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EXCAVATION OP CAVITIES DY CLASSES. 161

be placed and the cavity cleaned and dried. The cavity is now
in condition to be immediately sealed — after appropriate treat-
ment of the exposure — if the pulp should be exposed in the
removal of carious dentin from the occlusal cavity, which is the
danger point in this case.
Removal of remaining carious dentin. With spoon 20-9-12
loosen the decayed mass carefully some little way from the mar-
gins around the periphery of the cavity. Then, choosing the
most favorable position, force the spoon with such direction as
will follow close against the sound deutin across the cavity, the
endeavor being to turn out the whole of the softened material
at a single stroke. (See E"'igure 132.) Then scrape the remain-
ing softened material from every part of the area.
Resistance form and retention form. No pulp exposure
is found in this case, but the central area of decay is found to
be so deep, as shown in the remaining figures of the series, that
to cut the step to the same depth and to the full width necessaiy
would almost certainly expose the mesial horn of the pulp.
Moreover, the step as first cut between the two cavities is deep
enough, with the rise of its walls on the slopes of the cusps, to
give the required staliility. Or, in cases in which that seems to
be required, the cavity may be filled fuller, and the cusps of the
occluding tooth ground away to accommodate it. Therefore,
instead of cutting the step to the full depth of this part of the
cavity, a shelf or ledge is cut around it to the depth of the step
between the two, as shown in Figure 193 and the remaining fig-
ures, which gives the filling a sufficient seat to prevent any incli-
nation to tip or move under stress. Additional safeguarding
against the cutting of the recessional line of the mesio-buccal
horn of the pulp which is in greatest danger, is made by rounding
the cut about the mesio-buccal cusp, as shown in Figure 196 and
those preceding it.
Completion of outline form. The extension of the mesial
portion of the cavity is to be made to the gingival, the buccal
and the lingual, completing the outline form. The inverted cone
bur in the straight hand-piece of the engine is placed in the
deeper part of the gingival wall close against the dento-enamel
junction, as shown in Figure 193. Its angle is engaged in the
dentin and it is first carried out to the buccal wall and then
replaced and carried to the lingual wall in a series of cuts that
are supposed sufficient. In the last of these, it is well to press
the edge of the bur hard against the inner surface of the enamel
to weaken it so that it will be more easily broken. This is then