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CAVITY PREPARATION. 19

means from mesial to the distal. Bucco-lingually means from
the cheek toward the tongue, etc.
This use of words is so sim-
ple that it should only require mention to be perfectly under-
stood. Surgeons continually use this plan in speaking of the
location of fractures of the bones. As, for instance, the hum-
erus was broken at the junction of the middle and upper third,
or the radius was broken
in the middle of the lower third, etc.
There is no need of any specific rules for this use of words in
dividing the surfaces of the teeth in cavity descriptions, as any
divisions intelligently made will be readily understood, and the
If
portions of the surface involved quite accurately described.
it is said that a cavity in the mesial surface of a first lower molar
extends from the occlusal surface to the junction of the gingival
and middle third, and bucco-lingually from the mesio-buccal
angle to the junction of the middle and lingual third, it should
be understood. The same conception of the cavity should be
obtained if it is said that it occupies the buccal two-thirds bucco-
lingually, or that it occupies the buccal and middle third. There
is scarcely any limit to the use that may be made in cavity
descriptions of these divisions of the surfaces of the teeth.

Cavity Preparation.
GENERAL PRINCIPLES,
There are certain fimdamental principles that are general to
the e.xcavation of carious cavities in the teeth, the observance of
which will simplify and facilitate these operations.
Order of Procedure.
Obtain, first, the required outline form.
Obtain, second, the required resistance form.
Obtain, third, the required retention form.
Obtain, fourth, the recjuired convenience form.
Fifth, remove any remaining decayed dentin.
Sixth, correct the form of, and smooth the enamel wall, bevel
the cavo-surface angle, and make the toilet of the cavity.
The careful observance of this order of procedure by the
student will greatly facilitate his operations, lead to more careful
consideration of the requirements
in individual cases, and will
guide him to the use of the approjiriate instrument for the differ-
ent parts of the operation.