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EXCAVATION OF CAVITIES BY CLASSES. 131

into it. Then the same instrument may reach across to the oppo-
site side and its edge (which must be very sharp) engaged near
the margin of the pit, and, with a pulling motion, by hand pres-
sure, the undermined enamel may be rapidly split off, as shown
in Figure 130. This pulling motion in splitting off undermined
enamel is peculiarly effective and rapid if the enamel is touched
just right. The direction of force after engaging the edge of
the chisel is shown by the small arrow placed above the instru-
ment point in Figure 130. This kind of chipping should be con-
tinued until the enamel is found to be supported by sound dentin,
in which case it will not chip away so easily ; and if the use of
considerable force is persisted in, it will crum])le instead of split-
ting. The enamel wall should be smoothly trimmed parallel
to the long axis of the tooth in all of its parts, as shown in Fig-
ure 131. All of the circumference of the dento-enamel junction
should be carefully examined with the edge of hatchet 6-2-23,
by sweeping it around the cavity to see if any points of softening
remain, or if further undermining of the enamel can be dis-
covered. If the edge of the hatchet readily catches under the
margin of the enamel, it should be cut away until the hatchet no
longer catches. This completes roughly the outline form, except
as it may be necessary to follow out deep grooves, which these
illustrations are unfavorable for showing.
Removal of decay when exposuee of pulp is feaeed. The
opening of the cavity is such that if the pulp should be found
exposed at any point upon removal of the decay, it will be ready
for immediate treatment and protection by a temporary filling.
It is often best in such a cavity to remove the decay before com-
pleting the resistance and retention form and gain further
insight as to its depth. Up to this point it would be entirely
wrong to make more than the most superficial examination of
the depth of the decay, or to do anything that might possibly dis-
turb an exposed pulp. Any examination of the extent of the
ca^dty up to this time should be directed to the lateral extension
under the enamel and not in the direction of its depth. If the
rubber dam has not been placed in the beginning, it should be
placed before going further.
The next movement is shown in Figure 132. The walls have
been so trimmed that the lateral margins of the area of softened
dentin may be approached from any point. For a cavity of this
extent, spoon 20-9-12 should be chosen and the edge examined
to see that it is sharp. It should be introduced under the most
convenient margin of the decayed area, as shown in Figure 132,