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MOLARS.
145
palatal end, passing toward and into the crown, finally returning to-
the be and
ward the starting-point, cavity may rapidly easily filled.
To in the crown, however, and build over into the ex-
begin palatal
tension, will often be found most tiresome and perplexing.
I have elsewhere designated these two cavities as an exception to
the general rule of opening up sulci from end to end. Where con-
siderable caries is present, and the removal thereof discloses the fact
that the two cavities are united below the enamel by a narrow passage,
I think it best to open up the two into a single cavity, which would
then appear as shown in Fig. 163. Here it is to be observed that the
two cavities are connected only by a narrow passage. This should
only be done where the enamel at a, a is well supported by strong den-
tine beneath. Where the dentine is carious or absent, they should be
cut away freely till a strong wall is reached. This cavity will ordi-
be filled with either or the former, of
narily easily gold amalgam,
course, to be preferred, unless contraindicated by circumstances. No
FIG. 163. FIG. 164. FIG. 165.

special arrangement is necessary to assure retention, for each cavity
of retentive the two when united
usually presenting general shape,
form the that could be
strongest cavity designed.
In connection with a cavity ofabout the magnitude of that prepared
in the last case, we will sometimes find one so undermined
cusp by
decay, or so imperfectly calcified, that it must be removed in order to
reach strong walls. A point of special interest here is the palatal
Even that it should be non-carious, it is essential
groove. supposing
that at least a slight extension of the cavity should be made in that
direction. This will give us a cavity shaped as seen in Fig. 164, the
antero- palatal cusp being absent. From this it is at once apparent
that a point of weakness occurs at a, where we find that all that is left
of the cusp is frail. It is therefore obligatory to remove it, so that
the cavity ready for filling would appear as in Fig. 165. For the re-
tention of the a should be formed the
filling, deep groove along
approximal border and around the palatal portion as far as the palatal
groove, as indicated at a, a. At the palatal angle b, a depression is to
be made as deep as possible without danger to the pulp. Along the
base of the standing walls slight undercutting maybe resorted to, but