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FILLING BY CLASSES AND MODIFICATIONS. 187

under a projecting portion. In some cases, however,
where the enamel is thick and firm, it is admissible to
leave a slight projection, so as to form, as it were, a
shallow groove.
The walls of these cavities will be of various in-
clinations. If they converge, pits or grooves may be
required upon them for retaining points, especially if
the enamel be cut away at the orifice to the solid
dentine. If, however, the walls, or two opposite
walls, are parallel, or but slightly divergent, these

special retaining points will not be necessary, except
for the reception of the first pieces of gold. Small
cavities of this class may be opened up and formed
principally with the bur drill ; but in large cavities,
this instrument is not available. Thus the orifices of
the small cavities would be round, while those of the
large would be of various forms, determined by the
direction of the decay; as, round, square, triangular,

elliptical, parallelogramic. Cavities should not be
formed much larger within than at their orifices, unless
the filling can be consolidated so perfectly that it will
not yield in the least u.nder the greatest pressure of
mastication ; for, if there is any yielding in such cases,
under direct pressure, the filling being forced into a
larger part of the cavity, withdraws from the walls,
leaving an interval coextensive with the depression
it has undergone ; and thus fluids would be admitted