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EXCAVATION OF CAVITIES BY CLASSES. 137

these walls are so near the crest of the marginal ridges that the
enamel rods may be parallel with the cavity wall.
The chipping of undermined enamel in obtaining the outline
form in the lower molars is so similar to that described for the
upi)er molars that no separate description seems to be needed.
In principle it is the same. Differences in position give rise to
some little differences in instrumentation. These teeth fre-
quently have a lingual inclination which renders occlusal cavities
less easy of approach. In this they present great variation, many
being as easy of approach as the upper teeth, and some easier.
Yet, frequently there occurs a lingual inclination that renders
them very difficult. The rule is that the lower molars are most
accessible to instruments which approach them over the median
line at the upper front teeth, or when much inclined lingually a
line over the cuspid of the opposite side is often better. (See
Figure 52, finger positions.) In many cases these cavities, when
in the right lower molars, are easily approached from a position
left side behind with the instrument shaft close against the side
of the face. (See Figure 54, finger positions.)
If these are prepared for filling with gold by the use of
direct force, the mesial wall must be inclined to the mesial and
the mesial portion of the buccal wall must be strongly inclined
to the buccal to allow of direct force being used. It is in such
cases, however, that reverse pluggers have their greatest use,
and when such cavities are to be prepared for the use of these,
the mesial and buccal walls may be squared up in the axial plane
(any jilane one direction of which is parallel with the long axis
of the tooth) without inclination, by use of the square-ended fis-
sure bur in the contra-angle hand-piece, or with hoes 12-5-12 and
12-5-23, and the binangle chisels. Gold can then be well packed
with reverse pluggers, but not by direct mallet force.
Occlusal cavities in the upper bicuspids are so easy and
direct of access that little difficulty is experienced in their prepa-
ration. The principal points of instrmnentation are the same
as have been described. In these, however, the pits in which
decay starts are so close to the marginal ridges, especially in first
bicuspids, that the enamel of the marginal ridge, and not infre-
quently also of the proximal surface, is undermined very quicldy.
This often requires a proximo-occlusal filling in a case that
seemed to be only a slight pit cavity.
Occlusal cavities in the lower bicuspids are difficult only in
cases of strong lingual and distal inclination of these teeth. In
these cases, the instrumentation is similar to that described for
19a