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234 PRPJPARATIOy OF CAVITIES.

They usually oeoiir in oonsequenco of the wearing away of the latter
surface tlirouoh attrition or from the necessitated removal of the incisal
corner on aeeonnt of weakness, Botli the a|)])roximal and incisal cavi-
ties mav be prepared sei)arately as described in classes i*' and (r, after
which they should be connected, the walls made strong and smooth and
properly bevelled.
Atypical cavity of tliis class is siiown in Fig. 192. In all such
cases the labial ])late of enamel shonld be preserved intact as
far as possible for appearance sake, and if any cutting has to
be done to increase the size or depth of the incisal portion of
the cavity, it should be done at the expense of the lingual wall.
In order to protect the labial wall from possible fracture in
mastication the enamel should be bevelled outwardly (as men-
tioned under class F) so that when filled the gold alone will
come in contact with the opposing teeth in mastication.
The only anchorage needed for tins class of cavities is a slight
undercut along the cervical wall and a dovetailed form of the incisal
portion of the cavity.
In many cases there is no cavity upon the incisal edge, but where
opportunity offers for making one (as in the case of thick or worn teeth)
this method of forming a compound cavity affords the greatest possible
support and security for a large ai)proximal filling involving the ap-
proximo-incisal angle.
Where the crown is thin and unworn upon the incisal surface a com-
pound cavity of this character cannot be formed, but the same result as
to anchorage may be obtained by cutting an extension upon the lingual
surface of suitable size, form, and depth, as described on
^^'
p. 231. One form of such extension where the corner is
'^'
gone is shown in Fig. 187.^ Another form, rej)resented
in Fis;. 193,'"^ consists of giving the extension a cnrved or
hooked form. Both forms serve the same purpose, for
^^^^y ^ff'^^'^l in these cases ])erfectly secure anchorage which
"^t-inauchoraT
could not be obtained so well in any other way.
Class 0.— Mcdo-clkto-indsal Cavities.—Cavities of this character
differ from the preceding ones principally in extent. The method of
preparation in each case is similar and the operation requires the
exercise of great skill and care in order to produce the best results.
In both cases the following points will have to be observed :
As the operations are extensive in character, good strong walls are
needed on all sides to withstand the force exerted in the introduction of
the filling.
' Dental Review, vol. ix. pp. 812 and 819.
* I. C. St. John, D. D. S., Dental Cosmoss, vol. xxxvi. p. 198.