pulp, and in the ratio of the number, minuteness and aggregated
disposition of the cavities containing that earthy matter, is the facility
of detaching the ossified from the unossified part of the tooth
increased. But this facility of separation is quite inadequate to prove
an absence of organic connection between the separated parts, and
a formation of the calcified layer by transudation from a free secreting
surface. The calcigerous tubes of the mammiferous tooth have
distinct parietes in both the calcified and uncalcified portions of the
pulp ; these parietes are rendered brittle by the deposition therein of
earthy particles in the calcified part of the pulp, and separate readily
from the uncalcified continuation of the tubes in the remaining pulp;
while the minuteness of the ruptured tubes renders the irregularity
of the denuded surface of the pulp invisible to the naked eye ; hut
the appearance thus presented of a naturally free transuding surface is
deceptive. (1 )
(]) Since these facts and the general conclusions as to the nature of dentification deduced
rom them were mentioned in my Lectures at the College of Surgeons (May 1839), and subsequently
communicated with more detail to the French Academy, I have perused the work by
Dr. Schwann, entitled “ Untersuchungen ueber Einstimmung der Struktur und Wachsthum
der Pflanze und Thiere,” 8vo., 1839 : or, " Observations on the Correspondence between Plants
and Animals in their Structure and Growth.” Not anticipating, from the title of this work,, that
it contained observations bearing immediately on dental anatomy, I have to regret that some
months elapsed after its publication before I ordered it from my bookseller. Dr. Schwann
describes the results of microscopic observations, which he instituted on the development of the
dentine in mammalian teeth, and arrives at the conclusion that the process-like ossification, is one
of intus-susception. Thus the theory of dentification, which I applied analogically from observation
of the process in the shark, to the same process in the higher vertebrate animals, is established et I
visu by one of the most accurate and experienced micrographers of the present day. A full analysis j
of his observations will be given in the general introduction to the presentwork. Cuvier, after stating
that the teeth of fishes grow like those of quadrupeds, by layers, adds in the first edition of the
Leçons d’Anatomie Comparée, iii, p. 112, "Mais lea dents qui he tiennent qu’à la gencive
seulement, comme celles des Squales, croissent à la manière des epiphyses des os, c’est-à-dire
que toute leur substance est d’abord tendre et poreuse, et qu’elle se durcit uniformément, et finit
par devenir entièrement dure comme de l’ivoire.” I have never seen an instance of such uniform
hardening—if by this is meant a uniform deposition of the earthy particles through the
whole substance of the pulp—in the tooth of any shark or other animal ; and, were such actually
the case, it would not be like the ossification of an epiphysis. It seems that Cuvier himself had
withdrawn his confidence in the observation and conclusion above quoted, as the passage is sup-1
pressed in the second edition of the Leçons, by M. Duvemoy, in 1835. It is true, however, that
the teeth, not only of sharks, hut of the higher animals, are developed like bones ; the hardening
salts in both cases are deposited in preformed cavities organized in a pre-existing mould or
As a consequence of a formation of a tooth by conversion of,
instead of transudation from a pre-existing pulp, the successive formation
of these pulps necessarily follows, where a succession of teeth
is required ; these reproductive pulps are developed in the shark in
the vascular mucous membrane at the angle of reflection of the
thecal fold upon the groove at the basal line of the jaws. They
gradually advance from this situation towards the margin of the jaw,
the centripetal ossification extends as they advance, and consolidation
is completed by the time they are ready to change their recumbent
for the erect position, and take the place of the tooth previously
shed.This change of place and direction is well known to be not the
effect of muscular contraction, hut of partial absorption and deposition
operating upon the membrane to which the teeth are attached.
This membrane is gradually brought to the exterior of the jaw, and is
then removed together with the attached tooth, supposing the latter
not to have been already violently displaced- But the following
question now offers itse lfD o e s this movement of growth take place
simultaneously in the membrane and the jaw to which it is attached,
or is it a slow and gradual sliding motion of the dentigerous membrane
upon the jaw ?
To determine this question would require an experiment similar
to those by which Duhamel and Hunter traced the change of place
in the particles of growing bone ; a foreign body e. g. should be inserted
into the base of the jaw of the shark, and a tooth in the corresponding
place should be so marked, as at a subsequent period it might
be recognized, and its position compared with the perforated part of
the jaw. Such an experiment would not be very practicable in the
carnivorous inhabitants of the deep we are now considering, but accident
has satisfactorily supplied its place. The jaws of a large
Galeus passed into the private collection of an English anatomist,
in which the barbed spine of a sting-ray (Trygon) had been driven,
during a predatory attack of the shark, into the lower jaw through
matrix of animal matter ; but they differ as to the direction of the deposition, which in bone is
from the centre to the circumference, in tooth from the circumference to the centre, the pro
gress of calcification in the one is centrifugal, in the other centripetal.—See Comptes Rendus
dp VAcadémie des Sciences, 1839,jp* 784.