means does nature ensure in the often repeated process, that the
detached osseous parts in the fish should always be well set, and
the tooth transferred, not only to its right place in the series, hut to
its right relative position upon the bone ? The marvel is not diminished
when we consider that immobility is quite impracticable during the
uniting process, that the fish must eat with the teeth that remain
fixed, and that the bone to which the new tooth is in process of
becoming anchylosed must he daily rotated backwards and forwards.
The pharyngeal bones in the Cyprinoids have rarely the thickness
adequate to the lodgment of the matrix of a tooth beneath the
one in place; in this respect they differ materially from those of the
Lahroid fishes: they are, on the contrary, in general, so thin that
they are perforated on the side opposite the anchylosed base of the
teeth in place, and the nerves and vessels of the remaining pulp
pass by these orifices directly into the cavities of the teeth.
In all the Cyprinoid fishes the pulp-cavity of the pharyngeal tooth
is extensive, and traverses almost the whole of the tooth in the
barbel and other species, where the teeth present the elongated laniary
form. The whole of the dentine is composed of fine calcigerous
tubes, analogous to the structure of that of the simple mammalian teeth:
these tubes in the barbel radiate from the narrow and elongated pulp-
cavity, with a general course at right angles to the surface of the tooth,
and consequently to the axis of the tooth: their diameter is ^„th of
an inch at their origin with interspaces of thrice that diameter ; they
proceed with a slightly hut regularly undulating course, branching
dichotomously, but not frequently; and the branches, after slightly
diverging are continued nearly parallel with each other, and in the
direction of the trunk, until they approach the peripheral and denser
portion of the tooth; here they diverge and decussate each other,
and present a general inclination towards the base of the tooth. In
plate 58, fig, 1 is shown the origin of a portion of the calcigerous
tubes from, and in connection with, the uncalcified portion of the
formative pulp, magnified 600 linear diameters ; the honeycombed
surface of the pulp, from which some of the calcified tubes have been
displaced is shown at the lower part of the figure. The clear calcified
walls of the tubes are scarcely, if at all, discernible in sections, which,
like the present, are taken parallel with their axis; and the calcifying
cell or nascent tube appears to project from the surface of the pulp
in a pyramidal form at the base of each tube, and to pass by a contracted
apex into its cavity; this is, however, a deceptive appearance, for
its walls are in continuity with the transparent calcified walls of the
dental tube, and the more opake cavity of the cell alone communicates
with the corresponding cavity of the calcified tube. The true diameter
of the calcigerous tube, including its transparent parietes with its
sub-opake cavity,, nearly equals that of the superficial elongated cell,
at the expense of which it is about to be developed.
From the -circumstance of the section, here figured, including two
or three layers of the calcigerous tubes, their interspaces are made to
appear smaller and their decussation to commence sooner and to be
more frequent than is really the case in any given layer.
In fully formed teeth calcification proceeds in the ligamentous
basis of the pulp from the circumference towards the centre ; the union
of the tooth to the pharyngeal bone takes place in the laniariform
teeth, as in those of the barbel, by numerous slender, wavy, subcylin-
drical processes, corresponding with the former fasciculi of the ligamentous
substance • these calcified processes are implanted, like piles,
into the subjacent bone, slightly diverging as they penetrate it, and
gradually exchanging in it their dentinal for an osseous texture. The
centripetal course of calcification proceeds more rapidly at the basis
than in the body of the tooth, and the pulp-cavity thus becomes
closed below, (excepting at a small central aperture for the passage of
the nerve and vessels), while it remains widely open in the body of the
tooth. A section of part of the base of a pharyngeal tooth of the
barbel, exhibiting the numerous roots or fangs by which it is anchy-
losed to the bone, and the widely open pulp-cavity at the base of the
projecting body of the tooth, is represented, as seen by a moderately
magnifying power (1 inch focus of Ross’s compound achromatic) in
plate 58, fig. 2 : the figure is in the situation of the pulp-cavity:
opposite the right hand is a portion of the thin dentinal wall of the
base of the exserted part of the tooth, in which the transverse course
of the calcigerous tubes is indicated.