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FORMING CAVITIES. 145

superior incisors, and in proximal cavities of the
molars, in which it would be impossible to make
anything like an approach to this rule, without ex-
posing the pulp, and even cutting through its chamber.

A general direction, and one that we think good,
and applicable in many cases, especially in crown
cavities of the molars, and in almost any of the deep
perforations by decay, is, to make the walls of the
cavity as nearly as possible parallel with one another.
This rule is applicable in almost all small cavities.
In medium or large-sized cavities, it is admissible

to leave them slightly larger at the bottom than at
the orifice, if circumstances require ; a large cavity of
this form can be perfectly rilled, when a small one
could not, from the fact that, in the former, there is
more room to operate in introducing and consolidating
the filling.
Cavities that are larger within than at the orifice,
should have their walls perfectly plain, smooth sur-
faces, free from transverse grooves or depressions, so
that the gold may be perfectly adapted to them.

It is sometimes necessary to leave a cavity slightly
larger at the orifice than at the bottom. This may
be done by a converging inclination of the wall of
one or more sides of the cavity. When there is an
inward inclination of the wall at one side of the
cavity, the general form may be such as to retain a