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CARIES OF THE TEETH. 47 ;

decayed portion will produce pain : and thus we may
he led falsely to conclude that the softened dentine is
sensitive; and, indeed, it is maintained that in some
cases the partially decomposed dentine is so, on the
supposition that a small portion of the calcareous
elements may be removed, and yet the vitality of the
part, not be destroyed.
The progress of caries is far more rapid in the crowns
of the teeth than in the roots, for the reason that the

former are more exposed to the influences of external
It is true that the crowns are covered by
injuries .
enamel, which is designed to shield the dentine from
injury, but which is often defective, and on which are
accumulated agents that it can not resist, even when
it is perfect ; so that the enamel itself is sometimes
decomposed. The roots, too, possess a higher degree

of vitality than the crowns, and their ability to resist
the encroachments of decay is correspondingly greater
and hence we often find the roots of teeth solid and
free from decay, the crowns of which have been
removed by rapid decomposition. Injurious substances
are sometimes pressed into contact with the dentine,
through defects in the enamel, or under its projections,
and there retained till their mischievous effect is pro-
duced.
It is maintained by some writers that caries is con-
tagious. Dr. Koecker was of this opinion. The