268 TEETH OF THE PORCUPINE. 



longest, and usually having tbeir sockets extending from 

 the fore to the back part of the under jaw. The tooth 

 consists of a body of compact dentine, with a plate of 

 enamel laid upon its anterior or convex surface, and the 

 enamel commonly consists of two layers, of which the 

 anterior and external one is the densest. Thus the sub 

 stances of the incisor diminish in hardness from the front 

 to the back part of the tooth. The wear and tear from 

 the reciprocal action of the upper and lower incisors pro- 

 duce, accordingly, an oblique surface, sloping from a 

 sharp anterior margin formed by the denser enamel, like 

 that which, in a chisel, slopes from the sharp edge formed 

 by the plate of hard steel laid on the back of that tool, 

 whence these teeth have been called " chisel-teeth" {denies 

 scalprarii). Their growth never ceases while the animal 

 lives, and the implanted part retains the form and size of 

 the exposed part, and ends behind in a widely open or 

 hollow base, which contains a long, conical, persistent 

 forming pulp. This law of unlimited growth is uncon- 

 ditional, and constant exercise and abrasion are required 

 to maintain the normal form and serviceable proportions 

 of the scalpriform teeth of the rodents. When, by acci- 

 dent, an opposing incisor is lost, or when, by the distorted 

 union of a broken jaw, the lower incisors no longer meet 

 jthe upper ones, as sometimes happens to a wounded hare 

 lor rabbit, the incisors continue to grow until they pro- 

 Iject, like the tusks of the elephant, and the extremities, 

 in the poor animal's attempts to acquire food, also become 

 pointed like tusks. Following the curve prescribed to 

 their growth by the form of their socket, their points 

 often return against some part of the head, are passed 

 through the skin, cause absorption of the bone, and per- 

 haps again enter the mouth, rendering mastication im- 



