apart ; they are more plainly seen in transverse sections of the crown
than in longitudinal sections, and they have the same relation to the
fibres of the enamel which the contour-lines of the dentine bear to
the calcigerous tubes. Without doubt they indicate, in like manner,
strata of segments of the fibres and stages in the formation of
the substance. Where these strata, which are arranged very obliquely
to the vertical surface of the dentine, crop out upon that
surface they occasion those wavy transverse annular delicate
markings which Leeuwenhoek(l) noticed upon the exterior of the
enamel, and which he supposed to indicate successive stages in
the protrusion of the tooth through the gum, in taking its place
in the dental series. The various conditions of the enamel at
different periods of its formation have been mentioned in the
description of the development of the tooth. (2)
The cement in the human tooth,(3) as in other simple teeth,
is confined to the exterior surface, with the exception of a small
portion which, in old teeth, is usually reflected into the entry to
the pulp-cavity and sometimes closes it up. This third substance
is thinnest upon the crown and very gradually increases in thickness
as it approaches the end of the fang : it is only on the implanted
part of the tooth that the radiated cells, which demonstrate
the close analogy between cement and bone, exist ; elsewhere
the clear basis of the cement alone is present, and this is soon
worn away from the enamel of the crown. There are no vascular
canals in human cement, except when it happens to be abnormally
thickened ; in which case, as in the naturally thickened masses of
that substance in the teeth of Herbivora, it acquires also this additional
feature of resemblance to true bone. The chemical constitution
of both the animal basis and earthy salts of this dental
tissue and of bone is identical ; the hardening particles are chiefly
blended in the finest state of sub-division with the clear basis, a
portion being contained in a coarser state in the cells ; it is this
which occasions their milk-white colour when viewed by reflected
light. (4) The cemental cells are generally oblong, sometimes cir-
(1) | Select Works’ by Hoole, 4to. 1800, p. 115.
(2) Introduction, p. xlvi. (3) PI. 122, fig. 1—7, c.
(4) The calcareous matter does not occupy the whole of the cavity of the cell; space
is left as in the dentinal tubes, for the transit of fluids. Mr. Smee, (Med. Gazette,
cular, more rarely fusiform i they average about » th of an inch
in the long diameter: their contour is broken by the numerous
fine tubes which radiate from them, with wide angular beginnings,
but quickly contracting to the minute size of jpooth of an inch.
In transverse sections of the fang, the cells are generally seen to
be arranged in parallel concentric lines ; their long axis being in
the direction of the lines, which vary in clearness of definition,
like the concentric striae of enamel; and, like these, indicate
stages of formation in the cement. The radiating tubuli anastomose
together either directly or by their numerous fine branches;
which latter also communicate with the terminal ramifications of
the dentinal tubes, either directly or through the medium of the
fine granular cells dispersed through the boundary line between
the dentine and cement. In the deciduous teeth the cement is
relatively thinner and the cells fewer and less regularly arranged.
The thickened cement near the end of the fang of old permanent
teeth is occasionally perforated by a vascular canal, conveying
capillary vessels to the osteo-dentine and the small remnant of
the pulp. An increase beyond the usual thickness is usually accompanied
by the formation of vascular canals in the cement» and
it is the existence of this most highly organized of the dental
tissues which explains the possibility of engrafting teeth upon
vascular parts. (1)
In my Report to the British Association in 1838, which
contains the first announcement of some of the observations described
in detail in the present Work, I stated, with respect to the
component structures of a tooth, “ that in addition to those
usually described and admitted, there were other substances entering
into the composition of teeth and presenting microscopic
characters equally distinct both from ivory, enamel and cement,
Nov. 20tb. 1840), succeeded in injecting them with Canada balsam, and I have generally
found in fossil teeth that the mineral matter had penetrated into the cells and tubes of
the cement, as well as into the tubes of the dentine.
(1) Retzius rightly points out the cement as the seat of the abnormal growths called
‘exostosis,’ to which the fangs of teeth loosened by the scurvy or mercurial medicines are
more particularly liable. One of his figures of the thickened cement on the fang of the
tooth of an aged person is given in PI. 122, fig. 8.
H H 2