SALIVA. 



123 



Fig. 23. 



HUMAN TEETH UPPER JAW. a. Incisors. 6. Canines, 

 c. Anterior molars, d. Posterior molars. 



edge running from side to side. The canines (b), which are situated 

 immediately behind the former, are much less prominent and 

 pointed than in the car- 

 nivora, and differ less 

 in form from the inci- 

 sors on the one hand, 

 and the first molars on 

 the other. The molars, 

 again (c, d), are thick 

 and strong, and have 

 comparatively flat sur- 

 faces, like those of the 

 herbivora; but instead 

 of presenting curvili- 

 near ridges, are covered 

 with more or less coni- 

 cal eminences, like those 

 of the carnivora. In the 

 human subject, therefore, the teeth are evidently adapted for a mixed 

 diet, consisting of both animal and vegetable food. Mastication is 

 here as perfect as it is in the herbivora, though less prolonged and 

 laborious ; 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 mastication, so extensive and powerful a triturating ap- 

 paratus. Finally, animal substances are more completely masti- 

 cated in the human subject than they are in the carnivora, and 

 their digestion is accordingly completed with greater rapidity. 



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

 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 irritation 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. 



SALIVA. At the same time that the food is masticated, it is mixed 

 in the cavity of the mouth with the first of the digestive fluids, viz., 

 the saliva. Human saliva, as it is obtained directly from the buc- 

 cal cavity, is a colorless, slightly viscid and alkaline fluid, with a 



