STRUCTURE OF TEETH. DENTINE. 



283 



Fig. 73. 



lower Vertebrata, there is an actual continuity between the bone of the jaw 

 and the teeth projecting from it, notwithstanding that the latter form part of 

 the dermal skeleton, whilst the former belongs to the 

 neural or internal. In Man and the higher animals, 

 however, there is an obvious difference in their struc- 

 ture, as in their mode of development. These sub- 

 jects have lately received much attention; and the 

 practical importance of an acquaintance with them, 

 renders it desirable that they should be here treated 

 somewhat fully. The Teeth of Man, and of most of 

 the higher animals, are composed of three very dif- 

 ferent substances ; Dentine (known as ivory in the 

 tusk of the Elephant), Enamel, and Cementum or 

 Crusta Petrosa. These are disposed in various 

 methods, according to the purpose which the Tooth 

 is to serve : in Man, the whole of the crown of the 

 tooth is covered with Enamel (Fig. 73, 1) ; its root 

 or fang is covered with Cementum (2, 7), whilst the 

 substance or body of the tooth is composed of Den- 

 tine (3). In the molar Teeth of many Herbivorous 

 animals, however, the Enamel and Cementum form 

 vertical plates, which alternate with plates of Den- 

 tine, and present their edges at the grinding surface 

 of the tooth ; and the unequal wear of these sub- 

 stances the Enamel being the hardest, and the Ce- 

 mentum the softest occasions this surface to be al- 

 ways kept rough. 



275. The Dentine* consists of a firm substance, in which mineral matter largely 

 predominates, though to a less degree than in the enamel. It is traversed 

 by a vast number of very fine cylindrical, branching, wavy tubuli j which corn- 

 Fig. 74. 



Vertical section of Human Molar 

 Tooth : 1, enamel ; 2, 7, cementum 

 or crusta petrosa; 3, dentine or 

 ivory ; 4, osseous excrescence, aris- 

 ing from hypertrophy of cemen- 

 tum ; 5, cavity ; 6, osseous cells at 

 outer part of dentine. 



Section through the fang of a Molar Tooth : a, a, dentine traversed by its tubuli ; b, b, nodular layer ; c, c, 



cementum. 



mence at the pulp-cavity (on whose wall their openings may be seen), and radiate 

 towards the surface (Fig. 74, a a). In their course outwards, the tubuli occa- 



1 A structure exactly resembling Dentine has been found by the Author in the shell of the 

 Crab, especially at the tips of the claws ; and a less regular structure of the same kind in 

 the shells of many Mollusca. ("Princ. of Phys., Gen. and Comp.," |g 197, 199, Am. Ed.) 



