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EXCAVATION OK CAVITIES BY CLASSES. 167

such cavities and many years of observation have shown them to
be very safe for the tooth's pulp and vei-y effective in protecting
against recurrence of decay.
A point of considerable value in deciding which shall be
used, a very broad filling or a wide separation and prominent
contact, will be the condition of the proximatiug surface of the
next tooth. In case decay has not yet begun in this, a prominent
and small contact will do much to protect it from the beginning
of decay later.
MANAGEMENT OP WEAK CAVITY WALLS IN BICUSPIDS AND MOLARS.
ILLUSTRATIONS: FIGURES 211-245.
In the lower molars, especially when the contacts are very
broad, the proximal cavities are likely to be deepest toward the
lingual, and to burrow extensively along the lingual dento-
enamel junction. In these cases the lingual wall will be weak at
the mesio- or disto-lingual angle of the tooth, and this not infre-
quently extends to the central line of the lingual surface. In
all such cases the removal of the angle, including the lingual
cusp, mesial or distal, is imperative. In mesial cavities this
should usually be done by catching the enamel near the margin
of the cavity with the sharp edge of a binangle chisel, and with
a pull toward the cavity, split off the undermined enamel. This
may be thrown off, chip after chip, easily until a point is found
at which it is supported by sound dentin, or strong enough for
a filling to be built against it and restore the lost contour with
safety. Distal cavities of this class are very difficult to fill when
the whole of the cusp must be removed.
Description of case. When a prominent disto-buccal cusp
on the lower first molar makes contact well to the occlusal on the
second molar, the buccal cusp of the second molar is more often
badly undermined. Such a cavity of the worst type is illus-
trated in Figures 211-213, which may be taken as a type of the
preparation in such cases. In a considerable number of these,
the pulp will be living and may be retained alive and the filling
sufficiently anchored in the distal portion of the tooth. In the
case illustrated, however, the pulp is to be removed.
Outline form and removal of decay. The area of decay
is rapidly uncovered by the chisel, using hand pressure or the
mallet on the buccal portion and by the pulling motion over the
lingual portion of the occlusal surface. On the lingual, the cusp
is found to be supported by sound dentin, but the buccal cusp
is completely undermined. The question first to be determined