The enamel-fibres are smaller than those of the teeth of the
Carnivora : and the cement is much thicker upon the roots of the
teeth of the Hog, and can be more readily traced over the enamelled
crown, especially in hollows of the irregular grinding surface, and
upon those parts of the dentine of the tusks which are not covered
by enamel. Here, and upon the roots of the teeth, the Purkingian
cells were developed, and of a full oval figure, about ^ th of an
inch in long diameter: the cemental tubuli, communicating with
these cells on the one hand, and with the terminal plexuses of the
dentinal tubuli on the other, traverse the thicker layers of cement
in horizontal and parallel directions, resembling in this respect the
dentinal tubes.
The complex molar of the Phacochere yields a beautiful demonstration
of the microscopic characters of the several tissues of a
tooth.T
he dentine, "as we have seen, is broken up into detached, long,
and slender prisms. In a longitudinal section of one of these
denticles, the calcigerous tubes ascend vertically through the central
third part of the prism, and in the lateral parts gently diverge
or bend away towards the periphery; and when they are close to
the enamel increase their outward curve and the width of their
clear interspaces, bifurcating there once or twice : several of the
terminal branches dilate into opake cells close to the enamel; the
secondary undulations of the tubes are minute but well marked,
especially in the ends of the tubes. One or two medullary canals,
with a thin coat of osteo-dentine, extend along the centre of the
prism. In transverse sections the ends of the tubes, cut across,
occupy all but the marginal part: they rarely shew interspaces
wider than the breadth of a tube, and are in some places even closer
together. The peripheral terminations of the tubes, appear to
be more strongly bent than when seen in longitudinal sections.
The tubes are smaller, more numerous, and send off much fewer
lateral ramuli than in the dentition of the Horse.
In the middle part of the transverse section the dentinal cells
are sub-elliptic, with a clear outline, and include about twenty
of the dentinal tubes; towards the periphery they are indicated by
slightly curved lines, with the feeble convexity turned towards the
periphery.
In the middle of the transverse section of the dentinal prism,
a narrow tract of osteo-dentine is seen surrounding the area of
the central vascular canal: it is characterized by its more transparent
texture; its fewer and more contorted tubuli, and the minute
opake cellules dispersed through it. The clear substance is arranged
in concentric layers around the canal, and the outer boundary
of this small sub-circular patch of osteo-dentine is very well defined
from the true dentine | the central ends of the tubes of this
tissue are closely crowded round the clear boundary of the osteo-
dentine, but a very few are traceable into that substance.
The enamel-fibres, as exposed by a vertical section of the
complex tooth, extend obliquely upwards and outwards in a straight
line, for the first half of their course, then bend outwards to the
periphery of the enamel, which is very irregular. The transverse
section of the cylinder of enamel displays a strong wavy, or crenate
outline : the rounded lobes which project into the cement being
of nearly equal breadth with the intervening notches. The contourlines
of the enamel are well shown in this section, and alter their
course from the parallel of the even margin of the dentine to
that of the crenate outer margin of the enamel.
A very small outer part of the exterior cement of the molar
is nearly clear, with the fine cemental tubes penetrating the substance
at right angles to the surface. A thick and densely aggregated
stratum of Purkingian cells, mostly of a full oval figure, are next
seen, these cells being rather less thickly arranged in the middle
of the cement. The vascuar canals of the cement are chiefly
present near the enamel; some were seen forming loops turned
towards the outer surface of the cement.
200. Cheeropotamidce.—The dental formula of the Peccari
(Dicotyles,) is :—
Incisors canines^ 9 ; premolars ^ ; molars — : = 38.
The two middle upper incisors are the largest, and are thick,
short, curved and convergent, as in the Hog. The external surface