MASTICATION. 139 



in the human subject, even when the diet is the same ; and 

 it is found that while man must, as a rule, take food two or 

 three times in the day, the carnivorous animals are generally 

 best nourished when food, in proper quantity, is taken but 

 once in the twenty-four hours. In the carnivora, the propor- 

 tionate quantity of food is greater than in man, and digestion 

 is much more prolonged. 



The comparative anatomy of the organs of mastication 

 makes it evident that the human race is designed to live on a 

 mixed diet ; but experience has shown that man can be nour- 

 ished for an indefinite period on a diet composed exclusively 

 of either animal or vegetable principles. 



Physiological Anatomy of the Organs 'of Mastication. 



In the adult, each jaw is provided with sixteen teeth, all 

 of which are about equally well developed. The canines, so 

 largely developed in the carnivora, but which are rudiment- 

 ary in the herbivora, and the incisors and molars, so per- 

 fectly developed in the herbivora, are, in man, of nearly the 

 same length. Each tooth presents, for anatomical descrip- 

 tion, a crown, a neck, and a root or fang. The crown is 

 that portion which is entirely uncovered by the gums ; the 

 root is that portion embedded in the alveolar cavities of the 

 maxillary bones; and the neck is the portion, sometimes 

 slightly constricted, situated between the crown and the root, 

 covered by the edge of the gum. Thin sections of the teeth 

 show that they are composed of several distinct structures. 



Enamel of the Teeth. The crown is covered by the enam- 

 el, which is by far the hardest structure in the economy. 

 This is white and glistening, and is thickest on the lower 

 portion of the tooth, especially over the surfaces which, 

 from being opposed to each other on either jaw, are most 

 exposed to wear. It here exists in several concentric layers. 

 The incrustation of enamel becomes gradually thinner tow- 

 ard the neck, where it ceases. Microscopical examination 



