MASTICATION. 265 



of the tongue is derived from the hypoglossal nerve ; its sensation is 

 derived from the lingual branch of the fifth nerve and the glossopharyn- 

 geal, both of these nerves being also concerned in the special sense of 

 taste. 



We see from the above that the act of mastication differs very decid- 

 edly in nature according to the type of organization of the animal, and 

 its characters result from the configuration of the jaws, the pla}- of the 

 muscles, and the form of the teeth. Thus, we find that the movements 

 of mastication in carnivorous animals are restricted to a simple eleva- 

 tion and depression of the lower jaw, this mode of mastication being 

 dependent upon the mode of articulation of the lower with the upper jaw 

 and the overlapping of the upper molar and canine teeth. Mastication, 

 therefore, in these animals is reduced simply to a process of section, 

 laceration, and crushing. The incisor teeth have but slight functional im- 

 portance, and are confined in their action to cutting. The canine teeth 

 are the principal organs of mastication, and exert a lacerating or tearing 

 function, while the molars are crushing in function and, from the fact 

 that they are highly tuberculated on their free crown surface, no process 

 at all analogous to grinding can occur between them. When in these 

 animals bones are crushed, such an operation only occurs on one side at 

 a time. In animals of the cat tribe, from the highly pointed character 

 of their molar teeth, crushing of hard articles of food is performed with 

 greater difficult} 7 than in animals in whom the molar teeth have a more 

 blunt, tuberculated crown. Thus, animals allied to the dog can-^more 

 readily crush bones than the cat tribe. When the molar teeth are 

 brought into play, as in crushing a bone, the substance is usually fixed 

 by the fore paws, while the flesh is torn from the bones by the canine 

 teeth, and then the bones are drawn between the molar teeth by the action 

 of the tongue, the lips being loose and pendulous and enabling the mouth 

 to be opened back beyond the level of the molar teeth : so that, therefore, 

 even large bones may partly be placed between the molar teeth, while the 

 remainder remains without the mouth. Crushing is then accomplished 

 by powerful contractions of the temporal and masseter muscles on one 

 side of the jaw at a time, and are accompanied by motions of the head 

 on the side on which mastication is taking place, and usually by the 

 closure of the eye on the side in which this operation occurs. 



In the herbivora the movements of mastication are much more com- 

 plicated, and, as we find, differ in nature, the most marked extremes 

 being found in the rodents and the ruminants. In all herbivora the 

 lower jaw is always the narrower, and therefore both sides cannot act at 

 once. The jaw is longer, less powerful, and we find among the her- 

 bivora differences in the series of movements for the necessary com- 

 plete comminution of the food with which these animals are sustained. 



