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EXCAVATION OF CAVITIES BY CLASSES. 143

of food. If this can be had in narrow cavities, it is well. If it
can be obtained by separating the teeth and building prominent
contacts, that is well. If the case requires wide cutting to accom-
plish it, that should be done. If the case is one that lias l)ccome
immune to caries, that fact may have consideration.
Class 2. Cavities in the Proximal Surfaces in Bicuspids
AND Molaes.
ILLUSTRATIONS: FIGURES 154-245.
Case illustrating the meaning of extension for preven-
tion. Figure 155 shows a narrow decay penetrating the enamel
and burrowing in the dentin. Figure 156 represents the rounded
gingival portion in this case when the thinner margins of the
enamel, which had been undermined by the spreading of decay
along the dento-enamel junction, had been removed and the
cavity cleared of carious dentin. From the mechanical stand-
point, it is a form of cavity unfit to receive a substantial filling
because of its rounded contour. Figure 157 represents the same
cavity squared out into the box form, with a step cut in the
occlusal surface to give the filling additional stability, and Fig-
ure 158 exhilnts the filling from the same view. Figure 159
represents the abnormal relation of this tooth to its next neigh-
bor, which rendered this narrow cavity ample to remove the
margins of the filling from near contact. This is shown in the
occlusal view in Figure 160, which gives the outline of the occlu-
sal step after the removal of the sharper grooves of the occlusal
surface of the tooth. The excursions of food in chewing will
be over the margins of the filling to the gum line and thus the
requirements of extension for prevention are satisfied. The cut-
ting of the step in the occlusal surface has nothing to do with
extension for prevention. The step is designed to give the filling
the greatest possible stability, and, incidentally, to remove all
danger from occlusal pits.
The occlusal step. The demand for the cutting of this step
in the occlusal surface is absolute, because of the more secure
seating and rigidity which it gives the filling in all proximo-
occlusal cavities in bicuspids and molars, wherever the marginal
ridge is broken. It must also be cut for filling this class of cavi-
ties even if the marginal ridge is as yet unbroken by decay in all
cases where the proximating tooth is present, or is to be supplied
by a substitute, for the reason that otherwise a good opportimity
can not be had to make the filling, nor can a reasonably safe con-
tact point be formed. In a few cases in which the proximating