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122 THE TECHNICAL PEOCEDFRES IN FILLING TEETH.

most eases the bur will cut tlirougli the enamel and enter the
cavit}' within a few seconds. If it does not do so promptly, it
should be removed for a moment and allowed to cool — for heat
will develop quickly — and then reapplied in the same manner.
This should be repeated until the bur passes through the enamel
and enters the softened dentin. Immediately this occurs, the
bur should be removed from the hand-piece and round bur 10
chosen. This should also be passed through the opening in the
enamel. This completes the use of the round bur, and it is the
only use made of it in excavating cavities. Any further enlarge-
ment of the opening found necessary is made with hand instru-
ments. An examination should now be made with a curved
exiDloring tine to determine more nearly the extent of the decay
in the dentin. In case no softened dentin extending laterally
under the enamel is found, the rubber dam should be applied.
The cavity should now be sufficiently extended to include the
sharp slopes toward the pit without cutting it deeper. The
extent of this broadening should be determined and the enamel
undermined by pressing the edge of an inverted cone bur, 8 or
10, under it while in rotation, removing the dentin. This should
at once be carried around all parts of the cavity that require to
be made broader for the purpose named. Then the undermined
enamel should be chipped away with straight chisel 10 or 15 and
biuangle chisel 10-6-6 or 15-8-6, or enamel hatchets of similar
widths. This undermining and chipping is repeated until the
desired outline form has been ol)tained. In such a cavity this
should be only sufficient to smoothly obliterate the slopes of the
pit so that a smooth finish of the filling with the surface of the
enamel may be made. With straight chisel 10. or binangle chisel
10-6-6, the enamel is chipped away along the buccal groove as far
as it will readily split off. Then inverted cone bur 8 is passed
into the cavity and made to enter the dentin toward tlie buccal
groove just beneath the dento-enamel junction, and, while rotat-
ing, slowly drawn to the surface of the enamel. It is then entered
again at the same point and this motion repeated, making cut
after cut, following the line of the groove until the groove has
been opened to a point where it is sufficiently shallow, or the sur-
face of the enamel is sufficiently level for a good finish of a filling
to be made. Figure 124. Often it will he necessary to follow
this groove to the crest of the marginal ridge. This done, chip
away the mesial and distal walls of tlie groove with straight
chisel 10, or binangle chisel 10-6-6, sufficiently to remove the
inclines of the buccal groove.