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202 THE TECHNICAL PHOCEDURES IN FILLING TEETH.

the gingival wall of the cavity that it will be deeply covered by
the free border of the gum when jBnished. As the general rule,
only very slight extensions to the occlusal or incisal are required.
The rubber dam should now be placed.
Eesistance roKM. No resistance form is necessary in cavi-
ties of this class.
Ketention form. Eetention form in such a cavity recjuires
that the axial wall be flat and the four surrounding walls at
right angles to the axial wall, the opposing walls parallel to each
other, with a very slight undercut in the dentin entirely around
the axial wall. This undercut should not be greater than that
made by the flare of the blade of an inverted cone bur held with
its flat end against the axial wall. The retention form may be
made with inverted cone bur 10, or with hoe 12-5-12 or 12-5-23,
or smaller sizes of the same angles, and increase the retention
along both the occlusal and gingival walls, using these instru-
ments with a scraping motion. This cutting should not be deep
so as to undermine and weaken the walls, but just enough to hold
when gold is packed against them. In some cases it may be more
convenient to use the equivalent hatchets with the shaft perpen-
dicular to the axial wall, with a lateral scraping motion.
Convenience form. Sometimes it is well to place slight
convenience points in the axio-disto-gingival and axio-disto-occlu-
sal angles in which to start the filling.
Eemov.\l of remaining carious dentin. If there should be
any remaining decay in the axial wall, it should be removed with
spoons 15-8-12.
Finish of en.^^mel wall. The enamel wall should be planed
and the cavo-surface angle lightly beveled in all parts with
chisels 20. Make the toilet and fill the cavity. Figure 284
represents the prepared cavity, and Figure 285 the completed
filling.
The extension to the mesial and to the distal as given above,
is correct for the conditions presented in the illustrations. But
an otherwise similar case in which the whitening of the enamel
was practically confined to a spot in the mesio-distal center of
this area at the time of operating, the extension should reach
just as far toward the angles of the tooth as represented in this
case. Many of these cavities in which the spread of superficial
decay on the enamel is slower, will have penetrated the enamel
before the spreading has reached its full extent. If the central
part is cut out and filled without extension, the growth of micro-
organisms will recur on the filling, and, the support of the new