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COMPOUND CAVITIES. 237
In such instances the occlusal portion of the cavity should have a V
shape as shown in Fig. 199, and anchorage for the filling at this point
be obtained by slightly undercutting the approximo-occlusal walls at a
and B.
Fig Fig

sB B ^D
Prepared cavities and anchorages.
In the diagram Fig. 200 the black portion represents the floor
of the cavity ; A and b indicate the points to which the buccal and lin-
gual walls should be cut ; C and d show the curved form of cavity
after the occluso-approximal angles have been removed, while the
curved line outside of the cavity indicates the approximal contour of
filling, with contact point at H.
Fig. 201 represents a compound cavity (mesio-occlusal) in a lower
second molar. These cavities differ from similar ones in
Fig. 201.
bicuspids principally in having the occlusal portion of
the cavity exteud in different directions along the sulci.
All of the terminations should be ^vell rounded and in
no portion of the cavity should distinct angles be allowed Mesio-occiusai cav-
o
' i •' ity m lower sec-
tO remain. ond molar. Pre-
^^"^^
Class Q. — Disto-ocdusal cavities in either the bicus-
pids or molars are not essentially different from mesio-occlusal cavities
in the same teeth. Owing to their position they are more difficult
of approach, but their manner of preparation and their form are vir-
tually the same.
Class R.— Occluso-buccal cavities are more frequently met with
in the lower than in the upper molars. This is due to the general
presence of a pit upon the buccal surface in which decay by extension
reaches so near to the occlusal surface that the occluso-
Fig. 202.
buccal wall is weakened and has to be removed. Coin-
cident with this there is usually a cavity of some size
upon the occlusal surface, and the union of the two
cavities becomes necessary to insure a satisfactory
result in filling them. A common type of such cav-
ity is shown in Fig. 202. Occluso-buccal cav-
The channel connecting the two cavities is usually ity in lower molar.
Prepared.
narrower than either of the latter, and also more shallow,
thus conserving the strength of the tooth. As, however, the strain
upon the walls bordering this channel is very great in mastication they