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EXCAVATION OF CAVITIES BY CLASSES. 159

This is indicated by the prominence of tlie cusps of the tooth
and the age of the patient. The horns of the pulp are very
nearly under the highest points of the cusps, and the reces-
sional lines of the pulpal horns converge toward the angles of
the pulp chamber, usually converging a little more rapidly as
the pulp is approached. Tliese guides indicate the probable
position of the horns of the pulp and the localities to be avoided
in any of the cutting of sound dentin to obtain resistance form,
retention form or convenience form. They also indicate the
localities in which the horns of the pulp may be exposed in the
removal of carious dentin. The rule should be that the reces-
sional lines of the horns of the pulp should not be cut. There-
fore, in young persons the step in the occlusal surface should
be cut between the recessional lines of the pulpal horns. The
carious dentin must be removed, however, even though the pulp
is exposed in the process. But this should rarely come as a sur-
prise and the instrument used and the direction of cutting should
be such as will cause the least pain or do the least injury to the
pulp. The illustration being a central mesio-distal section, does
not show the length of the horns of the pulp, because it does not
cut through them, but tliis is indicated on the mesial in the pic-
tures by making the recessional line of the mesial crest of the
pulp diagrammatically prominent to represent the probable
length of the mesio-buccal horn, and the relation of the decayed
areas to this is shown.
Outline form. In Figure 188, the position of the chisel
for the first cuts in the chipping away of the undermined enamel
is shown in each of these cavities. No one should attempt to
remove this with the bur in the engine, because it is more diffi-
cult and more distressing to the patient. In this case the mesial
marginal ridge is undermined l)y decay but is still very strong.
The position of the chisel shown in the mesial cavity is a favorite
one for the first movement. The instrument blade is laid firmly
over the crest of the marginal ridge of the bicuspid, with its
bevel toward that tooth as shown. The edge is engaged in the
enamel over the decay, and with strong pressure the handle
raised (in the direction indicated by the small arrow) and a chip
split off and thrown into the cavity. After one or several strokes
in this way by hand pressure, the chisel should be turned with
its bevel in the other direction, the mallet used, and the chipping
continued until all of the undermined enamel is removed, uncov-
ering the area of decay as shown in Figure 189. Next the enamel
should be chipped from over the pit cavity, as shown in Figure