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RESISTANCE FORM 133

at definite line angles. Avoid exposing the pulp (see Figs.
i6o and i6i).
4. Cavities in Proximal Surfaces of Bicuspids and Molars.
—As a general rule, the cavity is extended onto the occlusal

surface in the form of a step, it frequently being necessary to
break down the marginal ridge. There are three methods of

accomplishing this:

Fig. 162. Fig. 163. Fig. 164.
Fig. 162—Proximo- occlusal cavity in an upper bicuspid. Step form.
Fig. 163.—Proximo-occlusal cavity on a bicuspid. Compound mortise or step
form.
Fig. 164.—Proximo-incisal cavity on a central incisor. It has been necessary
to involve both labial and lingual enamel plates owing to thinness of the tooth in
the incisal region.

First Method.—With a chisel and mallet cut a V-shaped

opening through the marginal ridge, enlarging it until free
access to the cavity is gained.
Second Method.—With a bi-bevelled drill inserted into the

occlusal pit or fossa, drill an opening through the dentin
toward the proximal cavity until the area of decay is reached.
Then cleave away the enamel of the marginal ridge as before,
by means of the chisel, or remove it with a dentate fissure
bur.
Third Method.—Adjust the separator and attack the