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180 THE TECHNICAL PROCEDtTBES IN FILLING TEETH.

trimmed away smootlily farther toward the gingival, using
strong hand pressure, and, by changing the direction, the cutting
can be continued along the gingival wall to or past the labio-
lingual center of the tooth. With the same instruments working
from the lingual, the lingaial wall is next cut down in the same
way to, and somewhat past, the linguo-gingival angle of the cav-
ity. For trimming the remainder of the gingival wall, hoe
12-5-6 will be required, or the 8-3-6 if the cavity is not opened
sufficiently to admit the larger instrument. This is done with a
scraping motion. Frequently hatchet 12-5-12 will do this well,
working from the labial. The gingival wall should be cut to
sound dentin and made as nearly flat in the horizontal plane as
practicable. This is likely, however, to produce a form of cavity
much like that shown in Figure 248, in which the gingival wall
is curved with its convexity toward the gingival and the labio-
gingival angle much rounded. In case the enamel of the mesio-
lingTial angle of the tooth is not undermined, as shown by its
having been cut away in this case, it will be similarly rounded.
Examine carefully as to the requirements of extension for
prevention. In cases in which the cavity is yet small, a line of
very superficial injury to the enamel will often be found running
away labio-gingivally from the labio-g-ingival angle of the cav-
ity near the gum line. Another line of superficial injury leads
away in similar fashion from the linguo-gingival angle. Often,
also, particularly in the smaller cavities, there is superficial
enamel injury to the gingival of the gingival wall. But whether
these injuries are apparent in the individual case or not, the
cavity must now be extended so that it will include the areas of
these superficial injuries frequently apparent. To do this, place
a small inverted cone bur 8 in the labio-gingival angle, intro-
ducing it from the lingual in most cavities ; enter the dentin close
to the enamel junction, and, swaying the hand-piece as much
as the position will allow, undermine the enamel in the direction
of the line of superficial injury toward the labial and gingival,
sinking it at first about the depth of the bur head. Then very
slowly withdraw the bur, pressing it toward the labial and incisal,
thus widening the cut in this direction. The bur should be kept
close against the dento-enamel junction in making this cut.
With the hoe 12-5-6 or 8-3-6, the undermined enamel may be
chi]iped away. These cuttings, first undermining and then chip-
ping, may be repeated until the extension in this particular direc-
tion seems sufficient. If it is desirable to extend the cavity to
the gingival, which should generally be done, the same bur may