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EXCAVATION OF CAVITIES BY CLASSES. 151

bucco-lingual center of the length of the gingival wall, and carry
it buccally to the buccal wall with several cuts, squaring out the
bucco-gingival angle. It should then be drawn along the buccal
wall toward the occlusal surface, undermining the enamel along
this wall also. The undermined enamel is easily cut away with
the enamel hatchets, or by clipping with the chisel used from
the mesio-buceal angle of the tooth through the embrasure. The
linguo-gingival angle of the cavity and the gingival wall should
be extended in the same way, using the same instruments. The
lingual wall should be extended to a line where its margin will
be in view past the proximating tooth, when looking across the
central incisors at the median line. The extension of the buccal
wall should be made to correspond. When the effect of this
rule is carefully examined by placing the edge of a straight
chisel in the embrasure of any two bicuspids, or the bicuspid and
first molar, it will be seen that the effect will vary the width
of cavity as the embrasures are deep and wide, or narrow and
shallow. The former makes a narrow cavity, while the latter
makes a broad cavity. Either places the margin of the cavity
where it will be well cleaned in chewing. In some instances it
will be found that after this extension has been made, a line of
superficial injury to the enamel extends beyond the margin of
the cavity at one or both of its gingival angles. This should
be carefully looked for, and, when found, the cavity should be
farther extended so as to include the injured portion.
Extensions gingivally. It must now be determined whether
or not the gingival wall has been sufficiently extended gingivally
for its margin and its buccal and lingual angles to be covered
by the free margin of the gums when the filling is finished. If
not, it must be extended farther to the gingival. An inverted
cone bur 8 or 10 should be placed with its flat end against the
gingival wall, and inclined sufficiently for its edge, or circum-
ference, to engage, and a cut made within the dentin along the
dento-enamel junction to the bucco-gingival angle. It should
then be inclined in the opposite direction and a cut made into
the linguo-gingival angle. In this cutting, the operator should
be very particular that the bur follows the dento-enamel junc-
tion and that it does not cut deeply into the dentin. The object
should always be to avoid very deep cavities. In this way the
enamel to the gingival is undermined and may be much weak-
ened by passing the bur a few times with pressure against its
inner side. It may then be removed with the enamel hatchets
and the gingival wall be made horizontal and smooth. The angle