104 ZOOLOGY OF THE VOYAGE OF THE BEAGLE. 



rally formed close to the fine dentine. In a few situations I have observed one 

 of the medullary canals continued across the fine dentine, and anastomosing with 

 the corresponding canals of the caementum. The interspaces of the medullary 

 als of the coarse dentine are principally occupied by calcigerous tubes which 

 have an irregular course, anastomose reticularly, and terminate in very fine cells. 

 The more regular and parallel calcigerous tubes, which constitute the thin layer 

 of hard dentine, are given off from the convexity of the terminal loops of the 

 medullary canals. The course of these tubes (b. fig. 1, PI. XXXII.) is rather 

 more transversely to the axis of the tooth than the medullary canals from which 

 they are continued. They run parallel to each other, but with minute undulations 

 throughout their course, in which they are separated by interspaces equal to one 

 and a half their own diameter. As they approach the caementum they divide and 

 sub-divide, and grow more wavy and irregular : their terminal branches take on 

 a bent direction, and form anastomoses, dilate into small cells, and many are seen 

 to become continuous with the radiating fibres or tubes of the cells or corpuscles 

 of the contiguous caementum. This substance enters largely into the constitution 

 of the compound tooth of the Megatherium : it is characterized, like the caemen- 

 tum of the Elephant's grinder, by the presence of numerous radiated cells, or 

 purkingian corpuscles, scattered throughout its substance, but may be distin- 

 guished by wide medullary canals which traverse it in a direction parallel with 

 each other, and forming a slight angle with the transverse axis of the tooth. 

 These canals are wider than those of the central coarse dentine, their diameter 

 being TsV^th of an inch ; they are separated by interspaces equal to from four to six 

 of their own diameters, divide a few times dichotomously 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 dense dentine. 



Fine calcigerous tubes are every where given off at right angles from the 

 medullary canals of the caementum, which form a rich reticulation in their inter- 

 spaces, and a direct continuation between the loops of the medullary canals and 

 the calcigerous tubes of the dense dentine. The caementum differs from the 

 coarse dentine in the larger size and wider interspaces of its medullary canals, 

 and by the presence of the bone-corpuscles in their interspaces ; but they are 

 brought into organic communication with each other, not only by means of the 

 tubes of the dense dentine, but by occasional continuity of the medullary canals 

 across that substance. The tooth of the Megatherium thus offers an unequivocal 

 example of a course of nutriment from the dentine to the caementum, and reci- 

 procally. Retzius observes with respect to the human tooth, that " the fine tubes 

 of the caementum enter into immediate communications with the cells and tubes 

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



