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FESTOON CA VITIES.
163
next step is to take a sharp excavator, and with a scraping motion
remove all of the chalky substance. This will leave the outlines even
defined. Now a rose bur be used in the
more clearly sharp may engine,
and a slight groove made around the whole extent of the cavity. This
groove, however, is not at first made in the nature of a retainer or
undercut, but is rather to perfect the edges and curve of the border.
With the same bur all caries may be cut from the cavity, if any has been
left after the use of the excavator. The groove must next be deepened
in order to acquire retentive shaping. This is done laterally only, and
in a direction sufficiently oblique that the pulp shall not be endan-
gered. The dotted lines at a, a show the manner of extending the
and indicate that the are farthest from the
grooves, deepest points
incisive edge. Along the incisive border of the cavity no under-
is to be ; but the must have been
cutting attempted original groove
deep enough to produce a distinct shoulder here, and avoid any-
thing like a bevel. This also must be true of all edges, which are
FIG. 189.

to be at right angles to the labial surface, and not beveled. This can
be better explained by showing a sectional view of the right and the
wrong way to treat this cavity. Fig. 187 shows a section through a
central incisor, in which the cavity has been properly shaped. At a, a
is seen the deepest part of the retaining groove, and its relation to the
and to the orifice of the can be noted. At 6, b are shown
pulp cavity
the right-angled edges. In the same kind of cavity, in Fig. 188, the
edges, b, l>, are prepared with a bevel. By this method it is plain
from the figure that the retainers, a, a, are weaker than in the other,
whilst it is also true that the thin edge of gold which will be the result
will eventually turn up and break, so that the borders of the filling
become defective and leakage follows. Moreover, in polishing, the
result pictured in Fig. 47 would easily occur. To fill this cavity with
use in one and then in the other. This done,
gold, pellets groove
build across, uniting them and covering the floor with heavy foil, with
which complete the filling. Care should be used not to proceed too
rapidly, so that thorough condensation may be had, as well as per-
fection of border. It is a pleasure, when such a filling is placed, to be
able to it like a mirror, and have the borders so well made that
polish