132 THE VITAL FUNCTIONS. 



that performed by these implements for grinding. 

 All these circumstances of difference are exempli- 

 fied 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 Rodeutia, 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 mate- 

 rials, from which but little nutriment can be ex- 

 tracted ; such as the bark, and roots, and even the 

 woody fibres of trees, and the harder animal tex- 

 tures, which would appear to be most difficult of 

 digestion. They are all animals of diminutive size, 



whose teeth are express- 

 ly formed for gnawing, 

 nibbling, and wearing 

 away by continued at- 

 trition, the harder tex- 

 tures 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 each jaw, 

 generally very long, and having the exact shape of 

 a chisel ; and the molar teeth have surfaces, irre- 

 gularly marked with raised zig-zag lines, rendering 

 them very perfect instruments of trituration. The 

 zygomatic arch is exceedingly slender and feeble ; 

 and the condyle is lengthened longitudinally to 

 allow of the jaw being freely moved forwards and 

 backwards, which is the motion for which the 

 muscles are particularly adapted, and by which 

 the grinding operation is performed. The Beaver, 



