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EXCAVATION OF CAVITIES BY CLASSES. »(

of the length of the enamel rods, but slightly inclined in a direc-
tion to throw the chips off from the free surface into the cavity.
The instrument must be very sharp to be effective. Then cut
away the enamel toward the central fossa, as far as it can well be
done by the chisel. At this point it is well to take the enamel
hatchets 15-8- 12, or 20-9-12 if there is sufficient room, and trim
the overhanging enamel from the buccal and lingual walls of the
cavity with a few strong, vigorous strokes, or the buccal wall
may usually be cut with the straight or binangle chisel, using
the enamel hatchet for the lingual wall. Then sweep the enamel
hatchets around the gingival wall with a scraping motion, cutting
to sound dentin. Note particularly that the enamel hatchet of
the right bevel, to cut the lingual wall, 'should cut to the
lingual in trimming gingival wall ; while the one with the
right bevel, to cut the buccal wall, should move to the buccal
in cutting the gingival wall. If the cavity is shallow mesio-
distally, this will incidentally remove most of the carious dentin ;
but that is immaterial. If the cavity is very deep mesio-distally,
no further effort should now be made to remove the carious
dentin. The outlines of the cavity are exposed, and that is
sufficient. The next procedure is to cut a step in the occlusal
surface for the retention, and complete the outline of the cavity
in this direction. Select an inverted cone bur not more than
one millimeter in diameter, and placing its shaft nearly or quite
parallel with the long axis of the tooth, enter the bur sidewise
into the dentin just beneath the dento-enamel junction, and
draw it slowly to the occlusal surface of the enamel. Repeat
this motion in cut after cut, following the mesial groove distally
to the pit in, the central fossa. Then with straight chisel 15 or
20, or with the enamel hatchets, chip away the enamel from
either side of the slot formed by the bur. Now pass the bur
along the slot again, first pressing it strongly to the buccal so as
to undermine the enamel, then pressing to the lingual, and again
chip away the enamel both to the buccal and to the lingual.
Repeat this until the step occupies the full middle third of the
occlusal surface bucco-lingually to and including the pit in the
central fossa. Now the buccal groove and other sharp grooves
should be cut out as in the preparation of occlusal cavities.
This gives a step with a flat pulpal wall and definite angles with
its surrounding walls, and of perfect retentive form.