148 THE VITAL FUNCTIONS. 



of the molar teeth are flattened and of great ex- 

 tent ; and they are at the same time, by a provi- 

 sion which will be hereafter explained, kept 

 rough, like those of mill-stones ; their office being 

 in fact very similar to that performed by these im- 

 plements for grinding. All these circumstances of 

 difference are exemplified in the most marked 

 manner, in comparing together the skulls of the 

 larger beasts of prey, as the tiger, the wolf, or 

 the bear, with those of the antelope, the horse, 

 or the ox. 



The Rodentia, or gnawing quadrupeds, which 

 I have already had occasion to notice, compose 

 a well-marked family of Mammalia. These 

 animals are formed for subsisting on dry and 

 tough materials, from which but little nutriment 

 can be extracted ; such as the bark, and roots, 

 and even the woody fibres of trees, and the 

 harder animal textures, which would appear 

 to be most difficult of digestion. They are 

 all animals of diminutive size, whose teeth are 



expressly formed for 

 gnawing, nibbling, 

 and wearing away by 

 continued attrition, 

 the harder textures 

 of organized bodies. 

 The Rat, whose skull 

 is delineated in Fig. 276, belongs to this tribe. 

 They are all furnished with two incisor teeth in 



