138 



DIGESTION. 



stantly repaired, and the animal is able to penetrate the hardest sub- 

 stances. 



In the human subject, the teeth combine the characters of those of the 

 carnivora and the herbivora. (Fig. 33.) The incisors (a), four in num- 

 ber in each jaw, have, as in other instances, a cutting edge running from 

 side to side. The canines (6), which are situated immediately behind 

 the former, are much less prominent and pointed than in the carnivora, 

 and differ less in form from the incisors on the one hand, and the 



first molars on the other. 

 The molars (c, d) are 

 thick and strong, and have 

 comparatively flat surfaces, 

 like those of the herbivora ; 

 but instead of presenting 

 curvilinear ridges, are 

 covered with more or less 

 conical eminences, like 

 those of the carnivora. In 

 the human subject, there- 

 fore, the teeth are evidently 

 adapted for a mixed diet, 

 consisting of both animal 

 and vegetable food. Mas- 

 tication is here as perfect 

 as in the herbivora, though 

 less prolonged and labo- 

 rious ; for the vegetable substances used by man, as already remarked, 

 are previously separated to a great extent from their impurities, and 

 softened by cooking; so that they do not require, for their masti- 

 cation, so extensive and powerful a triturating apparatus. Finally, 

 animal substances are more completely masticated in the human subject 

 than in the carnivora, and their digestion is accordingly completed with 

 greater rapidity. 



We can easily estimate, from the facts above stated, the importance, 

 to the digestive process, of a thorough preliminary mastication. If the 

 food be hastily swallowed in undivided masses, it must remain a long 

 time undissolved in the stomach, where it will become a source of irri- 

 tation and disturbance ; but if reduced beforehand, by mastication, to a 

 state of minute subdivision, it is readily attacked by the digestive fluids, 

 and becomes speedily and completely liquefied. 



HUMAN TERTH UPPER JAW. a. Incisors, b. Ca- 

 nines, c. Anterior molars, d. Posterior molars. 



The Saliva and its Action upon the Food. 



The saliva is a compound fluid, derived from the secretion of four 

 different glandular organs namely, the parotid, submaxillary, and sub- 

 lingual glands, and the muciparous glandules of the cavity of the mouth. 

 All these glands resemble each other in the essential points of their 



