose variety, and consist of small sacs or vesicles, 

 which are the terminal expansions of the smallest salivary ducts. Each 

 vesicle or acinus consists of a basement membrane surrounded by blood 



FIG. i. 



CELLS OF THE ALVEOLI OF A SEROUS OR WATERY SALIVARY GLAND. 



A. After rest. B. After a short period of activity. C. After a prolonged period of 

 activity. From Yeo's Text-Book of Physiology. 



vessels and lined with epithelial cells. In the parotid gland the lining cells 

 are granular and nucleated ; in the sub-maxillary and sub-lingual glands the 

 cells are large, clear and contain a quantity of mucigen. During and after 

 secretion very remarkable changes take place in the cells lining the acini, 

 which are in some way connected with the essential constituents of the 

 salivary fluids. 



In a living serous gland, e. g., parotid, during rest, the secretory cells 

 lining the acini of the gland are seen to be filled with fine granules, which 

 are often so abundant as to obscure the nucleus and enlarge the cells until 

 the lumen of the acinus is almost obliterated (Fig. i). When the gland 

 begins to secrete the saliva, the granules disappear from the outer boundary 

 of the cells which then become clear and distinct. At the end of the 

 secretory activity, the cells have become free of granules, have become 

 smaller and more distinct in outline. It would seem that the granular 



