344 MEGATHERIOIDS. 



running parallel with each other : they divide a few times dicho- 

 tomously in their course, and finally anastomose in loops, the 

 convexity of which is directed towards, and in most cases is in 

 close contiguity with the layer of hard dentine. Fine calcigerous 

 tubes are sent off, generally at right angles, from the medullary 

 canals, which quickly divide and sub-divide, form anastomosing 

 reticulations, and communicate freely with the similar tubes that 

 radiate from the calcigerous cells. These are dispersed throughout 

 the dentine, and present an oblong form, with the long axis transverse 

 to that of the tooth, measuring gith of an inch in diameter. The 

 cavity of the cell, which is not quite occupied by their opake 

 contents, is often very clearly demonstrated. The calcigerous 

 tubes, which radiate from the cells nearest the hard dentine, and 

 from the terminal loops of the vascular canals, intercommunicate 

 freely with the calcigerous tubes of the hard dentine. 



The tooth of the Megatherium thus offers an unequivocal 

 example of a course of nutriment from the dentine to the cement, 

 and reciprocally. Professor Retzius has observed, with respect to 

 the human tooth, that " the fine tubes of the coementum enter 

 into immediate communications with the cells and tubes of the 

 dentine (zahnknochen), so that this part can obtain from without the 

 requisite humours, after the central pulp has almost ceased to exist." 

 In the Megatherium, however, those anastomoses do not relate to 

 the performance of a vicarious office, since the pulp is maintained 

 in its full size and functional activity during the whole period of 

 the animal's existence. 



In the structure which the fossil teeth of the Megatherium 

 and its extinct congeners clearly demonstrate, we have striking 

 evidence of their rich organization, and that they were once pervaded 

 by vital activity. All the constituents of the blood freely cir- 

 culated through the vascular dentine and the cement, and the 

 vessels of each substance intercommunicated by a few canals 

 continued across the hard or unvascular dentine. With respect 

 to those minuter tubes, the more important as being more im- 

 mediately engaged in nutrition, which pervade every part of 

 the tooth, characterizing by their difference of length and course 

 the three constituent substances, and which are derived, like the 



