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186 CLASSIFICATION OF DECAYED CAVITIES.

enamel more rapidly than in any other direction; as
where there is an imperfect union between the enamel

and the dentine. In other instances, the cavity seems
to expand uniformly as it extends into the tooth.
The manner of opening up and preparing the
cavity for filling, will depend somewhat on the form
given to it by the decay. If it is larger at the orifice
than within, there will be little or no cutting of the
cavity about the orifice necessary, except to make it
even and smooth; and its preparation will consist in
an entire removal of the decay, and such a shaping of
the interior as will insure a retention of the filling.
This may be effected either by enlarging the cavity

within, till its walls are parallel with each other, or,
if these are left converging, by forming pits or grooves
upon them at proper points. Converging walls pre-
sent one or two advantages, which will be hereafter
considered. Usually, where the decay has formed a
cavity of nearly uniform diameter from the orifice to
the bottom, about all the preparation for filling that
is requisite, is a thorough removal of the decay. In
cases in which the decay burrows under the enamel,
the projecting portions are to be cut down, either with

a bur drill, or, what is generally better, a heavy
cutting instrument. In most instances, it is best to
cut away the enamel as far as the decay has extended
beneath it, since it is difficult to make a perfect filling