248 ORDER RODENTIA; GENERAL CHARACTERS. 



going on, instead of stopping at an early period of life. Each 

 tooth passes far backwards into the jaw, and is quite hollow at its 

 lower extremity, which is filled with pulp ; from the surface of 

 this, new ivory is continually formed, and thus the tooth is con- 

 stantly being pushed up, at about the same rate as it is worn 

 away at the top, so that its edge is kept up to its proper level. 

 If, however, in consequence of the fracture of the opposite tooth, 

 or from any displacement of the jaw (which sometimes results 

 from accident), the usual wear of the summit of the tooth does 

 not take place, its length continues to increase, often to the 

 great inconvenience of the animal. In fact, cases have been 

 known in which death was the result ; the unchecked growth of 

 the tooth having caused its point to advance in a curve upwards 

 and backwards, until it penetrated the skull and entered the 

 brain. The molar teetb, which are separated from the canines 

 by a wide interval, are composed of alternate plates of enamel 

 and ivory ; the former of which will, from their slower wear, 

 always remain standing up in ridges, so as to give a rasp-like 

 surface to these teeth. The ridges are always transverse, or in 

 a direction from side to side of the head ; and as the lower jaw 

 is endowed, by the peculiar form of its joint, with considerable 

 power of moving backwards and forwards, these teeth obviously 

 become very powerful instruments for the reduction of hard 

 vegetable substances. In the frugivorous (fruit-eating) species 

 of the order, however, such as the Squirrel, the surface of the 

 molar teeth is raised into rounded tubercles, as in the frugivorous 

 animals ; whilst in those which have any carnivorous tendency, 

 such as the Rat, they are raised into sharp points, so as to bear 

 some resemblance to those of the purely Carnivorous quadrupeds. 

 The digestive apparatus in this order is evidently adapted, 

 by the length and complexity of the alimentary canal, to the 

 reduction and preparation of vegetable food; and this is the 

 prevailing habit of all the animals which it includes. Their 

 function obviously is, to make the hard portions of plants, which 

 are rejected by other animals, available as nourishment ; but 

 there are some which are so omnivorous in their propensities, as 

 to devour with voracity almost anything that falls in their way. 



