TEETH OF ANIMALS. 261 



anterior surface of the tooth. The consequence 

 is, that the enamel stands up sharp and exposed, 

 so as to protect the bone of the tooth, and to give 

 the surface which is worn down a certain shape : 

 viz., that original shape suited to cut like an adze. 

 The attrition and the arrangement of the material 

 of the tooth so far correspond, that the cutting 

 form is preserved, however much the surface may 

 be worn down. 



Now a tooth cannot grow ; and as we have 

 seen that it is wasted by friction, how is the cutting 

 edge to retain its place ? When the steel plate 

 of the carpenter's plane is forced in by repeated 

 taps with his hammer, he projects the sharp edge ; 

 and when it is elevated above the plane in a just 

 degree, he fixes it there by a smart blow on the 

 wedge : but the cutting edge of the chisel-like in- 

 cisors of the rodentia is still more finely adjusted. 



In the first place, the tooth is very long, extend- 

 ing the whole length of the jaw, and it is of a curve 

 not easily described, not partaking of any section 

 of a cone ; still it is so adapted that the cutting 

 edge meets its opponent tooth, and although inces- 

 santly wasted, it is accommodated to the growing 

 jaw. We have said that a tooth does not grow. 

 It does not grow like a bone ; but sometimes ad- 

 ditions are given to it at the root ; such is the case 

 in this class of animals : the tooth of the gnawer 

 is thus pushed on along the jaw, owing to the 

 growth taking place at its root and at its proper 

 curve : so that the cutting edge is protruded in 



