326 STRUCTURE AND ADAPTATION OF TEETH 



hardest vegetable substances, and frequently feed 

 on wood or bark. The better to effect this object, 



Fig. 50. — Skull of Beaver. 



the incisors are furnished with extremely hard and 

 dense enamel on their anterior surfaces only. 

 Their reciprocal action maintains an oblique sur- 

 face by the more rapid wearing away of the 

 dentine, and the sharp edge of the enamel is thus 

 perpetually maintained, anticipating the ingenious 

 contrivance resorted to in the manufacture of a 

 carpenter's chisel, in which a layer of hard steel 

 is united to a plate of softer iron ; with this supe- 

 rior endowment, that these living chisels keep 

 each other sharp, and never require setting. 

 Like the canines of the hippopotamus, the im- 

 planted fang retains the form of the portion in 

 use, and the hollow base is filled by a persistent 

 formative pulp, the calcification of the dentinal 

 pulp and the deposit from the accompanying 

 enamel organ keeping pace with the abrasion of 

 the extremity. The upper incisor describes a 

 smaller circle than the lower, its basal extremity 



