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76 PATHOLOGY OF THE HAED TISSUES OF THE TEETH.

Peneteation OF Enamel in Pits.

ILLUSTRATIONS: FIGURES 75, 76, 77.
The beginning of caries in pits in the occlusal surfaces of
three molar teeth is shown in Figures 75, 76, 77. It is not diffi-
cult to conceive of a colony of the microorganisms present in
the mouth, establishing itself within one of these pits and grow-
ing there, producing acid sufficient to begin the solution of the
cementing substance between the enamel rods. These would
be covered with debris from foods that is forced in upon them,
very completely shielding the acid formed from being washed
away by the saliva. It seems to be true that there is a greater
intensity of the action of the acid in caries of enamel in this
position than in any other, for elsewhere it is very rare to see
the carious process extending across the length of enamel rods,
or a complete solution of any of the rods before they are loosened
from the dentin. Caries usually follows accurately the length
of the enamel rods in the penetration of the enamel in all axial
surface positions. But in beginning decays in pits, we occasion-
ally see progress across the length of the enamel rods, and in
Figure 77 particularly, less distinctly in Figure 76, we may note
that the pit has been enlarged at its deeper part by the solution
of the enamel rods about its walls. This kind of showing occur-
ring frequently, indicates that the locality is more completely
isolated, or less disturbed by solution of the acid in the oral
secretions than elsewhere, and therefore reaches the highest
percentage of acidulation. It seems to be for this reason that
in this position the rods themselves are often dissolved even
before the carious process has extended into the dentin. The
spreading across the enamel rods is, however, often more appar-
ent than real, for about pits the enamel rods are all inclined
toward the pits. It will be noted also, in the examination of
these illustrations, that there is no sign of the carious process
on the surface of the enamel outside the pit. It is confined
exclusively to the walls of the pits and usually to the deeper
part. Occasionally pits that are gradually narrowed from sur-
faces much inclined, or in a fimnel shape, will show caries extend-
ing a little out of the pit proper, but this is unusual. This absence
of superficial extension is because the surfaces immediately
about the pits are kept clean, or reasonably clean, in the process
of mastication; and further, because, if microorganisms were
growing in such a position, they would be subjected to continual