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TWENTY-FIFTH LECTURE.

Erosion.

In a course of lectures there are usually some odd subjects
that seem to belong nowhere, and yet are important. This
morning we will consider erosion.
Erosion is a term applied to a peculiar and very charac-
teristic loss of substance of the teeth, beginning in the enamel,
or upon the outside, and slowly working its way into the tooth,
destroying and removing the substance of the tooth as it goes.
At first it presents no symptoms whatever except this loss of
substance, and this looks almost exactly like a facet that would
be left after grinding it with a very fine stone. There is no
softening whatever, but simply a wasting of the substance,
leaving a perfectly smooth, polished surface; a surface so
smooth and polished and hard that an explorer passed over
the enamel and over the eroded surface will glide just as
smoothly upon the eroded surface as upon the enamel that
is perfect. This gradually deepens and widens, going very
slowly in most cases, until the enamel has been cut through.
Then the dentin wastes away in a similar fashion, and so
smoothly that there is no line of demarkation between the
enamel and the dentin that is being eroded. Usually the sur-
faces round up to the surface of the enamel, making dish-
shaped excavations. When the enamel has been penetrated
and the dentin begins to be eroded, the dentin becomes very
sensitive. This sensitiveness is characteristic of erosion, if in
living teeth, but teeth that have lost their pulps may suffer
from erosion in precisely the same way, except that in these
there will be no sensitiveness.
Erosion is usually slow in its progress. The facets may
appear upon the enamel and be seen for a considerable time
before the enamel is penetrated—a year or two years. It pro-
ceeds directly and steadily, in a large proportion of cases,
until the teeth are destroyed, requiring from three to ten years,
-or even more, to cut through and destroy a tooth. In other
cases the progress ceases spontaneously, or the progress may
Jbe intermittent.

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