CHAPTER V. 



DIGESTION (Cont'd). 



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 bit- 

 ing or crushing the food. The resulting comminution of the food 

 forms the first step in digestion. The manner of occlusion of the 

 cusps of the teeth in the performance of this act is not a problem 

 of Physiology, but rather of Anatomy and Orthodontics; never- 

 theless, the other factors which contribute to the efficiency of the 

 process and the condition into which the food is brought by it 

 are subjects to which we must devote some attention. The up 

 and down motion of the lower jaw results in biting by the in- 

 cisors, 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 suit- 

 able size of the mouthful is about five cubic centimetres, but this 

 varies greatly with habit. After mastication, the mass weighs 

 from 3.2 to 6.5 grammes, about one-fourth of this weight being 

 due to saliva. The food is now a semi-fluid mush containing par- 

 ticles which are usually less than 2 millimetres in diameter. 

 Some, however, may measure 7 and even 12 millimetres. 



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

 function of the tongue, gums and cheeks, for which purpose the 

 mucous membrane covering them is supplied with very sensitive 

 touch nerve endings (see p. 244). 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 swallow- 

 ing happen to be carried backward in the mouth, the tongue re- 

 turns them for further mastication. 



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

 solving some of the food constituents; (2) by partially digesting 



53 



