198 



DIGESTION. 



as to be readily acted upon by the gastric juice ; otherwise stomach-digestion is pro- 

 longed and difficult. Non-observance of this physiological law is a frequent cause of 

 what is generally called dyspepsia. In animals that do not masticate, as in some which 

 live exclusively on flesh, the process of stomach-digestion is much more prolonged than 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 proportionate quantity of food is greater than in man, and diges- 

 tion 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 nourished 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 pro- 

 vided with sixteen teeth, all of which are about equally well developed. The canines, 

 so largely developed in the carnivora but which are rudimentary in the herbivora, and 



FIG. 48. Permanent teeth. (Le Bon.) 

 The external portions of the maxillary bones have been removed to show the roots of the teeth. 



the incisors and molars, so perfectly developed in the herbivora, are, in man, of nearly 

 the same length. Each tooth presents for anatomical description 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 



