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152 THE TECHNICAL PEOCEDUEES IN TILLING TEETH.

of junction of the gingival with the buccal and lingual walls
should be sharp and defiuite in the dentin, but should be made
slightly curved in the enamel. The gingival enamel wall should
be examined carefully for extension of caries of enamel gin-
givally. This should complete the outline form. See Figures
167, 168.
Eesistance form and retention form. Resistance form
and retention form in this cavity require that in the step portion
the jjulpal wall be flat and the surrounding walls parallel, the
same as these would be in a simple occlusal cavity; and that
in the mesial portion the gingival wall be fiat and meet the axial,
buccal and lingual walls in the dentin in sharp angles and that
these three walls be practically at right angles to the gingival
wall, the buccal and lingual dentinal walls being parallel to each
other. Wherever the form of the occlusal surface will permit,
the occlusal step should be made in the form of a dovetail, which
will add materially to the retention form. After outline form has
been completed, the changing of the cavity to the boxlike form
for resistance and retention is largely accomplished by an
inverted cone or fissure bur 10 held with the fiat end against
the pulpal wall and carried around this wall, making sharp
angles where it meets the buccal, distal and lingual walls. Cen-
trally the depth of the pulpal wall should he little more than
the thickness of the enamel at the occlusal pit, but if it is made
flat, the depth will be greater to the buccal and lingual because
of the rising of the cusps. Figure 168.
In the proximal portion the same bur may be held in the
same position in relation to the gingival wall and carried along
the junction of the gingival with the buccal, axial and lingual
walls, making these angles sharp and at the same time making
the gingival wall flat. The squaring out of these angles in the
proximal portion may often be done with the enamel hatchets.
These instruments should generally be used to square out the
lines of junction of the axial with the buccal and lingual walls.
Convenience form. Most persons would prefer convenience
points in the gingival portion of the cavity for retention in
starting a gold filling, Figures 170-173. These are made, one
in tlie axio-bucco-gingival angle, and one in the axio-linguo-gin-
gival angle, by pressing an inverted cone bur into the dentin
slightly and drawing it a short distance to the occlusal along
the bucco-axial line angle and the linguo-axial line angle respec-
tively. These give suiBcient starting points and will retain the
filling while the building of this part is in progress.