54 PHYSIOLOGY FOR DENTAL STUDENTS. 



some of the starch; (3) by softening the mass of food so that it 

 is more readily crushed; (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 (p. 43). In some animals, such as the cat and dog, 

 there is no mastication, 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 the efficiency of digestion as a whole. 

 Soft starchy food is little chewed, the length of time required 

 for the mastication of other foods depending mainly on their 

 nature, but also to a certain degree on the appetite and on the 

 size of the mouthful. 



The crushing force of the molars, as measured by a dyna- 

 mometer, has been found to rise as high as 270 pounds, which is 

 far in excess of the force required to crush the ordinary food 

 stuffs. Thus cooked meats have a crushing point which varies 

 between 15 and 80 pounds on direct thrust, but is considerably 

 less when there is a side to side movement, as there is in chewing. 

 Candies have a crushing point of 30 to 110 pounds, and nuts 



55 to 170 pounds. Admixture of the food with saliva greatly 

 lowers the crushing point, especially in the case of such foods as 

 soft bread. Without such admixture this hardens into a solitl 

 mass when it is crushed, whereas it readily breaks up into small 

 particles in the presence of saliva. 



It cannot be too strongly insisted upon that the act of masti- 

 cation is of far more importance than merely to break up and 

 prepare the food for swallowing. It causes the food to be moved 

 about in the mouth so as to develop its full effect on the taste 

 buds; the crushing also releases odors which stimulates the ol- 

 factory epithelium. On these stimuli depend the satisfaction 

 and pleasure of eating, which in turn initiate the process of gas- 

 tric digestion (see p. 60). Thus it has been observed in chil- 

 dren with gastric fistula- that the chewing of agreeable fond 

 caused the gastric juice to be actively secreted, which, however, 

 was not the case when tasteless material was chewed. 



