DIGESTION. 87 



The movements of mastication, though orginating in an effort of the 

 will and under its control, are, for the most part, of an automatic or reflex 

 character, taking place through the medulla oblongata and induced by the 

 presence of food within the mouth. The nerves and nerve-centers involved 

 in this mechanism are shown in the following table: 



Nerve Mechanism of Mastication. 



Afferent Nerves. Efferent Nerves. 



1. Lingual branch of 5th pair. i. Third branch of 5th pair. 



2. Glossopharyngeal. 2. Hypoglossal. 



3. Facial. 



The impressions made upon the terminal filaments of the afferent nerves 

 are transmitted to the medulla; motor impulses are here generated which are 

 transmitted through motor nerves to the muscles involved in the movements 

 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. 



Insalivation is the incorporation of the food with the saliva secreted by 

 the parotid, submaxillary, and sublingual glands; the parotid saliva, thin and 

 watery, is poured into the mouth through Steno's duct; the submaxillary 

 and sublingual salivas, thick and viscid, are poured into the mouth through 

 Wharton's and Bartholin's ducts. 



In their minute structure the salivary glands resemble one another. 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-vessels and 

 lined with epithelial cells. In the parotid gland the lining sells are granular 

 and nucleated ; in the submaxillary and sublingual 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 the 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. (See Fig. n.) 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 been freed of granules and have become 



