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through the dentin, and cause them to form broad cavities,
or cavities of unusual form, so that we have variations of
form occurring among the cases that are presented to us.
But these things are things that give opportunity, not the
cause of decay.
*
Softness and Hardness of the Teeth.
HSoftness or hardness of the teeth has been much talked
of in the past and dental literature is full of it. It has been
regarded by the laity as a reason for decay of the teeth.
You will hear it continually spoken of among the people,
and you will hear it continually spoken of among dentists.
This person's teeth are soft and are melting down, chalky,
and all that. Now, the chemical constitution of the teeth
is not much different between person and person. Those
differences in the teeth that influence decay are the differ-
ences that we are able to see with the naked eye or with
the microscope; they are in the form of faults of physical
structure, not faults in the chemical constitution of the teeth,
that render one more soluble to acids than another; they
are faults that give opportunity; they are not chemical in
their nature. The idea that some teeth are hard and there-
fore do not decay, or decay very slowly, and that some
teeth are soft and decay rapidly because of that softness,
has come to the profession through the observation that
some teeth decay rapidly and break down into a chalk-like
detritus, while the teeth of other persons stand firm and
do not decay. This originally gave the impression that the
teeth of one person were soft, while the teeth of another were
hard. That which was seen has been reported correctly, but
the interpretation has been wrong. Again, the teeth of chil-
dren have been observed to decay much more rapidly than
the teeth of adults. This is the general rule as we see caries
among our patients. The child's teeth decay rapidly, but
if the decay is controlled by proper filling a time comes
when they decay very much less rapidly, become very much
less prone to the beginning of decay, and finally decay
will cease almost entirely. The explanation has been that
the teeth were soft before ; that they have now become hard
and dense. Then, again, another observation has been made.
A person whose teeth have seemed to stand firm during
their youth, have been practically immune to decay, have

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