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136 THE TECHNICAL PKOCEDTIBES IN PILLING TEETH.

increases the difficulty of making a good filling. Wlien the prepa-
ration is for amalgam fillings, this is not demanded but it is still
desirable.
When the distal pit in the occlusal surface of the upper
molars is the seat of operation, the procedure is not essentially
different, except that the approach should be rather more from
the buccal. The same instruments and the same methods should
be employed. In these, it is very generally necessary to cut out
the disto-lingual groove to the crest of the lingual marginal ridge,
and frequently to follow it over onto the lingual surface of the
tooth. This should be done with the inverted cone bur, as previ-
ously described, followed by the necessary chipping of the enamel
with hand instruments. In the lower molars the grooves are gen-
erally deeper and more deeply sulcate than in the upper molars
(except the disto-lingual groove), and will much oftener require
cutting out to the crests of the marginal ridges.
In the lower molars there will often be a decay starting in a
pit to the mesial and a pit to the distal of the central pit. When
this occurs and the decay has burrowed rapidly along the
dento-enamel junction, nearly the whole occlusal surface must be
removed, maldng a very broad cavity, as shown in Figure 145.
Often these are comparatively shallow, and again the central pit
decay only will be deei^, so that a ledge may be formed to the
mesial and distal, on which to rest the filling after the deeper
central part has been filled with cement in cases in which a non-
conductor seems desirable.
When the walls of cavities approach the crests of the mar-
ginal ridges, as in that shown in Figure 145, care must be taken
as to the direction of the enamel rods forming the wall, to see
that there are no short ends left at the cavo-surface angle. It is
best to bevel the cavo-surface angle slightly in all such cases with
a chisel held lightly in the fingers, using with a planing motion, as
shown in Figure 146. This motion should be along the length of
the cavo-surface angle. This lessens the liability of cheeking this
angle in i^aeking gold over it, and is also especially important in
preventing checking in usage in mastication. Very many occlu-
sal fillings come to have imperfect margins within a few years
from the checking of the cavo-surface angle of the enamel by
usage, and this should be prevented. This checking of the enamel
in usage has become especially prominent in connection with the
use of porcelain inlays in occlusal surfaces, and will be so with
the more recent gold inlays imless this matter be looked to with
great care. This applies to all cavities in occlusal surfaces where