98 



HUMAN PHYSIOLOGY. 



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 sali- 

 vary ducts. Each vesicle or acinus consists of a basement mem- 

 brane surrounded by blood-vessels and lined with epithelial cells. 

 In the parotid gland the lining cells are granular and nucleated ; in 

 the submaxillary and sublingual glands the cells are large, clear, 



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

 (Yeo's "Text-book of Physiology.") 



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

 period of activity. 



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 sali- 

 vary fluids. 



In the living serous gland, e. g., parotid, during rest, the secre- 

 tory 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 



