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EXCAVATION OF CA^rtTIES BY CLASSES. 147

vent the recurrence of similar conditions. In this case the axial
wall B, and the pulpal wall of the step a, have heen outlined on
the photomicrograph. This figure, while showing a little less
than the mesial half of the crown of the tooth, shows clearly the
relation of the cavity to the pulp and the amount of cutting
required to make the case safe against recurrence of caries to
the gingival of the filling.
The detection of this form of extension of caries while exca-
vating must usually be done by carefully noting the appearance
of the enamel forming the gingival wall, and especially the
appearance of its cavo-surface angle. If caries of enamel has
extended, as shown in this illustration, the margin of the gingi-
val wall will show a little whitish, or possibly will be quite white,
distinguishing it sharply from the solid vitreous appearance of
the sound enamel. In the case illustrated, this whitish portion
would be slightly more than one-fourth the thickness of the
enamel wall at d, which would appear as a fine whitish line on
the immediate margin of the cavity wall. 'When excavating
proximo-ocelusal cavities, this should always be especially looked
for, and, in cases in which it is discovered, the cutting gingivally
must be continued until the whitened enamel has been removed.
Then the enamel wall will show the solid vitreous appearance
to its cavo-surface angle.
Similarity or these extensions to undermining of gingi-
val WALLS OF fillings. The occurrence of this class of exten-
sions of the beginning of caries to the gingival is very exactly
similar in principle to those new beginnings of caries at the gingi-
val margins of proximal fillings that have wrecked so many so-
called contour proximal fillings in the past. In making these, the
form of the contact point has not been so perfectly suited to the
requirements as to prevent food from clinging between the teeth.
The result has been that the food particles so held have been
pushed onto the gum septum, causing a little absorption of it, and
have formed a pocket in which acid fermentation has occurred.
This has been sufficiently hidden away between the teeth to pre-
vent the free dissipation of this acid in the surrounding saliva and
it has acted upon the enamel, causing the beginning of caries.
In the case illustrated in Figure 164, the decaying enamel just
to the gingival of the original contact has become swollen, which
the photomicrograph shows perfectly, broadening the contact
and roughening it. This causes the contact to grasp food par-
ticles and hold them, to be finally forced deeper into the inter-
Ijroximal space with the result related. In this view, the two