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354 DENTAL ANATOMY.

coriiim, with an exposed part, which is covered with a substance indis-
tinguishable from the enamel of the teeth. The structure of the body-
is likewise coincident with true dentine, and becomes fused with a basal
plate of osseous material. Their development is as follows : First, a
papilla arises from the uppermost layer of the corium, being covered in
by the epidermis (see Fig. 187). From the deepest layer of the epi-
dermis, or that which corresponds with the Malpighian layer, a special
epithelial covering is furnished, which eventually becomes, by a process
of histological differentiation, the enamel of the exposed part. The
papilla, before the conversion of its substance into dentine, exhibits a
central cavity, from which fine branched canals radiate to the surface.
Eventually, calcification takes place, beginning at the summit, and the
salts of lime are deposited in the substance of the papilla, giving rise to
the dentine. Gegenbaur observes : ' " The placoid scale has therefore the
structure of dentine, is covered by enamel, and is continued at its base
into a plate formed of osseous tissue ; as they agree with the teeth in
structure, they may be spoken of as dermal denticles."
Now, in the early embryonic stages the integument bearing these
dermal denticles is pushed into the oral cavity, where they become
somewhat enlarged, and appear in the adult form as teeth. Tomes
says : ^ " No one can doubt, whether from the comparison of the adult
forms or from the study of the development of the parts, that the teeth
of the shark correspond to the teeth of other fish, and these again to
those of reptiles and mammals ; it may be clearly demonstrated that
the teeth of the shark are nothing more than highly-developed spines of
the skin, and therefore we infer that all teeth bear a similar relation to
the skin." Thus the generalization is reached that teeth are but spe-
cialized dermal apj^endages.

With this statement of the nature of teeth in general, we are now pre-
pared to begin a more special inquiry into the organization of a single
tooth. For this purpose I have selected the third lower premolar of
the dog as an average and easily-procurable example of a generalized
type among the higher forms, which will serve to illustrate the compo-
sition and nomenclature of the several parts of which all teeth, w ith few
exceptions, are made up.
For convenience of description, the several parts of most teeth can be
divided into crown, fang, and neck, although there are many in wliich
no true fangs are formed, owing to the persistent and continuous growth
of the tooth ; in all such no distinctions of this kind can be recognized.
In the particular tooth under consideration, however, we can distinguish
w-ithout difficulty an enamel-covered crown, which corresponds with the
exposed ])art of the tooth in the recent state ; two more or less cylindrical
fangs or roots, by which the tooth is implanted in the aveoli and attached
to the jaw bone ; and a slight constriction at the point where the fangs
join the crown, known as the neck (see Fig. 188). The crown in form
resembles a laterally compressed cone, with an anterior and posterior
cutting edge. It is covered by a dense shiny white substance of great
hardness, the enamel, which ceases at the point where the fangs com-
^ Elements of Cuwparative Anatomy. 2 A Manual of Dental Anatonui.