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EXCAVATION OF CAVITIES BY CLASSES. 139

the central part of the tooth surface l)ut inclining toward the
occlusal as the occlusal margin of the surface is approached. See
Figures 106, 107. Therefore, the inclination of the enamel rods,
or the direction of the cleavage, should l)e closely noted while
chipping away the enamel, and the occlusal enamel wall finished
in inclination to the occlusal, so as to be parallel with the line
of the cleavage. If in any case it is found that because of close
approach to the occlusal surface, the inclination of the enamel
wall to the occlusal will render the filling material too thin at its
mai'gin to have sufficient strength, the buccal groove should be
opened over the crest of the marginal ridge and the filling car-
ried onto the occlusal surface in the form of a step, as shown in
the upper second molars in Figures 147, 148, and in the lower
second molars in Figures 149, 150. Neglect of this precaution is
causing the loss of many otherwise good fillings in this position.
It is only occasionally that a good finish can be made along
the line of the buccal groove, as shown in Figures 151, 152. Gen-
erally the extension should be made to include the central pit of
the occlusal surface, including more or less of the grooves, as
shown in Figure 153. It so generally happens that there is also
decay in the central pit of either the upper or lower molar when
decay has begun in the buccal pit, that it is not inconvenient to
cut out the buccal groove between the two and unite them as
shown in Figure 153. This gives the safest treatment of these
pit cavities when they have been so neglected as to have bur-
rowed deeply, or have undermined considerable enamel near the
marginal ridge. The judgment between a simple filling and this
extension must be based altogether on the conditions revealed
in individual cases in the excavation of these cavities.
Pit or fissure cavities on lingual surfaces do not occur in
the lower molars, but do occur occasionally as independent cavi-
ties in the lingual grooves of the upper molars and rarely in the
mesio-lingual groove of the upper first molars where there is
a fifth cusp. The instrumentation in these is on the principles
already given and requires no special description.
PIT CAVITIES IN THE LINGUAL SURFACES OF THE UPPER INCISORS.
In the preparation of these cavities two points of special
caution need to be mentioned. First, the location of the cavity
is such that the pulp is easily reached, and accidentally and
unnecessarily exposed, unless especial caution be observed. The
use of burs should be limited strictly to the first opening of pits
but little decayed. Neither inverted cone nor fissure burs should