ANIMAL FUNCTIONS. 



and spreading on the lower jaw, and by which the fatter 

 is chiefly moved, is small and feeble when compared 

 with the corresponding part in the feline race. The ar- 

 ticulation of the lower jaw, instead of being such as to 

 allow only of a hinge-like action, is so connected as to 

 play laterally with a grinding motion. 



Such are the differences which the Creator has made 

 between the masticating organs of the flesh-eating, and 

 the grain-eating races of quadrupeds. 



Teeth of the Gnawers. There is still another family 

 of Mammalia, which are remarkably well distinguished 

 by their teeth, and which remove them very decidedly 

 from either of the above classes. These are the Roden- 

 tia or gnawing quadrupeds, as the Squirrel, Rat, Beaver 

 and Rabbit. 



These animals appear to be formed for gnawing the 

 hardest of vegetable bodies, as the shells of nuts, or for 

 living on dry tough materials, as the bark of trees, and 

 even on the woody fibres. They are generally of 

 diminutive size, and reside mostly either in hollow trees, 

 or in burrows which they dig for themselves. 



As an example of the chewing mechanism of this 

 family, the skull and teeth of the Rat are represented by 

 Fig. 78. Fig. 78. The cutting teeth 



are two in number in each 

 jaw. They project forward 

 ' so as merely to admit of be- 

 ing covered by the lips, and 

 are exceedingly sharp, having 

 at the edge the exact shape of 

 a chisel. The roots are large, 

 curved, and set in solid bone, 

 so that in the 'dry skull they cannot be extracted without 

 breaking the jaws. The grinding teeth are marked with 

 raised,, angular lines,, by which they are rendered very 

 perfect instruments for the trituration of hard substances. 



Masticating Organs in Man, compared with those of the 

 Mammalia. In comparing our own organs of mastica- 



What is the difference between the teeth and jaws of canivorous and 

 herbivorous animals t What peculiarities do the teeth of the gnawers 

 present ? 



