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EXCAVATION OF CAVITIES BY CLASSES. 135

times the oblique ridge must be cut tlirougli along the distal
groove with the bur in the same manner that grooves are cut out
elsewhere before the enamel can be chipped away, connecting the
two cavities. When this is done, however, it is easily removed.
It is also necessary to follow out the disto-lingual groove to tlie
crest of the marginal ridge, and often over it onto the lingual
surface. These, however, are minor differences in the same plan
of procedure. In this case the mesial half of the buccal wall and
the mesial wall, including the mesio-buccal angle, have been
strongly sloped toward the marginal ridges for convenience in
packing gold against them.
In second and third molars some differences of procedure
and of insti'umentation are necessary on account of the differ-
ences of position. Often the bur can not be brought into position
for effective use in squaring up the surrounding and pulpal
walls, and this is done best with the hoe 12-5-12, or, in some posi-
tions, especially in lower molars, hoe 12-5-23, used with a scrap-
ing motion. In some cases the bur may be used effectively in the
contra-angle hand-piece. Figure 25, but this is an awkward and
generally ineffective instrument and its use instead of hand
instruments should be reserved for cases of the greatest neces-
sity. This instrvmient is, however, much better than the right-
angle hand-piece formerly used, for the reason that the working
point is in the line of the shaft, which gives it the proper balance
in the hand. Often one can not follow the progress of the work
well with the eye and there is much greater danger of inaccurate
cutting, which increases the risk of pulp exposure.
The mesial wall, and especially the mesio-buccal angle of
occlusal cavities in the second and third molars, should be so
inclined to the mesial and buccal as to allow of packing gold
against them easily. The amount of this inclination will depend
upon the position and the character of the approach to the cavity.
If the mouth of the patient opens well, giving a good view, and
allows instruments to be placed very nearly parallel with the
long axis of the tooth operated, very little inclination mesially
and buccally will be required. If, on the other hand, the mouth
does not open well and the approach of the instruments must be
at a considerable inclination to the distal, the mesial wall and
the mesio-buccal angle must be inclined in proportion, or sufli-
ciently to allow of mallet force being applied parallel with the
plane of this wall and angle. Any failure in this will require, in
filling with gold, that the gold be packed against this wall with
reverse pluggers or by lateral hand pressure, which greatly