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EXCAVATION OF CAVITIES BY CLASSES. 125

molars the lingual groove will oftenest require cutting out to the
crest of the marginal ridge.
In any case in which the cavity in the dentin is of consider-
able breadth and depth, the danger of cutting the recessional
lines of the horns of the pulp and of exposing the horns of tlie
pulp in squaring out the pulpal wall should be carefully consid-
ered and such changes of form made as will certainly prevent such
an accident. It should be remembered that teeth with long, sharp
cusps will have long pulpal prolongations pointing toward the
points of the cusps, and that these are much longer in the teeth
of children than of adults. Therefore, the lines of these horns
from the pulp toward the points of the cusps (the recessional
lines of the pulpal horns) are dangerous lines in the excavation
of cavities. Of these, the recessional line of the mesio-buccal
horn in molars is especially dangerous.

LAKGE PIT CAVITY IN CENTRAL, FOSSA OF AN UPPER FIRST MOLAR.
DESCRrPTiON. The pit is open so that the exploring tine
readily passes in, giving room to be turned about, revealing a
considerable decayed area; some ashy discoloration shows
through the enamel, which also indicates undermining of the
enamel.
Outline form. Begin the excavation by chipping away the
overhanging enamel with straight chisel 15. Or, if the orifice
in the enamel is still very small, begin with straight chisel 10,
and use the 15 later. This may be done by hand pressure, but
mallet pressure is much better. In using mallet pressure for
the purpose of chipping enamel, the instrument should be held
between the fingers with the third finger resting against the
adjoining teeth whenever practicable. The instrument so held
should be placed lightly on the enamel near the margin of the
cavity and so directed as to throw the chips into the cavity. A
single quick, sharp stroke should be used. The grasp upon the
instrument should be sufficient to prevent the stroke of the mallet
from driving it into the depths of the cavity and causing pain by
striking its more sensitive parts. Continue this chipping as far
as the enamel can be readily cut in this way, or until soimd den-
tin is found supporting the enamel at every point. Look care-
fully to the relations of the outline of the cavity to the surface
and proceed at once to cut out any grooves that will interfere
with a perfect finish of the filling. Take hatchet 8-3-23, or, if
the opening into the cavity is too small for this, take hatchet
6-2-23, and sweep its edge around the dentin wall of the cavity