concentric striæ, of which some are more clearly and others more
faintly visible, as if the cement were deposited in fine and coherent
layers. The layer of cement is found in the deciduous teeth, but
is relatively thinner and the Purkinjian cells are more irregular.
“ In growing teeth with fangs not fully formed, the cement is
so thin that the Purkinjian cells are not visible : it looks like a fine
membrane, and has been described as the periosteum of the fangs,
hut it increases in thickness with the age of the tooth, and is the
seat and origin of what are called exostoses of the fang which are
wholly composed of it.” These growths are subject to the formation
of abscess, and all the other morbid actions of true bone.
It is the presence- of this osseous substance which renders
intelligible many well-known experiments of which human teeth
have been the subjects ; such as their transplantation and adhesion
into the combs of cocks, and the establishment of a vascular connection
between the tooth and the comb ; the appearances which the
Hunterian specimens of these experiments present, and of the reality
of which Professor Müller satisfied himself during his visit to
London, are no longer perplexing, now that we know that the surface
of the tooth, in contact with, and adhering to the vascular comb,
is composed of a well organised tissue, closely resembling bone.
This correspondence of the cement, which, when it exists in
sufficient quantity, becomes almost identity, with true bone, is illustrated
by the varieties of microscopic structure which the cement
presents in different classes of animals, and which always correspond
with the modifications of the osseous tissue of the skeleton in those
animals ; thus the cement in the osseous fishes, in which the bone
is not characterized by the radiated calcigerous cells, likewise ceases
to present that character ; and, in reptiles and mammals in which
the radiated cells are present in the bone of the skeleton and in the
dental cement there is a close conformity as to their size and shape
in both tissues.
The most remarkable modification of mammalian cement is
presented by the thick layer of that substance which invests the
molars of the extinct megatherium ;(1) besides abounding in calcigerous
cells it is here traversed by straight, parallel and occasionally
bifurcated medullary canals, arranged with regular intervals, and
directed from the exterior of the tooth somewhat obliquely to the
surface of the unvascular dentine, close to which they anastomose
by loops, corresponding with, and opposite to those formed by the
medullary canals of the vascular dentine of the same tooth (2).
Under every modification the cement is the most highly organized
and most vascular of the dental tissues, and its chief use is to
form the bond of vital union between the denser and commonly unvascular
constituents of the tooth and the bone in which the tooth is
implanted. In a few reptiles (now extinct) and in the herbivorous
mammalia the cement not only invests the exterior of the teeth, but
penetrates their substance in vertical folds, varying in number, form,
extent, thickness and degree of complexity, and contributing to maintain
that inequality of the grinding surface of the tooth which is
essential to its function as an instrument for the comminution of vegetable
substances.
The higher an animal is placed in the scale of organization, the
more distinct and characteristic are not only the various organs of
the body, but the different tissues which enter into their composition.
(1) Plate 84, b. (2) PL 84, a.