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40 PATHOLOGY OF THE HAED TISSUES OF THE TEETH.

general rule is that it does not spread to other surfaces, i. e., if
it has begun upon a labial surface there is rarely a disposition
to begin upon a proximal surface or a lingual surface, but it will
it will, however, spread to other
be confined to labial surfaces ;
labial surfaces in the neighborhood. For instance, if it begins
upon a labial surface of a central incisor— which is not an
uncommon place of .beginning — it is likely to spread to the
labial surface of the other central incisor and to the labial sur-
faces of the lateral incisors and cuspids, spreading from before
backward. If it begins upon a first molar it is liable to spread
to the second and third molars and to the bicuspids, and in most
of these cases it will be bilateral, though occasionally we find it
It may
unilateral, not occurring upon the opposite side at all.
also apparently spread from the upper teeth to the lower, or
vice versa, but continuing upon the -labial or buccal surfaces,
not involving other surfaces of the teeth. If it begins on proxi-
mal surfaces, proximal surfaces only will be affected. But it
will spread from tooth to tooth. "

Diagnosis of Erosion.
One who has carefully studied the conditions of the eroded
areas in a few cases should have little difficulty in recognizing
it in any of its forms. It should be remembered that the form or
location of the eroded area is not distinctive, for the reason that
a great variety of forms of area and of location are presented.
The area of eroded surface is always smooth and glossy, and a
sharp explorer held lightly in the fingers glides over it the same
as over enamel. This distinguishes erosion from beginning
caries, but does not distinguish it from abrasion. All facets
occurring on the occlusal surfaces of the teeth should be regarded
as abrasion. An examination should be made that will certainly
exclude abrasion from any cause such as the rubbing of the par-
ticular part against another tooth, or rubbing by some artificial
appliance, or the possible grinding by a stone for any purpose.
With these excluded, a definite facet of any form that is hard,
smooth and glossy, is distinctive of erosion. So long as this is
in the enamel only, it will be the only symptom. A tooth with a
living pulp will usually become sensitive when the dentin is
reached, but in cases that have made much progress slowly in
dentin, the pulps of the teeth may become much calcified, and
the sensitiveness will disappear on account of the cutting off
of the dentinal fibrils by the calcification. Obviously there will
be no sensitiveness when erosion occurs in pulpless teeth. In