TEETH. 



867 



mj/Jg. 547., in which c is the thick external 

 cement, d the hard dentine, and o the osteo- 

 dentine ; sometimes developed in loose stulac- 

 titic-shaped nodules. 



In the teeth of the Sloth, and its great 

 extinct congener, the Megatherium, the hard 

 dentine is reduced to a thin layer, and the 

 chief bulk of the tooth is made up of a 

 central body of vaso-dentine, and a thick ex- 

 ternal crust of cement. Fig. 548. represents a 



Fig. 548. 



Section of tooth of Megatherium. 



longitudinal section of a lower molar of the 

 Megatherium, of half the natural size : v is 

 the vaso-dentine, d is the hard dentine, and c 

 is the cement ; p is the base of the wide per- 

 sistent pulp-cavity. 



The hard dentine is, of course, the firmest 

 tissue of a tooth so composed, and forms the 

 crest of the transverse ridges of the grinding 

 surface, like the enamel plates in the elephant's 

 grinder. It has, consequently, been described 

 to be enamel *, but its relation to that tissue 

 is only one of analogy or function. 



The human teeth, and those of the carni- 

 vorous mammals, appear at first sight to be 

 composed of dentine and enamel only, as they 

 were described to be by the Cuviersf, who 



* Cuvier, Ossemens Fossiles, 4to, t. v., pt. 1., 

 p. 172. ; and Clift, Transactions of the Geological 

 (Society, 1835, p. 438. 



t F. Cuvier, Dents de Mammiferes, p. 1. 8vo, 

 1825; G, Cuvier, Lecons d'Anat. Comp, iv. (1836), 

 p. 199. 



called them, therefore, simple teeth ; but their 

 crowns are originally, and their fangs are 

 always, covered by a thin coat of cement. 

 There is also commonly a small central tract 

 of ostco-dentine in old teeth. 



In Jig. 7, pi. 122, of my Odontography is 

 given a longitudinal section of a human molar 

 tooth, in which d is the dentine, c the enamel, 

 and c the cement. 



The teeth, called by Cuvier compound or 

 complex in Mammalia, differ, as regards their 

 composition, from the preceding, only by the 

 different proportion and disposition of the 

 constituent tissues. Fig. 549. is a longitudinal 

 section of the incisor of a horse ; d is the 

 dentine, e the enamel, and c the cement ; c' is 

 the layer of cement reflected into the deep 

 central depression of the crown ; and s is the 

 coloured mass of tartar and particles of food 

 which fills up that cavity, forming the u mark" 

 of the horse-dealer. The characteristic struc- 

 ture of the three tissues is shown in the 

 magnified part of the section, t /%. 550. 



Fig. 549. 



Section of incisor of a Horse (Equus). 



A very complex tooth may be formed out 

 of two tissues by the way in which these may 

 be interblended, as the result of an original 

 complex disposition of the constituents of the 

 dental matrix. 



Certain fishes, and a singular family of 

 gigantic extinct Batrachians, which I have 

 called " Labyrinthodonts,"* exhibit, as the 

 name implies, a remarkable instance of this 

 kind of complexity. Fig. 551. is a view of a 

 canine tooth of the Labyrinthodon salaman- 

 drdidcs, of the natural size : and Jig. 552. is a 

 slightly magnified view of a transverse section 

 across the part of the crown marked a. At 

 first view, the tooth appears to be of the simple 

 conical kind, with the exterior surface merely 

 striated longitudinally, but every streak is a 

 fissure into which the very thin external layer 

 of cement (c) is reflected into the body' of 



* Proceedings of the Geological Society, Jan. 20 

 1811, p. 257. 



3 K 2 



