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db HUNTER OX THE TEETH.
laid all around, on the outside of the bony, or internal substance.

together by a partial filling of their interspaces by cement, whilst a
fourth substance is added, in the form of a vascular osteo-dentine, which,
produced by the ossification of the base of the pulp, serves to unite the
denticles with each other, and with the subjacent pharyngeal bone.
The tooth, thus composed of four substances, exhibits the highest degree
of complexity yet observed in the animal kingdom. Many other varia-
tions in the structure of the teeth, and in the arrangement of the dental
tissues, are to be found amongst vertebrate animals ; but sufficient
examples for the purpose of illustration have been adduced. (1)
In the human tooth, the dentine forms the principal mass both of the
fang and crown : in the latter situation it is covered by enamel, in
the former by cement. As no separate mention is made of this last-
named substance in the text, it will be, perhaps, convenient to describe
its disposition and structure in the human tooth before commenciug our
commentary on the enamel.
The Cement, Crusta Petrosa, Substantia Ostoidea, is a layer of true
bone which in Man covers the fang or fangs of the tooth. In some
instances it appears to commence where the enamel ceases, but in many
others it manifestly overlaps the enamel for a short distance from its
edge. The layer of cement is extremely +hin at the neck of the tooth
;
but, in tracing it down the fang, it is found to become thicker as it
descends, until it attains its maximum of depth at the apex. That con-
fluent condition of the fangs which is so common in certain of the
molar teeth amongst the leucous and xanthous varieties of Man, is
produced by the cement which fills the interspaces, just as in the com-
pound molar of the Elephant it unites a series of denticles into a
single mass. It is, of course, found of considerable thickness along
the sulci which indicate the division of fangs so enjoined. Accord-
ing to some of the best observers, a thin layer of cement may be
traced in the unworn tooth covering the enamelled crown. In a paper
read before the Medico-Chirurgical Society in 1839, Mr. Nasmyth
announced the discovery of a membrane separable by the action of
dilute hydrochloric acid from the surface of the enamel, which, he
expressly states, is continuous with the cement, and which, he believed,
was at least analogous to, if not identical with, a layer of that substance
in a rudimentary condition : this layer he terms the persistent capsular
investment. (2) Nasmyth' s observation was confirmed by Professor Owen,
who, in writing of the so-called simple teeth, "says their crowns are
originally and their fangs are always covered by a thin coat of

(1) Owen, op. cit., Odontography.
(2) Mod. -Chir. Trans., vol. xxii., pp. 312-315. Researches on the Develop-
ment, Structure, and Diseases of the Teeth, by Alex. Nasmyth, p. 79 ; 1S19.