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298 OPERATIVE DENTISTRY
Avoid the Exact Circle in outline, as this will bewilder the oper-
ator as to the position when setting. In case the outline is so near
a circle as to make position questionable, the axial wall should
have a small rounded pit at one side to guide the operator in set-
ting.
The Axial Wall should, in large cavities, be the miniature of the
tooth surface in which it occurs. The axial wall of small cavities
should have a rounded groove cut around the entire circumference.
The Surrounding Walls should meet the axial at an obtuse angle
to relieve any undercuts before the matrix is formed. When the

Fig. 160.—A Class Three cavity labial Fig. 161.—A Class Three cavity labial
approach for porcelain inlay. a|)proach for porcelain inlay.

inlay is ready to set give the cavity retentive form by making the
base line angles acute.
Cavities in Proximal of Bicuspids and Molars. Class Two. Ex-
perience has taught us that porcelain is not indicated in this class
of cavities. Their location subjects the filling to extreme crushing
strain which porcelain will not stand. The occlusal surfaces are of
an irregular shape and made up of a great varietj^ of forms with
surfaces in any number of planes. This makes the right angle
cavo-surface angle demanded in porcelain filling improbable and
results in exposing porcelain margins of an acute angle. (Fig.
159 may be used.)