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greatly depends upon the different proportion of the ani-
mal and earthy constituents of the bony structure of the
tooth ; and also on the chemical state of the saliva, which is
naturally much influenced by the state of the other teeth and
parts of the mouth, as well as by the general state of the
health of the individual.
As the carious matter increases in its corrosive qualities,
and the affected part becomes softer, the disease causes a
cavity in the crown of the tooth, similar to that produced by
internal caries ; excepting that the cavity produced by the
latter is generally large and round, whilst that produced by
superficial caries is frequently narrow, like a tube.
When seated in the necks and roots of the teeth, caries
rarely forms such a cavity, but extends itself on the surface,
and becomes broad and more irregular in its progress, and
sometimes in the neck of the tooth it has the appearance of
undermining the enamel towards the crown, so as to form an
oval or oblong cavity, ending in a point at each extremity,
such as might be cut into it artificially, by a triangular file.
After the disease has penetrated through the enamel, its
progress and effects, as well as symptoms, are precisely like
those of deep-seated caries. It is subject to all the same
general and local influences, with this difference, however,
that such teeth as are affected by external caries being of a
stronger original construction than such as are affected with
deep-seated caries, they are acted upon more slowly than
the latter ; consequently, if we suppose that the diseased ac-
tion of deep-seated caries requires from one to five years to
penetrate through the bony structure of the tooth, and to de-
stroy the life of its lining membrane, superficial caries may
require from four to ten years : and the chemical destruction
of a tooth, the death of which has been effected bv the lat-