THE TEETH OF MARSUPIAL ANIMALS. 
405 
and then upwards. Near the surface numerous small opaque cells are found irregularly 
interspersed amongst the enamel fibres, or arranged in contour lines. 
In the molar teeth the enamel fibres which arise from the depressions on the mas- 
ticating surface describe several curves, while those from the sides of the tooth and 
of the tubercles have but one flexion, and even this is lost where the enamel thins 
previous to its termination on the neck of the tooth. 
The tubes here, as in the incisors, accompany the fibres, but those which arrive 
near the apex of the tubercles wind round in a spiral course, fig. 3 . When the enam.el 
becomes thin previous to its cessation, the tubes are less abundant ; and at its termina- 
tion few, if any, are seen in this or any of the marsupial teeth. When speaking of 
the enamel in the incisors of the Kangaroo, Professor Owen says*, “The fibres of the 
enamel which invest the crown of the large lower incisor are likewise unusually 
minute; viewed in a transverse section, as in plate 102 ee, they describe an abrupt 
curve at their commencement, and then proceed in a nearly straight course to the 
surface; but at the trenchant margins of the tooth their course is curved, and they 
decussate one another, as represented in the figure-f'. Some of the enamel lines at 
this part seem to be as fine as the dentinal tubes.” Professor Owen has evidently 
seen the enamel tubes in this tooth, but from some cause has failed to recognise them 
as such. In the same page he says, “The terminal branches of the (dentinal) tubuli 
open into minute irregular cells, forming a thin boundary layer between the dentine 
and enamel.” These cells are certainly not between the dentine and enamel in any 
of the sections I have made, but are distinctly enough visible in the enamel, and form 
part of the continuous dentinal and enamel tubes. 
In conducting the examination of this tooth and of the teeth I am about to describe, 
I have made at least half-a-dozen sections, taken from different parts and in different 
directions of the tooth for the purpose of avoiding error. 
Professor Owen states, that the dentinal tubes are the ixWolli of joch in dia- 
meter. In the specimens I have examined, they have averaged i o.ooo fh of an inch at 
their commencement at the pulp-cavity, have gradually diminished, and on entering the 
enamel have not exceeded the y o ' ;l) ~ oo fh of an inch. On dilating they reach e^^eth, 
and again contract to the 2^07^0^^ of an inch, from which they are reduced gradually 
fo the 30,000 th and are lost. 
The enamel fibres measure oeVe th and are cylindrical or oval in their transverse 
section. The dentinal tubes in the root of the teeth vary in diameter indifferent 
parts of their course, and are oval in a transverse section. At their commencement 
they are the iVsT^t, but at the middle of their course they reach the -gi^th of an 
inch in diameter, in addition to which they are subject to short local dilatations. In 
this part of the tooth branches are given off almost from the commencement of the 
* Odontography, p. 397. 
f Plate 102, fig. 1. “ One-half of a transverse section of the lower incisor of a Kangaroo (Macropus major), 
showing the course of the dentinal tubes at d and the fine fibres of the thick enamel at e.” 
3 G 
MDCCCXLIX. 
