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232 rnEPARATION OF CAVITIES.
the occlusal surface, and the other from the buccal aspect. Usually the
former is chosen, as it involves less sacrifice of tooth structure, although
by it the difficulties are increased owing to the limited space in which
we arc obliged to operate.
These cavities can usually be best opened and mainly ])reparcd with
a round bur. After the decay has been removed and the walls defined
and prepared, the cavity may be made retentive in form by sli<>ht under-
cutting throughout its entire circumference, or it may be enlarged at
two o[)posite })oints only. The cervical wall can be inwardly deepened
by an obtuse-angle hatchet excavator as illustrated in Fig. 188, and the
lower or occlusal wall be slightly imdercut by an acute-angle excavator
like Fiff. 189.
Fig. 188. Fig. 189.

Obtuse-angle hatchets. Acute-aiiKle hatcliets.
The sharp angles on the cutting edges of these excavators should
be rounded before being used, so as to avoid the formation of angles in
the cavity.
As the enamel rods on this surface radiate outwardly at such an
angle as to give the proper bevel to the orifice of the cavity, a careful
following of their lines in the preparation of the cavity margins will
be all that is necessary to give them the desired form and strength.
Occasionally these cavities, instead of being round or nearly so, have
a decided oval or oblong form, their greater diameter being in a bucco-
lingual direction, in which event the cavity may generally be best ap-
proached, for preparation and filling, from the buccal aspect.
When this seems desirable, the cavity should be extended so as to
open at the approximo-buccal angle. A round bur is best suited for
this purpose, and when the extension has thus been made the cervical
and occlusal walls of the cavity may be slightly grooved with a hoe
excavator and the inner or lingual wall be made abrupt and also slightly
undercut.
In all cases where sufficient space cannot be gained to operate satis-
factorily from the direction of the occlusal surface, an extension of the
cavity to the buccal aspect is the only alternative.