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EXCAVATIOX OF CAVITIES BY CLASSES. 89

with the enamel hatchets and the gingival wall be made horizon-
tal and smooth. Its angle with the axial wall should be sharp
and definite.
Any decay remaining on the axial wall is now removed with
spoons 20-9-12, or in this particular position with the discoid.
The enamel walls are now planed to form. ' In all of the occlu-
sal portion these may be perpendicular. On the buccal, the
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 buc-
cal and lingual enamel walls will depend upon their relations to
the angles of the tooth. Generally the plane of the enamel wall
should be perpendicular to the surface of the tooth at the partic-
ular point where the enamel margin is laid. The apparent
inclination will therefore vary with its position. If the cavity
margin is carried far around the angle onto the buccal surface,
the plane of the enamel wall will be nearly or quite in the bucco-
lingual plane of the tooth. If a less distance, the inclination
will be less. The gingival enamel wall should be inclined a little
to the gingival. 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 chisels. A special instrument is required for
beveling the gingival cavo-surface angle, the gingival margin
trimmers 20- ( 80) -g- 12 for mesial cavities and 20- (95) -9- 12 for dis-
tal cavities. These instruments are used for no other purpose
whatever. They have the angle of the edge cut purposely for
making this bevel. A few strokes lightly made, and carried
around the angle first with the right-hand instrument in the one
direction and then with the left-hand instrument in the other
direction, are sufficient.
The cavity now only requires the toilet to be made^ to be
ready for filling.
However, most persons would prefer some convenience points
in the gingival portion of the cavity for retention in starting a
gold filling. These are made, one in the axio-bucco-gingival
angle, and one in the axio-linguo-gingival angle, by pressing an
inverted cone bur into the dentin slightly and drawing it a short
distance to the occlusal along the bucco-axial line angle and the
axio-lingual line angle respectively. These give sufficient starting