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EXCAVATION OF CAVITIES BY CLASSES. 185

tin, when, of course, it refuses to split away tlirougli its thick-
ness. The outline of the prepared cavity should by this time be
quite fully determined. With inverted cone bur 6 or 8, the enamel
is undermined, first to the lingual and gingival and chipped away;
then the l)ur is introduced from the lingual and the enamel to
the labial undermined and chipped away, and this continues until
the outline form shown in Figure 252 has been attained. The
decay in the dentin, though as broad as has been mentioned, is
not more than a millimeter deep at any point, and that depth,
with the addition of the thickness of the enamel, should be the
greatest depth given the cavity at any point. Every part should
be kept very shallow, except, perhaps, in some cases, more depth
might be wanted at the incisal anchorage and the convenience
points. The widely spreading form given to the angles in the
gingival portion of the cavity is the essential point in extension
for prevention; for it is especially at these angles that caries
has so often recurred and destroyed otherwise good fillings.
AVhen this form has been attained and the incisal anchorage and
the convenience points have been cut, the enamel walls should
be finished and the cavo-surface angle beveled as previously
described. The toilet is then made, all in the same manner sub-
stantially as described in connection with the first series of illus-
trations of cavity preparation.
This cavity is intended to represent the minimum of exten-
sion for prevention in incisal cavities. The outline form is that
which will most generally do the best service in protecting the
tooth. The filling is shown in Figure 253, and the appearance in
labial view in Figure 254. So small a cavity should be made
only after close study of the probable coming of practical immun-
ity to caries at an early date. In the opposite condition of
extreme susceptibility to caries, the cavity should be enlarged,
the gum septum pressed away and the cavity cut to the outline
shown in Figure 257. This cavity should be cut no deeper than
the first, except to deepen it just a little along the labial and
lingual walls to obtain better ledges to lodge the filling against
in building.
The forms of the convenience points are shown in Figure
256 in cross section of the tooth, with the filling, cut close to the
gingival wall. The form of the incisal anchorage is shown in
Figure 255 in mesio-distal section. The slope of the enamel wall
to the incisal at the summit of the curve at the incisal angle is
also made apparent. It must be specifically understood that
the gingival wall is left flat labio-lingually in all cases, or if
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