34-8 FOOD AND DIGESTION 



of which exercise a digestive influence upon the portion of the food not already 

 digested and absorbed. In the large intestine some further digestion and 

 absorption take place, and the residue of undigested matter leaves the body 

 in the form of feces. 



Mastication. The act of mastication is performed by the biting and 

 grinding movement of the lower range of teeth against the upper. The 

 simultaneous movements of the tongue and cheeks assist by crushing the 

 softer portions of the food against the hard palate and gums, thus supple- 

 menting the action of the teeth, and by returning the morsels of food to the 

 action of the teeth as they are squeezed out from between them until they 

 have been sufficiently chewed. 



The simple up-and-down or biting movements of the lower jaw are per- 

 formed by the temporal, masseter, and internal pterygoid muscles, the action 

 of which in closing the jaws alternates with that of the digastric and other 

 muscles passing from the os hyoides to the lower jaw, which open the jaws. 

 The grinding or side movements of the lower jaw are performed mainly by 

 the external pterygoid muscles, the muscle of one side acting alternately with 

 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, ninth, 

 and tenth, and the efferent are the motor branches of the fifth and the twelfth 

 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 submaxil- 

 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. 



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 

 nuclei and smaller lumen. During activity the cells become smaller and 

 their contents more opaque. 



