122 



DIGESTION. 



Fig. 20. 



edges, each presenting several sharp points, arranged generally in 

 a direction parallel with the line of the jaw. In these animals, 



mastication is very imperfect, since 

 the food is not ground up, but only 

 pierced and mangled by the action 

 of the teeth before being swallowed 

 into the stomach. In the herbi- 

 vora, on the other hand, the inci- 

 sors are present only in the lower 

 jaw in the ruminating animals, 

 though in the horse they are found 

 in both the upper and lower max- 

 illa. (Fig. 21.) They are used merely 

 for cutting off the bundles of grass 

 or herbage, on which the animal feeds. The canines are either 

 absent or slightly developed, and the real process of mastication is 



Fig. 21. 



SKULL OF POLAR BEAR. Anterior 

 Tiew ; showing incisors and cauines. 



Fig. 22. 



SKTLL or THK HORSE. 



performed altogether by the molars. These are large and thick 

 (Fig. 22), and present a broad, flat surface, diversified by variously 

 folded and projecting ridges of enamel, with shal- 

 low grooves, intervening between them. By the 

 lateral rubbing motion of the roughened surfaces 

 against each other, the food is effectually commi- 

 nuted and reduced to a pulpy mass. 



In the human subject, the teeth combine the 

 characters of those of the carnivora and the herbi- 

 TOOTH OF V ora. (Fig. 23.) The incisors (a), four in number 



THE HORSE. Grind- _ V -.--* 



ing surface in each jaw, have, as in other instances, a cutting 



