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EXCAVATION OF CAVITIES BY CLASSES. 153
In making these, the bur should not be sunk into the gingi-
val wall, but the square end should be kept level with that wall,
as shown in Figure 170. In some eases in which a i^articularly
strong hold on the gold is thought to be needed in starting the
filling, at the last the bur head may be sunk half its length into
the gingival wall, as shown in Figure 171. This will hold the
gold driven solidly into it very strongly and is the full limit
of cutting into the gingival wall that should ever be done. The
cutting into the gingival wall for the anchorage of fillings, either
as grooves or pits, as has been practiced, is pernicious, in that it
causes many checks and breaks of the enamel in a position not
easily seen. They have been the cause of the loss of many other-
wise excellent fillings. Figure 172 gives another form of con-
venience point as cut with the chisel. This latter is the best
form. If the operator has a finger thrust with the pen grasp of
eighteen to twenty pounds or more, he can make this form easier
than that made by the bur and with much less pain to the patient.
For this purpose, a chisel 15, beveled to an edge on three sides,
is the best instrument. The completed convenience points as
made with the bur are shown in the cavity in Figure 173. This
is to be taken as the type of the convenience points in all proxi-
mal cavities, wherever they occur, including the incisors.
Eemoval of remaining carious dentin. Any decay remain-
ing on the axial wall should be removed with spoons 20-9-12,
or in this particular position, with the discoid. Generally, how-
ever, in decays treated in their early beginning, there will be no
remaining decay.
Finish of enamel wall. The enamel walls are now planed
to form. In all of the occlusal portion these may be parallel in
the axial plane. In the mesial portion of the cavity, the buccal
enamel wall must be inclined strongly to the buccal to agree
with the enamel cleavage. The lingual enamel wall must be
inclined similarly to the lingual. The inclination given the buccal
and lingual enamel walls will depend upon their relations to the
angles of the tooth. The gingival enamel wall should be inclined
slightly to the gingival. Generally, the plane of the buccal or
the lingual enamel wall should each be either perpendicular to
the surface of the tooth at the particular point where the enamel
margin is laid, or inclined more outward. The inclination will
therefore vary with the position.
The cavo-surface angle of all parts of the cavity outline
must now be beveled. All, except the gingival and the rounding
of the bucco- and linguo-gingival angles, may be done with the

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