whilst those from the base of the pulp are transverse to the same
axis : the intermediate canals have an intermediate course Processes
of the vascular pulp are continued along these canals, which
consist essentially of tracts of the primitive pulp, which have
remained uncalcified. The medullary canals are cylindrical; but
not of equal diameter throughout, sometimes presenting alternate
expansions and constrictions : they everywhere give off minute
calcigerous tubes, which have a wavy course, and the finest branches
of which terminate in the minute cells of the intertubular
tissue. The medullary canals occasionally bifurcate, and also,
but more rarely, anastomose together by loops, the convexity of
which is turned towards the hard dentine. Their usual termination,
in Bradypus, is by splitting into a pencil of smaller wavy branches,
close to the hard dentine, the calcigerous tubes of which, are formed
by the ultimate sub-division of those branches, as shown in Plate
82, fig. m on the left hand-side of the section figured.
The hollow cylinder, or thin layer of hard dentine, resembles
very closely that of the human tooth in structure: it consists of a clear
substance permeated by the calcigerous tubes given off from the peni-
cillate terminations of the medullary canals: the calcigerous tubes
have a diameter of j^th of a line, and their interspaces are equal to
about twice that diameter. At the sides of the tooth, they proceed in
beautiful sigmoid curves parallel with each other, and almost transverse
to the cement, but a little inclined towards the summit of
the tooth: their first curve is convex towards the same part, their
second curve concave, and they then proceed straight to the cement,
in which they terminate, either by immediately penetrating, or by
sending into it their finer sub-divisions. In a few instances, I
have detected looped anastomoses of the peripheral ends of the
calcigerous tubes. With a magnifying power of 600 linear dimensions,
the minute branches sent off by the calcigerous tubes into
their clear interspaces may be discerned. The axis of vascular
dentine is not conformable with the conical pulp; but, in the
molar of the young Ai examined by me, (the third of the lower
jaw) expanded at its summit, which is bifid. The calcigerous tubes
of the hard dentine sent off from this part of the vascular dentine,
do not present the curvatures observable in the lateral layer, but
pass almost in a straight line to the grinding surface of the tooth,
slightly converging and decussating each other at the middle, and
slightly diverging near the margin of the crown. The coronal portion
of the hard dentine, which is originally nearly three times the thickness
of the cylindrical or parietal portion, is worn away almost before
the Sloth reaches maturity; and, as the pulp, when once it has
taken on the incomplete process of calcification which produces
the vascular cement, is never afterwards subject to the primitive
and more complete process, the coronal hard dentine is never reproduced.
The depressed centre of the grinding surface is then
formed by the vascular dentine only; that surface having its inequality
maintained by the edge of the cylinder of hard dentine, in place of
enamel, and being thus adapted to the comminution of the softer
vegetable substances, as the leaves and tender buds of trees. (1)
The external coat of cement has an average thickness of 3th of
a line ; it gradually thins off to the sharp margin of the pulp-cavity,
forming the inserted base of the tooth. It is principally characterized
by the numerous calcigerous cells, represented by the dark
or opake dots in Plate 82, fig. 1. These present a more or less
oblong form, with the long axis parallel with that of the tooth |
those next the hard dentine making the nearest approach to the
circular form. Their average diameter is ^ th of an inch in the
long-diameter, and js»th in the short diameter; but the extremely
numerous minute tortuous tubes, which open into every part of
their circumference, give the cells a greater apparent size than they
present when viewed by a sufficiently deep power for the clear
analysis of the rich and minute tubular system, with which they
are everywhere connected. The principal trunks of this system,
which, nevertheless, do not exceed the diameter of the calcigerous
tubes of the hard dentine, affect a parallel course in many parts
of the cement, at nearly right angles to its surface. They are most
conspicuous at that part of the cement, which is in contact with
(1) Cecropia peltata and Achra sapota have been particularized as affording nutriment
to the Sloths ; hut, probably, few of the trees forming the denser forests of tropical America
are exempt from their attacks.