582 
PROFESSOR OWEN ON THE MEGATHERIUM. 
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 X^iVI. fig. 3, v) is traversed throughout by medullary 
canals, s^^th 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. /, Z), 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 obse^’ved 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 j^ - ^^- ^ th of an inch, 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 T^^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 dipho- 
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 tubules are sent off, generally at right angles, from the medullary 
canals, which quickly divide and subdivide, form anastomosing reticulations, and 
communicate freely with the similar tubules that radiate from the lacunse or calci- 
gerous cells, ib. r, r. These cells are dispersed throughout the dentine, and present 
an oblong form, with the long axis transverse to that of the tooth, measuring 
