

39 



cement (c, c), which is of great thickness on the fore and hind surfaces, but is thin 

 where it covers the outer and inner sides of the tooth. 



As the outer layer of the vaso-dentine is first formed by the centripetal calcification 

 of the pulp, the thin crust of that substance at the open base of the tooth includes a 

 space equal to the vaso-dentine at the crown of the tooth : the contraction of the 

 base of the tooth is due to the progressively-diminishing thickness of the cement as 

 it approaches that part ; the intervening vacancy (m, m) in the socket indicating the 

 primitive thickness of the vascular capsule, by the ossification of which the cement 

 is formed. 



The vaso-dentine (Plate XVII. fig. 3, v) is traversed throughout by medullary 

 canals, TcToth of an inch in diameter, which are continued from the pulp-cavity and 

 proceed, at an angle of 50° to the plane of the hard dentine, parallel to each other, 

 with a slightly undulating course, having regular interspaces, equal to one diameter 

 and a half of their own area, generally anastomosing in pairs by a loop (ib. /, I), the 

 convexity of which is turned toward the origin of the tubes of the hard dentine, 

 forming a continuous reflected canal. 



The loops are situated near, and for the most part close, to the hard dentine. In 

 a few places one of the medullary canals may be observed to extend across the hard 

 dentine, and to anastomose with a corresponding canal in the cement. The inter- 

 spaces of the medullary canals of the vaso-dentine are principally occupied by den- 

 tinal tubes, which have an irregular course, form reticulate anastomoses, and termi- 

 nate in very minute cells, at least one hundred times smaller than the calcigerous 

 radiated cells of the cement. 



The more regular and parallel tubes, which traverse the thin layer of unvascular 

 dentine (ib. d), are given off from the convexity of the terminal loops of the medullary 

 canals. The course of these tubes is more directly transverse to the axis of the tooth 

 than is that of the medullary canals from which they are continued. They run par- 

 allel with each other, but with fine undulations throughout their course. They have 

 a diameter of 10 ooo tn °^ an mcn > and have interspaces of about twice that diameter. 

 As the dentinal tubes approach the cement they divide and subdivide, and become 

 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 tubes of the cells of the contiguous cement. 



The cement (ib. c), which enters so largely into the composition of the grinders of 

 the Megatherium, is characterized in that extinct animal by the size, number, and 

 regularity of the vascular or medullary canals (ib. m, m) which traverse it. They pre- 

 sent the diameter of ia 1 o th of an inch, and are separated by intervals equal to from 

 four to six of their own diameters. Commencing at the outer surface of the cement, 

 they traverse it in a direction slightly inclined from the transverse axis towards the 

 crown of the tooth, 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 



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