sively decreases in size, fewer nuclei are developed in the cells, and
these do not acquire so large a size. The diminution in both
respects proceeds, however, unequally, in the cells of the same
stratum. Here and there the linear tract formed by the nuclear
matter in a part of a smaller calcifying cell, containing fewer
nuclei, may be observed to unite with the converging extremities
of two residuary tracts (areæ of dentinal tubes) of a calcified cell
in advance (PI. 1. fig. 1, g). It is thus that the bifurcation of the tubes
is produced, and a repetition of this confluence, which becomes more
frequent as the calcifying process approximates the centre and base
of the pulp, gives rise to the dichotomous divisions of the main
tubes. In some of the cells at and near the central and basal
part of the pulp, the nucleus has undergone no division, but has
become merely elongated and sometimes angular or radiated. In
others it has disappeared ; such cells occur not unfrequently close
to the field of calcification, when the process has made much
advance in its centripetal course. The altered mode of action or
change in the nuclei of the smaller central cells of the pulp is
the first and essential step in the modification of the dentinal tissue
which produces the substances which I have termed osteo-dentine
and vaso-dentine. In the former many of the cells retain their
nucleus undivided, and the hardening salts are impacted around it
in the interior of the cell, but enter only partially into the granular
substance of the nucleus, in the minutely disgregated form, which
produces the opacity and whiteness of the resulting corpuscle. In
the formation of vaso-dentine many of the cells lose their nucleus
which seems to have become dissolved. In both the latter modifications
of dental tissue the blood vessels remain, and establish
the wide tubular tracts in the calcified substance to which the
name of ‘ vascular canals’ is given. In true, hard, or unvascular
dentine no trace of the blood vessels remains; all has been converted
into a much more minute calcified tubular tissue by the
assimilative or intus-susceptive properties of cells, and by the modification
of their nucleolar contents. (1)
(1) That the dentine is the ossified pulp is, as Dr. Schwann observes, an old opinion ; but
an opinion is not a theory. Almost every true theory has been indicated, with various degrees
of approximation, before its final establishment; but he has ever been held, in exact philosophy,
to be the discoverer of a theory, by whom it has been first clearly enunciated and satisfactorily
proved. Thus established, on the basis of careful and sound induction, it is sooner or later
received to the exclusion of the, till then, prevalent and generally accepted erroneous doctrine,
at which period the truth of the antecedent hints and indications of the true theory begins to
be perceived, and it is not uncommon to find their value exaggerated in quotations by the
emphasis of type. Thus the remark of B l a k e “ As the bone of the tooth increases in
thickness, the pulp is proportionally diminished: and seems as it were converted into bone,”
(Essay, 8vo. 1801, p. 7) is quoted in the article " Zoology,” ‘ Encyclop. Metropolis,’ vol. vxi,
p. 232, with “ converted into bone” in italics. So also Mr. Conybeare s observation that the
interior cavity of the teeth of the Ichthyosaurus was obliterated “ by the ossification of the
pulpy nucleus” ; and that “ the ossified pulp has become a spongy mass of reticulated bony
fibres,” (Geological Transactions, Second Series, vol. i, 1824, p. 107) might be cited in italics,
as the older hypothesis of Rau (De ortu et regeneratione dentium) has been, in depreciation of
the value or necessity of researches establishing the true relation of dentification to ossification.
But the actual value and bearing of such casual expressions would have been more fairly and
truly set forth, if, when the theory of the formation of dentine by successively excreted layers,
as promulgated by Hunter and Cuvier, universally prevailed in the systems of Physiology,
that theory had been formally combatted on the strength of such facts and observations in the
development of dentine, which it could be shown that Raw, Blake, Conybeare, and others,
had advanced in support of their expressions of the seeming, or actual conversion of the pulp
into bone.
Such expressions are, however devoid of scientific value, in regard to the question
of development on which they are quoted to bear, precisely because they are unsupported by
the observations requisite to prove what they affirm, and they have, therefore, been deservedly
neglected by the best authorities in Physiology, who have treated ex professo on the development
of teeth, prior to 1839.
In the edition of the English translation of “ Muller’s Physiology” of 1837, many facts
are cited from Blake’s excellent Treatise, but not his idea of the seeming conversion of the
pulp into bone. The Translator, indeed, adds to the text the microscopic observations ot