190 



STRUCTURE AND ENDOWMENTS OF ANIMAL TISSUES. 



Fig. 54. 



usually destitute of tubuli ; but Mr. Tomes has shown that it is occa 

 sionally penetrated by prolongations of the tubuli of the dentine, and 

 that this peculiarity, which is occasional and abnormal in Man, is cha- 

 racteristic of the teeth of many Marsupials. In density and resisting 

 power, the Enamel far surpasses any other organized tissue, and ap- 

 proaches some of the hardest of mineral sub- 

 stances. In Man, and in Carnivorous animals, 

 it covers the crown of the tooth only, with a 

 simple cap or superficial layer of tolerably uni- 

 form thickness (Fig. 54, -1), 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 

 Herbivorous animals, however, the Enamel 

 forms (with the Cementum) a series of vertical 

 plates, which dip down (as it were) into the sub- 

 stance of the dentine, and present their edges 

 alternately with it, at the grinding surface of 

 the tooth ; and there is in such teeth no con- 

 tinuous layer of dentine over the crown. The 

 purpose of this arrangement is evidently to pro- 

 vide, by the unequal wear of these three sub- 

 stances, of which the Enamel is the hardest 

 and the Cementum the softest, for the constant 

 maintenance of a rough surface, adapted to tri- 

 turate the tough vegetable substances on which these animals feed. 

 The Enamel is the least constant of the Dental tissues. It is more fre- 

 quently absent than present in the teeth of the class of Fishes ; it is 

 wanting in the entire order of Ophidia (Serpents) among existing Rep- 

 tiles ; and it forms no part of the teeth of the Edentata (Sloths, &c.) 

 and Cetacea (Whales) amongst Mammals. 



319. 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 consequently pass to- 

 wards those, which radiate from the central cavity towards the surface 

 of the dentine, where it possesses a similar vascularity, as was remarka- 

 bly the case in the teeth of the extinct Megatherium. In the Human 

 tooth, however, the Cementum has no such vascularity. It forms a thin 

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

 mination of the capping of Enamel (Fig. 54, 2). This layer is very sub- 

 ject to have its thickness increased, especially at the extremity of the 

 fangs, by hypertrophy, resulting from inflammation ; and sometimes 

 large exostoses are thus formed (Fig. 54, 4), which very much increases 

 the difficulty of extracting the tooth. When the tooth is first developed, 

 the Cementum envelopes its crown, as well as its body and root ; but 



Vertical section of human 

 molar tooth : 1, enamel ; 2, ce- 

 mentum or crusta petrosa; 3, 

 dentine or ivory; 4, osseous ex- 

 crescence, arising from hypertro- 

 phy of cementum ; 5, cavity ; 6, 

 osseous cells at outer part of den- 

 tine. 



