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CABIES OF ENAMEL. 77

washings by the saliva, which would dissipate the acid formed.
Therefore, decays of the enamel, beginning in pits, are in the
form of a cone, having the base on or toward the dento-enamel
junction, and the apex toward the surface.

Peneteation of Enamel is Peoximal Subfaces of Incisoes.
ILLUSTRATIONS : FIGURES 78-85.
The conditions are entirely different on the smooth surfaces
of the teeth, as the proximal surfaces. Here there is no pit or
fissure, depression or fault in the enamel. It is a smooth, rounded
surface. The teeth round together normally in such a way that
the interproximal gum tissue fills the space to the contact point,
and the contact point proper is fairly well rounded in most cases.
"While this condition continues perfect, there is no place for the
lodgment of microorganisms upon the proximal surfaces, but,
whenever the gum tissue fails to fill this space completely and a
little opening is left to the gingival of the contact point, it forms
a harbor or nidus at which microorganisms may lodge and begin
their growth. It is probable that by this growth alone they may
have something to do with forcing the gum tissue a little bit
further away, and, if they are sufficiently protected from wash-
ings by the saliva, they may form sufficient acid to produce an
action upon the calcium salts of which the enamel is formed.
Figures 78-81 are a group showing the manner of the pene-
tration of enamel by caries in the proximal surfaces, as may be
well seen in split teeth with an ordinary hand magnifier. Fig-
ures 82-84 illustrate the same thing in photomicrographs* pro-
duced by low powers of the microscope. The first beginning of
decays upon these surfaces is often small, as is shown by Figure
78, a photograph of the distal surface of a central incisor with
the spot of beginning decay very exactly in the position men-
tioned, just to the gingival of the contact point. This was a very
white spot on a tooth that was rather dark in its general color,
so that the engraver has been able to show it very distinctly. "We
might ask here, by what power, circumstance or condition has the
action of the acid been confined to this one small spot, while all
the rest of the surface of the tooth is free from any action of
the acid?
* The difference between the photograph and the photomicrograph, constantly-
observed in their use in this work, is that the photograph is taken from an opaque
object by reflected light, while the photomicrograph is taken from a thin section by
transmitted light. Usually the object is much less enlarged in the photograph.