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EXCAVATION OF CAVITIES BY CLASSES. 121

rence of decay. All the extenBion that is necessary is such cut-
ting away of the cavity walls as will fully uncover the carious
area and present a surface upon wliich a good, smooth finish
may he made; it is necessary that all sharp grooves connecting
with the cavity he cut out to a point where the enamel is suf-
ficiently level to make a smooth finish.
As a primary study of the (-onditions presented in this class
of cavities, the student should select a number of teeth from a
collection of extracted teeth and open the pit cavities. This is
best done, when a suitable jeweler's lathe with slide rest can be
had, by sawing the tooth through the central area of decay with
an aluminmn disk charged with carborundum powder in soapy
water. "VVlien this can not be had, much good work can be done
by filing or grinding away one-half of the tooth, retaining the
other half with its exposed cavity for study. Much familiarity
with the conditions that will be presented by these cavities as
they occur in the mouth may be obtained also by opening and
forming these as it would be done in the mouth in preparation
for filling. Enough of this work should be done to render the
student familiar with the directions and the extent which decays
in the occlusal surfaces and other pit cavities burrow under the
enamel.
For this work to be effective, the teeth must not have been
dried. A dried tooth has become cheeked by shrinkage. In any
effort to prepare a cavity, the enamel, particularly, is liable to
split off from the dentin, and the dentin is also liable to go to
pieces. Therefore, dried teeth are unfit for this purpose.
SMALL PIT CAVITY IN CENTRAL POSSA OF AN UPPER FIRST MOLAR.

Description. Upon examination, a small pit is found in the
enamel and the point of the exploring tine passes through it
into the dentin, which seems to be softened, showing that caries
has begun. The surface of the enamel about the pit is clear and
clean. The buccal groove is deep and sharp, but the mesial and
distal grooves are shallow, fairly smooth and well closed. (Fig-
ure 123.)
Outline form. In opening this cavity, a round bur, *8, in
the engine is placed in the pit while in rotation and fairly strong
pressure applied, while the hand-piece is swayed to and fro, In

* The tenth of a millimeter ig the unit of measurement for cutting edges of
excavators, sizes of plngger points and diameters of burs. Figures are not used as
arbitrary numbers, but as formulas indicating measurement of size, as diameter,
width, length, angle, etc. See instrument gauge, Figure 17.