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EXCAVATION OF CAVITIES BY CLASSES. 83
dark cavity. A little further extension to the labial relieves this
and gives the appearance of a filling. A little careful observa-
tion of this point will prevent mistakes that are often very
annoying to sensitive patients. The appearance of sharp angles
in cavity outlines that are exposed to view should be avoided.
Slighdy curved lines are better than straight lines. But the key
to good appearance of operations on the incisors is perfect tooth
form, including particularly the full mesio-distal breadth of the
tooth. In the incisors, and especially in the laterals, the labial
embrasure is usually well rounded out, and open, because of the
rounded form of the labio-mesial and labio-distal angles of these
teeth. Upon the lingual the embrasures are very shallow and
close, because of the acuteness of the mesio-lingual and disto-
lingual angles of the teeth, and the flatness of the lingual surfaces
from mesial to distal. For this reason the proximate cavities in
these teeth often begin well toward the lingual and burrow much
more under the enamel of the linguo-mesial or linguo-distal angle
of the tooth than toward the labial. The result is that the enamel
to the lingual is often undermined and very thin from injury of
the inner half of its thickness from backward decay, or decay of
its inner surface. Also the lingual marginal ridges are often
quite sharp and the course of the enamel rods uncertain, so that
a good margin can not certainly be made upon the sharp curves
of these ridges. Therefore, when the enamel margin of
the cavity must be laid against the enamel of the lingual
marginal ridge, it is best to cut away the marginal ridge
and lay the enamel margin of the cavity on the lingual
surface of the tooth. In this case this enamel wall will be
cut parallel with the axial wall of the cavity, or in the labio-
lingual plane of the tooth. Frequendy this enamel wall will
be level with the axial wall of the cavity, and this renders the
placing of the filling more than ordinarily difficult. When the
amount of tissue will allow, it is well to sink the lingual portion
of the axial dentin wall sufficiendy to obtain a slight ledge at the
lingual. This is readily done with a few strokes of the 6-2-12 hoe
for the gingival third, and with the 6-2-23 hoe for the incisal
two-thirds of the lingual wall, introducing these instruments from
the labial.
Class 4.— Cavities in the proximate surfaces of the
incisor teeth involving the loss of the mesial or distal