ANATOMICAL PECULIARITIES OF TEETH. 201 



majority of mammals the teeth when fully formed cease 

 to grow ; in some e.g., the rodents the teeth continue to 

 increase in length throughout life : such teeth are said 

 to possess persistent pulps. This is of great advantage 

 to the animal, as it compensates for the continual wear- 

 ing down of the tooth in consequence of the rough work 

 to which such teeth are subjected. Under abnormal 

 conditions a persistent pulp may be, and often is, of 

 great disadvantage. If from accident to a tooth, or 

 some injury to the jaw, the antagonism of such teeth 

 be interfered with, the tooth or teeth which fail to 

 antagonize cannot be used in mastication, and as their 

 continual growth is not checked by wear, they may attain 

 a length of several inches. In such cases the enlarged 

 teeth may prevent the animal feeding, and thus bring 

 about a fatal result from starvation ; or the elongated 

 tooth may re-enter the head or mouth, producing pain 

 or even death, by piercing the brain. This last event is 

 rare ; the usual mode of death is from starvation either 

 the animal cannot bite its food, or the abnormality of 

 the teeth prevents the mouth being opened. Such cases 

 are exceedingly frequent in rabbits and rats ; every game- 

 keeper of experience has met with many examples, and 

 also those who keep white rats and rabbits as pets. 



The study of such aberrant teeth is instructive, as it 

 serves to throw light on the mode of origin of tusks. 

 In many mammals it is common to find in the upper 

 or lower jaw, sometimes in both, one tooth larger and 

 more projecting than its fellows ; this conspicuous tooth 

 has a pointed extremity and is known as the canine. 

 It is exceedingly well developed in most carnivorous 



