the tooth-bone in the relation of a gland to its secretion ; that the
formative virtue of the pulp resided in its surface ; that the dentine
was deposited upon and by the formative or secretive surface of the
pulp in successive layers ; and that the pulp, exhausted as it were,
by its secretive activity, diminished in size as the formation of the
tooth proceeded ; except in certain species, in which the pulp was persistent,
and maintained an equable secretion of the dentine throughout
the life-time of the animal(l).
This idea of the pulp’s function, modified only by the phraseology
required to express the later-acquired knowledge of the form
and condition of the newly-developed dentine in contact with the
pulp, has predominated in the minds of most subsequent writers on the
development of teeth.
The successive steps_ to the establishment of the doctrine that
the cells of the ivory, under which form Dr. Schwann has described
the nascent dentine to make its first appearance, are actually part
of the pulp itself, pre-existing in that body before their calcification
and confluence, and continuing in organic connexion therewith after
their conversion into the tubular dentine, are few, well-marked, and
easily traced. The first advance was made by Purkinjé and Rasch-
kow in submitting to careful microscopical observation the structure
of the dentinal pulp prior to the formation of the dentine, and
in similarly tracing the changes which it undergoes during that
process.
(1) Cuvier, by whom this opinion of the formation of dentine is most clearly set forth,
premises the following acknowledgment: “ Quant à la manière dont lés dents en général
naissent et croissent, nos observations nous paroissent confirmer la théorie de Hunter, plutôt
que toutes les autres, dans ce qui concerne la partie de la dent qu’on nomme substance
osseuse.”—Ossem. Foss. 4to. 1812, p. 59.
These authors describe the parenchyme of the dentinal pulp as
being composed of minute uniform spherical granules, without any
of the characteristic filaments of cellular tissue, and, in this respect,
differing from the enamel-pulp. The free surface of the granular
tissue is covered by a peculiarly dense, structureless pellucid membrane,
which they term the ‘ preformative membrane ’ because the
formation of the dentine commences therein. Blood-vessels soon
penetrate the granular pulp, form several anastomoses in their course,
through its substance, and terminate in a rich and delicate net-work
of capillaries on that part of the surface of the pulp where the
dentine has begun to be formed; the rest of the pulp’s surface is
covered by the preformative membrane and does not display any
capillary reticulation. True nervous filaments cannot be distinguished
in the pulp until after its vascularity has been established. The
granules of the pulp immediately beneath the preformative membrane
have a more elongated form than the rest, and are placed either
vertically, or at an acute angle with the membrane.
The formation of the dentine is preceded by the development
of numerous minute elevations on the surface of the pulp, at and
near its apex ; these are conjectured to be subsequently transformed
into the undulating ridges in which the enamel-fibres are firmly
inserted. The preformative membrane becomes of a stony hardness,
except at the margin of the recently formed dentine, where it is
soft and easily rent. The dentine begins to be formed at the apex
of the pulp immediately beneath the preformative membrane.
Of the exactness of the preceding observations by Purkinje and
Raschkow I have had repeated evidence. The more obscure parts
of their description of the development of the dentine are quoted
and commented on by Dr. Schwann, whose observations on this