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GABIES AS A WHOLE. ITS CLINICAL FEATUEES. 101

to protect colonies of microorganisms. In the examination of
many teeth, such unusual positions of beginning decay are not
very rare.

SECONDARY EXTENSIONS GINGIVALLY OF PROXIMAL DECAYS.

ILLUSTRATIONS: FIGURES 121, 122, 123.
In proximal surfaces another condition arises frequently
that deserves especial attention on account of its great clinical
importance. Figure 121 shows a large carious area of enamel
at X that has just passed through the enamel and begun the solu-
tion of the calcium salts of the dentin. In this case, a secondary
beginning of caries of the enamel has occurred toward the gingi-
val at Y, running farther gingivally than the illustration shows.
This occurs in somewhat less breadth in a considerable number
of cases on account of lodgment and retention of food between
the teeth. Because of roughening of the surfaces, by the swelling
of the decayed area, or because of the falling away of enamel
rods in one or 'both of the proximating teeth, the food, instead
of gliding out laterally in the normal way, will be held, and will
be forced more and more onto the gum tissue as other food is
forced in upon it. In this way the interproximal gum tissue will
be absorbed and a pocket will be formed between the teeth that
will be well enclosed by the festoons of the gum to the buccal
and lingual, and by coverings of debris from washings by the
saliva. Acid fermentation will become established in this pocket.
The acid formed by this fermentation will be in contact with the
surface of the enamel and its calcium salts will be dissolved.
Another case of similar character is illustrated in Figure 122.
The progress toward the gingival line is less extensive, but in
depth its progress is more pronounced and it has a more evident
separation from the original beginning point of decay in this
surface. As in most of the decays of enamel at this stage, the
swelling of the carious enamel is very apparent in the principal
decayed area. There seems to be some effect upon the partially
dissolved enamel rods that causes them to lengthen slightlj',
causing this swollen appearance. Further, when these rods are
disturbed, they often collapse into a tangled mass in which they
seem bent and twisted together in such a way as to suggest that
they have become softened and in a degree pliable. See Figure
116. This swelling appears in a number of the photomicro-
graphs. I do not remember that any other writer has mentioned
it. In the hand grinding that I had done before this had not been