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34 — HUNTER ON THE TEETH. ;
one another ; in which case it is evident, that the Lower-Jaw
will move downwards and backwards. But if the Lower-
Jaw be fixed, as in the case supposed, and the Vertebrae
G G G be also fixed, the Condyle will move upwards and
forwards upon the eminence in the joint, the fore-part of
the head will be pushed upwards and backwards by the
Condyle, and the hind-part of the head will be drawn down
so that the whole shall make a kind of circular motion upon
the upper Vertebra; and the Digastric Muscle pulling the
hind-part of the head towards the Lower Jaw, and at the
same time pushing up the Condyles against the fore-part of
the head, acquires, by this mechanism, a very considerable
additional power.
OF THE STRUCTURE OF A TOOTH. AND, FIRST, OF THE
ENAMEL.
A Tooth is composed of two substances, viz. Enamel and
Bone. (I) The Enamel, called likewise the vitreous, or cortical

(I) [It may l>e stated, in general terms, that a tooth is a hard body-
primarily distinct from the skeleton, situated in the mouth or at the
commencement of the alimentary canal, composed of organic and inor-
ganic matter, and enclosing a vascular pulp which occupies a cavity in
its interior. In examining the teeth of the greater number of vertebrate
animals, we are able to discriminate two or more tissues, of differing
structure and hardness. The principal dental tissues are the dentine or
ivory (bone of the text), the enamel, and the cement or crusta pdrosa. Of
these, the dentine forms the chief mass of the body of the tooth, the
cement the outer crust, whilst the enamel, where it exists, is external to
the dentine, or between it and the cement. Besides the three principal
dental tissues, various modifications of the dentine not unfrequently
occur, to which names have been applied indicative of their structural
peculiarities e.g., osteo-dentine, vaso-dentine. Although structures ana-
logous to dental organs are met with in the invertebrate classes, it is
amongst the vertebrata alone that true calcified teeth are found. In the
teeth of vertebrate animals, the number of component tissues, and the
manner of their arrangement, exhibit considerable variation. Amongst
the class Pisces, there are a few instances (e.g., the Wrasse Ldbrus) in
which the teeth consist of but one tissue, which is always a modification
of dentine. It is more common, in the same class, to find the tooth