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132 PREPARATION OF CAVITIES

groove across the cutting edge, in between the labial and
lingual enamel plates, into the dentin, which has been exposed

by the carborundum stone. Then remove a portion of the
lingual enamel plate, leaving the labial plate untouched as
far as possible, to avoid exposure of gold, and then deepen
the groove further, continuing in this manner until the step
is of sufficient size to admit of the necessary strength for

Fig. 160. Fig. 161.
Fig. 160.—^Lingual step form in an upper lateral incisor. The gingival wall,
instead of being made flat, has been cut to follow the curv^ature of the gingival
line somewhat, in order to provide a better seat and anchorage for the filling.
Fig. 161.—Labial view of the mesio-incisal cavity with lingual dovetail, shown in
Fig. 160.

resistance and to allow of a sufficient body of gold to with-
stand all strain (see Figs. 158, 159, 164, 165, 166 and

167).
(c) Lingual Step Form.—This form is used where the tooth
is too thin labio-lingually to warrant the incisal step. With
an inverted cone bur cut an auxiliary mortise form or step on
the lingual surface of the tooth. The floor of this step is
made flat and the lateral walls plane surfaces, joining the floor