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EXCAVATION OP CAVITIES BY CLASSES. 155

taking the form sliown in the series of Figures 178-181. This
also shows the form of the cutting in the treatment of a begin-
ning decay in the lower molar similar to that shown in the upper.
It will be noted that the form varies only with the outline
required by the grooves to be cut out. In this the outline in the
lower second molars will also differ from that of the lower first
because of the difference in the number and direction of its
grooves. This does not require any special differences in instru-
mentation. This boxlike cavity is to be taken as the typical
form to be made in all bicuspid and molar proximal cavities
wherever situated, and wherever there is sufficient tooth tissue
remaining for its formation and to give ample strength to all
of its parts.
In distal cavities in the second molar and mesial cavities
in the third molar, it frequently happens that the inverted cone
bur in the straight hand-piece can not be made to approach
the cavity at the proper angle to square out and extend the buceo-
and linguo-gingival angles. In these cases the bur in the contra-
angle hand-iiiece may be sul)st:tuted, or the cutting may be done
entirely with the hand instruments. However, in the greater
number of cases in which distal cavities in the second molar
are excavated at the same time with mesial cavities in the third,
the straight hand-piece may be used for both. This is facilitated
by cutting the step in the second molar first and carrying it well
to the mesial. This gives additional room for the application
of instruments to the distal portion of the cavity and also to
the mesial cavity in the third molar.
Occasionally there is difficulty in so placing the inverted
cone bur as to begin cutting the step in the occlusal portion
from a distal cavity in a second molar. The approach is such
that the instrument is inclined so much to the distal that the bur
cuts the enamel from the surface inward instead of cutting the
enamel from within outward, and for this reason it will not cut
well. The proper position of the bur may be obtained with the
contra-angle hand-piece, but if the enamel proves very hard,
this instrument does not work well. It is better in such cases to
enter the occlusal surface through the pit in the central fossa
in the usual way of opening pits, or with a drill, if the jjit is
fully closed, and then enter the inverted cone bur at this point
and cut to the distal along the line of the distal groove until
the distal cavity has been entered. This slot may then be broad-
ened to the buccal and to the lingual in the usual way to form
the step.