CHAPTER LI 

 THE MECHANISMS OF DIGESTION 



MASTICATION. DEGLUTITION, VOMITING 



Mastication 



By the movements of the lower jaw on the upper, the two rows of 

 teeth come together so as to serve for biting or crushing the food. The 

 resulting comminution of the food forms the first step in digestion. The 

 up and down motion of the lower jaw results in biting by the incisors, 

 and after the mouthful has been taken, the side to side movements enable 

 the grinding teeth to crush and break it up into fragments of the proper 

 size for swallowing. The most suitable size of the mouthful is about 

 5 c.c, but this varies greatly with habit. After mastication, the mass 

 weighs from 3.2 to 6.5 gm., about one-fourth of this weight being due to 

 saliva. The food is now a semifluid mush containing particles which 

 are usually less than 2 mm. in diameter. Some, however, may measure 

 7 or even 12 mm. 



Determination of the proper degree of fineness of the food is a func- 

 tion of the tongue, gums, and cheeks, for which purpose the mucous 

 membrane covering them is supplied with very sensitive touch nerve 

 endings (see page 794). The sensitiveness of the tongue, etc., in this 

 regard explains why an object which can scarcely be felt by the fingers 

 seems to be quite large in the mouth. If some particles of food that are 

 too large for swallowing happen to be carried backward in the mouth, 

 the tongue returns them for further mastication. 



The saliva assists in mastication in several ways: (1) by dissolving 

 some of the food constituents; (2) by partly digesting some of the 

 starch; (3) by softening the mass of food so that it is more readily 

 erushed; (4) by covering the bolus with mucus so as to make it more 

 readily transferable from place to place. The secretion of saliva is 

 therefore stimulated by the chewing movements, and its composition 

 varies according to the nature of the food (page 431). In some animals, 

 such as the cat and dog, mastication is unimportant, coating of the food with 

 saliva being the only change which it undergoes in the mouth. In man 

 the ability thus to bolt the food can readily be acquired, not, however, 

 without some detriment to 1 lie efficiency of digestion as a whole. Soft 



444 



