MASTICATION. 



137 



Fig. 30. 



molars, eight or more in number on 

 each side, are larger and broader than 

 the incisors, and provided with serrated 

 edges, each presenting several sharp 

 points, arranged generally in a direc- 

 tion 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 

 herbivora, on the other hand, whose 

 food is more easily obtained, but is 

 generally more hard and resisting in 



texture, the teeth are adapted especially for mastication. In the rumi- 

 nating animals generally, the canine teeth are wanting, and the incisors 

 are present only in the lower jaw. In the horse and allied species, 

 the incisors are present in both upper and lower jaws (Fig. 31), and 

 are used simply for cutting off the herbage upon which the animal feeds. 



Fig. 31. 



OF POLAR BKAR. Anterior 

 view ; showing incisors and canines. 



SKULL OF THE HORSE. 



The canine teeth are absent in the female, and only slightly developed 

 in the male, and the real process of mastication is performed altogether 

 by the molars. These are large and thick (Fig. 32), and present a broad, 

 flat surface, diversified by variously folded and projecting ridges of 

 enamel, with shallow grooves between them. By 

 the lateral rubbing motion of the roughened sur- 

 faces against each other, the food is effectually com- 

 minuted and reduced to a pulpy mass. 



In the gnawing animals, rats, mice, squirrels, rab- 

 bits, and hares, the incisor teeth are developed to a 

 remarkable extent, presenting two chisel-like edges 

 opposed to each other in the upper and lower jaws, 

 and growing from permanently vascular roots ; so 

 that their waste from mechanical attrition is con- 

 10 



Fig. 32. 





MOLARTOOTH OF 



THK HORSE. Grind- 

 ing surface. 



