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Diseases of the Teeth. 83
action is kept up, not so much by being in contact with the
dead portions of bone, as with the various irritants and chemi-
cal agents to which it is exposed.
INTERNAL CARIES,
Origin, always in the internal bony structure of the teeth.
Symptoms—The first appearance of this species of caries,
will be marked for the most part, by a dark bluish spot,
appearing or shining through the transparent enamel, the
integrity of which, at this stage of the disease, appears perfect.
This spot is usually found in the centre or irregularities of the
crowns of the molares, and on their side midway between the
gum and their grinding surfaces. In the bicuspides, it appears
on the grinding surfaces, and in the incisores of the upper jaw,
the spot is seen on the inner surface, near the gum. In this
stage of the disease, if the tooth be sawn through, a dark
brown streak will be seen extending deep towards the internal
cavity, and in the direction, we may suppose the vascular and
nervous fibres run. As the disease progresses, these appear-
ances are more strongly marked, till at length, the enamel
being undermined, gives way while masticating some hard
substance, and exposes a cavity more or less extensive. The
disease now progresses more rapidly, from the action of mat^
ter admitted into the cavity, till the internal membrane is ex-
posed and irriated, and tooth ache follows.
When internal caries has attacked a tooth, its progress will
be governed in a measure, by its structure, or its predispose
tion to decay, sometimes extending to the nerve in a p few
months, and at other times, being the work of years.
EXCITING CAUSES OF INTERNAL CARIES.
Climate—The teeth are said to decay much earlier and
more frequently in America than in Europe. The great and
sudden changes of temprature to which our climate is subject,
is one principal reason that is to be assigned, to account for
these facts.