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CARIES OF ENAMEL. 85
pieces at a touch, and most generally through the whole thick-
ness of the enamel at once. This seems to be due to the complete
loosening of the ends of the rods from the dentin. In unpro-
tected places, however, such as decays of buccal or labial sur-
faces, and decays of proximal surfaces in which there has been
interference by the crowding of food through the contact, a
considerable number of cases are found in which the outer ends
of the enamel rods are broken before the enamel has been pene-
trated. In the greater number of cases, however, in any of these
positions, a considerable solution of the dentin has occurred by
the acid which has penetrated through the enamel before any
enamel rods have fallen away. This is well attested in many
of the illustrations presented.
The Relation op the Occlusion to the Localization
of Caries.
ILLUSTRATIONS: FIGURES 99-103.
When we come to study carefully the forms of the teeth
in their relations to each other as they stand in the arch, and
their uses in the mastication of food, we find that there are
certain points or areas of their surfaces that are comparatively
free from rubbing, or abrasion, while much the larger part of
the tooth surface is exposed to friction and washings by the
fluids in the mouth. The occlusal surfaces particularly are
exposed to severe abrasion in the mastication of food. The
lingual surfaces, both above and below, are exposed to the fric-
tion of the tongue and of the food that is washed over them
during mastication, so that these surfaces are fairly well
cleaned. The buccal surfaces are less exposed to friction during
mastication and to washings by the saliva during the motions
of the tongue, cheeks and lips, particularly in their gingival
thirds. The proximal surfaces are shielded from abrasion and
from washings by the saliva by the contact and areas of near
approach of these surfaces with adjoining teeth. Decays begin-
ning in the depths of the pits of the occlusal surfaces have no
opportunity to spread upon the surface of the enamel, for the
reason that these surfaces are so continually cleaned by abra-
sion in mastication and by washings by the saliva. It seems
to be for these reasons, principally, that decay does not spread
superficially upon these surfaces. It is prevented by the forms
and the uses of the teeth. It seems to be purely these local con-
ditions that are the basis of the strict localization of the begin-