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 tecth 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 
