THE TEETH OF MARSUPIAL ANIMALS. 
407 
The dentinal tubes are, at the pulp-cavity, about the of an inch in dia- 
meter, and at the peripheral ends so.ooo ^h of an inch. The tubes, as they leave the 
pulp-cavity, advance upwards towards the surface of the tooth, but in the latter part 
of their course they bend outwards, and thus describe large curves, the convexities of 
which are directed towards the masticating surface. Until within the 
inch of the enamel the dentinal tubes give off very few branches, but when they have 
arrived at that point, and for a short distance onward, they give off numerous tubules 
which form a dense meshwork of tubes in a contour line with the enamel, fig. 5 . 
Professor Owen, in the work already referred to — the standard work on Odonto- 
graphy in our language, — mentions that medullary canals are prolonged into the sub- 
stance of the dentine, and also that many of the dentinal tubes terminate in cells on 
the periphery of the dentine. Of this I have now to speak. 
Out of this meshwork of branching and anastomosing tubes a few are continued 
into a comparatively clear space about the of an inch broad, and terminate 
either in irregular cells which vary from the to the 1 s.ooo lh of an inch in 
diameter, or by anastomosis. None however cross into the enamel. That portion of 
the tooth which lies towards the mouth is not invested with enamel, but is covered by 
a thin layer of cement, between which and the dentine there is no distinct line of 
demarcation. Into the cells of this run the terminal branches of the dentinal tubes. 
The cement is continued over the surface of the enamel in a layer of about the 
of an inch in thickness, and this is tenanted by a single line of cells. 
These teeth closely resemble, as well in structure as in external form, those of the 
Rodents, and especially the Hare and Rabbit. 
Phalangista vulpina . — The dentinal tubes of the molar teeth of this creature arise 
at the pulp-cavity, with a diameter of about the 10,0 00 th of an inch, and pursue a 
slightly undulating course until they have accomplished two-thirds of their whole 
distance ; they then bend downwards from the crown, and give off innumerable 
minute tubules. When within a short distance of the enamel they dichotomize once 
or twice, and enter that structure with a diameter of about the 3o, - o ~ 6 uth of an inch, 
fig. 6. 
On passing into the enamel the tubes occasionally dilate into an oval or oblong 
cell of from the ysVoth to the r o.ooo t^^ of inch in diameter, but far more com- 
monly they undergo no dilatation. Others again dilate into an oval or long cell. 
When they have passed half or two-thirds through the enamel, some few divide once 
or twice into two branches. After following an undulating course until near the 
surface of the tooth, they terminate in small cells or become imperceptibly minute. 
The tubes in the enamel have a diameter varying from the T s.ooo th to the -2 5.0 00 th of 
an inch. 
The dentinal tubes, near their commencement at the pulp-cavity, in about the 
middle part of the fang, commonly reach a diameter of the of an inch. They 
give off branches during the whole of their course, and terminate in a dense anasto- 
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