590 



VERTEBRATED ANIMALS. 



FIG. 305. 



Vertical section of Human 

 Molar Tooth : 1, enamel ; 2, 

 cernentum or crusta peirosa ; 

 3, dentine or ivory; 4, osse- 

 ous excrescence, arising 

 from hypertrophy of cemen- 

 tum; 6, pulp-cavity ; 6, osse- 

 ous lacunae at outer part of 

 dentine. 



more or less wavy ; and they are marked by numerous trans- 

 verse striae, resembling those of the pris- 

 matic shell-substance, and probably origi- 

 nating in the same cause, the coalescence 

 of a series of shorter cells, to form the 

 lengthened prism. In Man, and in Car- 

 nivorous animals, the enamel covers the 

 crown of the tooth only, with a simple cap 

 or superficial layer of tolerably uniform 

 thickness (Fig. 305, a), which follows the 

 surface of the dentine in all its inequalities ; 

 and its component prisms are directed at 

 right angles to that surface, their inner 

 extremities resting in slight but regular 

 depressions on the exterior of the dentine. 

 In the teeth of many of the Herbivorous 

 animals, however, the Enamel forms (with 

 the Cementum) a series of vertical plates, 

 which dip down into the substance of the 

 dentine, and present their edges alternately 

 with it, at the grinding surface of the tooth ; 

 and there is in such teeth no continuous 

 layer of enamel over the crown. The pur- 

 pose of this arrangement is evidently to 

 provide, by the unequal wear of these 

 three substances, of which the enamel is the hardest, and the 

 cementum the softest, for the constant maintenance of a rough 

 surface, adapted to triturate the tough vegetable substances on 

 which these animals feed. The enamel is the least constant of 

 the dental tissues. It is more frequently absent than present 

 in the teeth of the class of Fishes ; it is wanting in the entire 

 order of Ophidia (serpents) among existing Reptiles ; and it 

 forms no part of the teeth of the Edentata (sloths, &c.) and 

 Cetacea (whales) amongst Mammals. The Cementum, or Crusta 

 Petrosa, has the characters of true bone ; possessing its distinctive 

 stellate lacunae and radiating canaliculi. Where it exists in 

 small amount, we do not find it traversed by medullary canals ; 

 but, like dentine, it is occasionally furnished with them, and 

 thus resembles bone in every particular. These medullary canals 

 enter its substance from the exterior of the tooth, and conse- 

 quently pass towards those which radiate from the central cavity 

 in the direction of the surface of the dentine, where this pos- 

 sesses a similar vascularity, as was remarkably the case in the 

 teeth of the extinct Megatherium. In the Human tooth, how- 

 ever, the cementum has no such vascularity ; but forms a thin 

 layer, which envelopes the root of the tooth, commencing near 

 the termination of the capping of enamel (Fig. 305, b). In the 

 teeth of many Herbivorous Mammals, it dips down with the 

 enamel to form the vertical plates of the interior of the tooth ; 



