106 MICROSCOPIC ANATOMY OF THE TEETH 



The Enamel of Rodents 



There are certain peculiarities in the enamel of the 

 Rodents which render it advisable to consider its structure 

 in a separate section. 



The incisors of Rodents are of great length and very large 

 in proportion to the other teeth ; they extend a long way 

 back in the jaws and describe a sharper curvature in the 

 upper than in the lower jaw. 



The exposed portions of these incisors terminate in a chisel- 

 shaped edge produced by the unequal wear of the three 

 tissues which make up the tooth. The dentine core is 

 coated with enamel, which is confined to the outer or 

 anterior surface, and with cement beneath. The cement 

 does not cover the surface of the enamel as was originally 

 stated by Owen, but in the Rodent-like marsupial, the 

 Wombat, the enamel has an external covering of cement. 

 The unequal wear of the three tissues gives rise to the sharp 

 cutting edge, the cement wearing down most rapidly, the 

 dentine less so, and the enamel least, so that the latter 

 tissue projecting beyond the others forms a sharp chisel 

 edge to the tooth. As the wear rapidly proceeds at this 

 free edge the tooth is maintained at the same level by 

 constant advance from behind, the additional growth taking 

 place during the whole life of the animal from its persistent 

 pulp, and the curvature of the teeth in the jaws preventing 

 injury to this pulp from direct pressure. The incisor teeth 

 of many rodents show a deep orange coloration of the 

 enamel on the outside. This pigmentation is confined chiefly 

 to the outer surface, but penetrates some little distance 

 within the enamel substance, gradually fading away towards 

 the dentine. This coloration is very strongly marked in 

 the Porcupine, Beaver, and Squirrel, but is absent in the 

 Hares and Rabbits. The molar teeth of the Capybara and 

 Beaver and of the Hares and Rabbits also grow from 

 persistent pulps. In the Capybara (HydrocJicerus) the large 

 molar tooth is built up of a series of plates or laminse, each 

 consisting of dentine surrounded by enamel, the separate 

 plates being united by cement. The wear of the tissues 

 being unequal, as in the incisors, a sharp grinding surface 



