211. Microscopic Structure.—The body of the long molar teeth of
the Horse consists of columns of fine-tubed unvascular dentine
(PL 137, a), coated by enamel (b) which descends in deep folds into
the substance of these teeth ; the enamel is covered by cement (c),
thickest in the interspaces of the inflected enamel-folds and upon the
crowns of the molars, where it is permeated by vascular canals,
thinnest on the crowns of the canines and incisors. At the roots of
these teeth, and on those developed from the worn down molars,
the dentine is immediately invested by cement.
In a vertical section of the incisor, as in PI. 136, fig. 11, the pulp-
cavity, contracting as it approaches the vertical enamel-fold, divides
near the end of that fold, and extends a little way between it and
the periphery of the incisor, or leaves a few medullary canals and
a modified thin tract of irregularly formed dentine, between the
reflected and the outer coat of enamel but rather nearer the former.
Above this tract, near the Summit of the crown, the dentinal tubes
proceed in a nearly vertical direction, with a gentle sigmoid primary
flexure, where they diverge from the perpendicular; lower down
the dentinal tubes diverge in opposite directions, curving from the
remains of the pulp-fissure towards the outer and the inner enamel;
and are described by Retzius as being bent in the form of the Greek
c (1); but the course of two distinct series of dentinal tubes, and
not that of a single tube is illustrated by this comparison; when the
pulp-cavity becomes single and central, as at the lower half of the
tooth, the tubes diverge to the periphery, with one principal primary
curve, convex towards the crown. Each tube is bent in minute
secondary gyrations to within a short distance of its peripheral
termination, where it is much diminished in size, ahd is dichoto-
mously branched. The tubes at their commencement from the
upper calcified tracts of the pulp-cavity, which usually retain some
remnants of that vascular receptacle in the form of medullary canals,
are strongly and irregularly flexuous, before they fall into the
ordinary primary curves. Those tubes proceeding towards the inner
reflected fold of enamel, are more vertical than the tubes going
to the periphery..
(1) Loc. cit. p. 27.
A transverse section of the incisor of a young Horse or Ass
taken across the part marked a in fig. 11, shows a long oval island
of vascular cement in the centre, bounded by a border of enamel,
with an irregular crenate edge next the cement, and an even edge
next the dentine ; which is here clearly seen to be divided into an
inner and an outer tract by an irregular series of the vascular canals
continued from the summit of the pulp-cavity, and by the irregularly
tortuous dentinal tubes which, with the canals, indicate the last
converted remnant of the pulp in this part of the crown. The inner
tract of dentine next the island of enamel is well defined, and a
little broader than the section of the enamel itself, and shows the
extremities of the tubes cut transversely across, which tubes, as
before observed, were at this part directed chiefly in the axis of
the incisor towards the working surface of the crown. The tubes
in the outer tract of dentine, inclining more towards the sides of the
tooth, are more obliquely divided and at the ends of the section they
are seen lengthwise elegantly diverging towards the sides of the
section. This tract of dentine is bounded externally by a layer
of enamel, one sixth part thicker than that forming the central
island; and the enamel is coated by an outer layer of cement, of its
own thickness at the sides, but thinning off at the two ends of the
section. The dentinal tubes proceeding from the residuary pulp-
tract make strong and irregular curvatures, diverging to include
the divided arese of the vascular canals, and in the outer layer
at one side of the section, they describe strong zig-zag curves at
the middle of the outer division of the dentine.
The diameter of the dentinal tubes at their central and larger
ends is pretty regularly, about th of an inch ■ at the middle
of their course sffioth of an inch ; thence decreasing, and very rapidly
after the terminal bifurcations commence.
The dentinal tubes are separated from one another by intervals
varying between once and twice the thickness of the tubes ; in some
parts of the dentine of the incisor they are more closely crowded
together, especially near their origin from the pulp-cavity. The
secondary gyrations of the dentinal tubes describe a curve about ^ t h
of an inch in length; these subside^ in the slender terminations
p p