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112 THE TECHNICAl, PEOCEDUEES IN FILLING TEETH.

cavity and the extension of the cavity outlines to include the area
of the surface that may have suffered superficial injury, or is in
danger of decay in the future. This will often require that sound
enamel and dentin be cut away to obtain the correct outline form,
and is known as extension for prevention of the recurrence of
the decay. The study of the case should be made, the outline
determined upon and the cavity cut to the outline form required
as the first procedure.
If the student will take a large number of decayed teeth and
select all of those that have small cavities in some one or more
of their surfaces, which properly rejiresent the place of the begin-
ning of decay, he will find a remarkable uniformity as to the par-
ticular spot in each individual surface in which decay begins.
In all of the pit and fissure cavities, he will find that the decay of
the enamel, superficially, is confined to the immediate pit or fis-
sure, but that it spreads in the internal parts of the tooth and the
enamel finally is decayed from the inside outward; backward
decay of enamel. In all, or nearly all, of the smooth-surface
decays, when small, he will find a different condition, the decay
beginning in the surface of the enamel tends to spread super-
ficially on the surface from some point, usually central to the
surface, toward the margins of the surface. In this superficial
spreading of decay, the widest possible variations will be found,
from decays of exceedingly small superficial area to those that
are very broad. It is only after the decay has penetrated the
dentin considerably that the enamel is undermined, as in the
pit and fissure cavities. In this study, the student should note
particularly the number of decays he can find beginning upon
the axial angles of the teeth and make a careful comparison of
the number of these with those beginning centrally upon the sur-
faces. This will demonstrate very certainly that the axial angles
of the teeth are, on account of their position, comparatively
immune to decay, and point out to him the directions and extent
to which extensions of cavities should be carried in their prejiara-
tion for filling to prevent the recurrence of decay after the filling
has been made. In this, he will find that extensions should be
made toward the angles, and that such extensions should never
be carried past the angles. The central area is most liable to the
beginnings of decay, the angles least liable to decay. Hence, any
extension beyond an axial angle would l)e an extension into a
region of greater liability to decay.
Resistance foem is that sha]ie given to a cavity intended to
afford such a seat for the filling as will best enable it to with-