Bauhaus-Universität Weimar

Titel:
The Cyclopaedia of Anatomy and Physiology, vol. 4: Pla [corr.: Ple] - Wri
Person:
Todd, Robert Bentley
PURL:
https://digitalesammlungen.uni-weimar.de/viewer/image/lit29465/889/
TEETH. 
and teeth, that close the posterior aperture of 
the mouth. 
The superior dentigerous pharyngeals (Jigs. 
558. and 565.) present the form of an elon¬ 
gated, vertical, inequilateral, triangular plate ; 
the upper and posterior margin is sharp and 
concave ; the upper and anterior margin forms 
a thickened articular surface, convex from side 
to side, and playing in a corresponding groove 
or concavity upon the base of the skull ; the 
inferior boundary of the triangle is the longest, 
and also the broadest ; it is convex in the 
antero-posterior direction, and flat from side 
to side. It is on this surface that the teeth 
are implanted, and in most species they form 
two rows ; the outer one consisting of very 
small, the inner one of large dental plates, 
which are set nearly transversely across the 
lower surface of the pharyngeal bone, and are 
in close apposition, one behind the other : 
their internal angles are produced beyond the 
margin of the bone, and interlock with those 
of the adjoining bone when the pharyngeals 
are in their natural position ; the smaller 
denticles of the outer row are set in the ex¬ 
ternal interspaces of those of the inner row. 
The single inferior pharyngeal bone con¬ 
sists principally of an oblong dentigerous 
plate, of the form represented in fig. 3, pi. 
51, of my “Odontography;” its breadth 
somewhat exceeds that of the conjoined 
dentigerous surface of the pharyngeals above, 
and it is excavated to correspond with their 
convexity. This dentigerous plate is prin¬ 
cipally supported by a strong, slightly curved, 
transverse, osseous bar, the extremities of 
which expand into thick obtuse processes 
for the implantation of the triturating muscles. 
A longitudinal crest is continued downwards 
and forwards from the middle line of the 
inferior pharyngeal plate, anterior to the 
transverse bar, to which the protractor mus¬ 
cles are attached. 
A longitudinal row of small oval teeth 
alternating with the large lamelliform teeth, 
like those of the superior pharyngeals, bounds 
the dentigerous plate on each side ; the inter¬ 
mediate space is occupied exclusively by the 
larger lamelliform or wedge-shaped teeth, set 
vertically in the bone, and arranged trans¬ 
versely in alternate and pretty close set series. 
The dental plates are developed in wide 
and deep cavities in the substance of the 
posterior part of the lower, and of the an¬ 
terior part of the upper pharyngeal bones. 
Each denticle is developed in its proper cap¬ 
sule, which contains an enamel-forming pulp 
and a dentinal pulp, in close cohesion with 
each other and with the thin external capsule. 
The teeth exhibit progressive stages of form¬ 
ation as they approach the posterior part of 
the upper and the anterior part of the lower 
pharyngeal bones : as their formation advances 
to completion they become soldered together 
by ossification of their respective capsules 
into one compound tooth, which soon be¬ 
comes anchylosed by ossification of the den¬ 
tinal pulp to the pharyngeal bone itself. 
The dentine of the pharyngeal teeth of the 
879 
Scarus consists of calcigerous tubes and a 
clear intermediate substance. The calcigerous 
tubes average a diameter of of an inch, 
and are separated by interspaces equal to 
twice their own diameter. The course of 
these tubes is shown in fig. 558, d., in which 
they are exposed by a vertical section through 
the middle of two of the superior denticles. 
They all, on leaving the pulp-cavity, form a 
curve with the convexity turned towards the 
base of the tooth, and then bend slightly in 
the opposite direction; the sigmoid curve 
being most marked in the calcigerous tubes 
at the base of the denticles, whilst those to¬ 
wards the apex become longer and straighter. 
Besides the primary curvatures exemplified 
in the figure, each calcigerous tube is minutely 
undulated ; it dichotomises three or four 
times near its termination, sends off many 
fine lateral branches into the clear uniting 
substance, and finally terminates in a series 
of minute cells and inosculating loops at the 
line of junction with the enamel. 
This substance (fig. 558, e.) is as thick as the 
dentine, and consists of a similar combination 
of minute tubes and a clear connecting sub¬ 
stance. The tubes may be described as com¬ 
mencing from the pheripheral surface of the 
tooth to which they stand at right angles, 
and, having proceeded parallel to each other 
halfway towards the dentine, they then begin 
to divide and subdivide, the branches crossing 
each other obliquely, and finally terminating 
in the cellular boundary between the enamel 
and dentine. 
The teeth which present this complex 
structure are successively developed at one 
extremity of the bone, in proportion as they 
are worn away at the other ; not, however, 
as Cuvier describes, from behind forwards, in 
both upper and lower pharyngeal bones, but 
in opposite directions in the opposite bones, 
the course of succession being from before 
backwards in the upper, and from behind 
forwards in the lower pharyngeal bones. In 
the progress of the attrition to which they 
are subjected, the thin coat of cement result¬ 
ing from the ossification of the capsule is first 
removed from the apex of the tooth, then the 
enamel constituting that apex, next the den¬ 
tine, and, finally, the coarse central cellular 
bone, supporting the hollow wedge-shaped 
tooth ; and thus is produced a triturating 
surface of four different substances of different 
degrees of density. The enamel, being the 
densest element, appears in the form of ellip¬ 
tical transverse ridges, inclosing the dentine 
and central bone ; and external to the enamel 
is the cement which binds together the dif¬ 
ferent denticles. 
There is a close analogy between the dental 
mass of the Scarus and the complicated 
grinders of the elephant, both in form, struc¬ 
ture, and in the reproduction of the compo¬ 
nent denticles in horizontal succession. But 
in the fish, the complexity of the triturating 
surface is greater than in the Mammal, since, 
from the mode in which the wedge-shaped 
denticles of the Scarus are implanted upon,
        

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