304 



FOOD AND DIGESTION 



the other. When both external pterygoids act together, the lower jaw is pulled 

 directly forward, so that the lower incisor teeth are brought in front of the level 

 of the upper. 



The act of mastication is voluntary. It will suffice here to state that the 

 afferent nerves chiefly concerned are the sensory branches of the fifth and 

 tenth or glosso-pharyngeal, and the efferent are the motor branches of the 

 fifth and the twelfth, or hypoglossal, cerebral nerves. 



The act of mastication is much assisted by the saliva, which is secreted by 

 the salivary glands in largely increased amount during the process. The 

 intimate incorporation of the saliva with the food is termed insalivation. 



The Salivary Glands. The glands which secrete the saliva in 

 the human subject are the salivary glands proper, the parotid, the submaccil- 

 lary, and the sublingual, and numerous smaller bodies of similar structure, 

 and with separate ducts, which are scattered thickly beneath the mucous 

 membrane of the lips, cheeks, soft palate, and root of the tongue. 



FIG. 248. 



FIG. 249. 



FIG. 248. Section of the Submaxillary Gland of a Dog, Resting Stage. Most of the Alveolar 

 cells are large and clear, being filled with the material for secretion (in this case, mucigen), which 

 obscures their protoplasm; some of the cells, however, are small and protoplasmic, forming the 

 crescents seen in most of the alveoli. (Ranvier.) 



FIG. 249. Section of a Similar Gland after a Period of Activity. The mucigen has been dis- 

 charged from the mucin- secreting cells, which consequently appear shrunken and less clear. Both 

 the cells and the alveoli are much smaller, and the protoplasm of the cells is more apparent. The 

 crescents of Gianuzzi are enlarged, c, Crescent cells; g, mucus-secreting cells; /, lumen of alveolus. 

 (Ranvier. ) 



Histological Structure. The salivary glands are compound tubular 

 or tubulo-racemose glands. They are made up of lobules. Each lobule con- 

 sists of the branchings of a division of the main duct of the gland, which 

 are generally more or less convoluted toward the extremities, that form the 

 alveoli, or proper secreting parts of the gland. The salivary secreting cells 

 are of cubical or columnar form and are arranged around a central canal. 

 The granular appearance frequently seen in the salivary cells is due to the 

 numerous zymogen granules which they contain. 



During the rest period the cells are larger, highly granular, with obscured 



