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146 FILLING TEETH.

filling perfectly, for there may be two opposite sides
parallel, or even divergent ; in that case, the axis of
the cavity will not be in the direction of the center

of the crown.
Two opposite sides may converge and the others

diverge, and a filling be retained firmly. When two
contiguous sides have the same converging inclina-
tion, making the orifice larger than the interior, if
the walls are smooth, plain surfaces, a filling will not
be retained ; but retaining points may be made by
forming transverse grooves, or pits upon them, and
by this means the filling be firmly retained. As a

general rule, it will be necessary, when the orifice is
larger than the cavity within, to make grooves or pits
on the walls. For this purpose the diamond point
excavator is invaluable.
If the cavity is large, and the walls near the orifice
thin, and liable to be broken, the situation of the
grooves or under-cutting should be farther within the
cavity than if the walls are firm out to the edge.
Sometimes it is best to make little pits at the bottom
of such cavities for retaining points. In cases where
it is necessary to make an under-cutting, one or two

little transverse grooves upon one side will be suffi-
cient, and in no case on more than two sides, leaving

the others perfectly plain surfaces.
In the formation of retaining points in difficult