332 SLOTHS. 



the dentine. They communicate freely with the dentinal tubes, 

 and, by their fine sub-divisions, with the calcigerous cells. I have 

 not observed any canal or vessel in the cement of the Sloths' 

 tooth, capable of carrying red blood. It is, therefore, harder than 

 the vascular dentine, but softer than the unvascular dentine, and 

 wears away, so as to form a bevelled edge at the circumference 

 of the grinding surface. 



The structural appearances of the Sloth's tooth, with unaided 

 vision, are, as might be expected from its unusual composition, 

 different from those presented by ordinary teeth. Cuvier was misled 

 by these appearances to describe the teeth of the Sloths as " being 

 of the most simple structure imaginable. A cylinder of bone en- 

 veloped by enamel, and hollow at both ends ; at the outer end by 

 attrition, at the inner end by default of ossification, and for lodging 

 the remains of the gelatinous pulp which has served for their 

 nucleus. " Voilk," he concludes, " toute leur description." Cuvier, 

 however, adds, that " it may be further remarked that the plates 

 which compose their osseous substance are but imperfectly united 

 to each other. In sawing a tooth longitudinally, these plates are 

 seen to be all distinct, piled one upon another like pieces of money 

 or draughtmen in a tube : which tube is formed by the enamel (1)." 



The parallel and nearly equi-distant vascular canals occasion 

 the appearance of intervals separating distinct layers of vascular 

 dentine. M. F. Cuvier(2) describes the substance surrounding the 

 central axis as being " analogous to, but less hard than enamel." 

 Dr. Richard Harlan(3) appears to have first distinguished the 

 cemental constituent of the Sloth's tooth ; " in which tooth," he 

 says, " there exists first, a central cylinder of bone which is sur- 

 rounded by enamel, which itself is surrounded or enveloped by a 

 regular layer of cement." The intimate structure of the Sloth's 

 tooth was first accurately described by Retzius(4). 



(1) Ossemens Fossiles, torn. v. part 1, p. 84. 



(2) Dents de Mammiferes, 1825, p. 193. 



(3) Medical and Physical Researches, Svo. 1835, p. 315. 



(4) Mikroskopiska undersokningar ofver Tandernes struktur, Svo. 182/, p. 23. 



