330 SLOTHS. 



whilst those from the hase of the pulp are transverse to the same 

 axis : the intermediate canals have an intermediate course. Pro- 

 cesses of the vascular pulp are continued along these canals, which 

 consist essentially of tracts of the primitive pulp, which have 

 remained uncalcified. The medullary canals are cylindrical ; but 

 not of equal diameter throughout, sometimes presenting alternate 

 expansions and constrictions : they everywhere give off minute 

 calcigerous tubes, which have a wavy course, and the finest bran- 

 ches of which terminate in the minute cells of the intertubular 

 tissue. The medullary canals occasionally bifurcate, and also, 

 but more rarely, anastomose together by loops, the convexity of 

 which is turned towards the hard dentine. Their usual termination, 

 in Bradypus, is by sphtting into a pencil of smaller wavy branches, 

 close to the hard dentine, the calcigerous tubes of which, are formed 

 by the ultimate sub-division of those branches, as shown in Plate 

 82, fig. "2, on the left hand-side of the section figured. 



The hollow cylinder, or thin layer of hard dentine, resembles 

 very closely that of the human tooth in structure: it consists of a clear 

 substance permeated by the calcigerous tubes given off from the peni- 

 cillate terminations of the medullary canals : the calcigerous tubes 

 have a diameter of j^th of a line, and their interspaces are equal to 

 about twice that diameter. At the sides of the tooth, they proceed in 

 beautiful sigmoid curves parallel with each other, and almost trans- 

 verse to the cement, but a little inclined towards the summit of 

 the tooth : their first curve is convex towards the same part, their 

 second curve concave, and they then proceed straight to the cement, 

 in which they terminate, either by immediately penetrating, or by 

 sending into it their finer sub- divisions. In a few instances, I 

 have detected looped anastomoses of the peripheral ends of the 

 calcigerous tubes. With a magnifying power of 600 linear dimen- 

 sions, the minute branches sent off by the calcigerous tubes into 

 their clear interspaces may be discerned. The axis of vascular 

 dentine is not conformable with the conical pulp ; but, in the 

 molar of the young Ai examined by me, (the third of the lower 

 jaw) expanded at its summit, which is bifid. The calcigerous tubes 

 of the hard dentine sent off from this part of the vascular dentine, 



