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32 THE TECHNICAL PBOCEDURES IN FILLING TEETH.

in the direction of the length of the blade, or shave a wall by
being inclined toward it at about the angle of the bevel, or again
they may be used laterally with a scraping motion. This form
of instrument has a great range of usefulness in all of the more
delicate cutting in the preparation of cavities, and especially in
the incisor teeth. In the angle of 6 centigrades much use can
be made of them with the push motion somewhat similar to the
use of a chisel. Indeed, it has the greatest variety of applica-
tions of any instrument the dentist has in his outfit.
The hoes are essentially pull motion cutting instruments
and in the angles of 12 and 23 centigrades are used most with
a scraping motion. In the angle of 6 centigrades they are
used most with a push motion or as chisels. This is a very
important form of the hoe, especially in the sizes 8-3 and 12-5.
The hoe is beveled on the distal side of the blade only, and the
bevel should be rather short, much like the bevel of chisels.
Efforts have been made from time to time to form a right angle
hoe to work with a push motion by beveling on the mesial side
of the blade instead of the distal. This has not been sufficiently
successful for any such instrument to be continued in use.
Beveled eights and lefts. Beveled rights and lefts are
direct cutting instruments made into rights and lefts by placing
a bevel on one side of the blade of one, and on the other side
of the blade of another, forming a pair. These are not double
plane instruments, however, neither are they lateral cutting,
but are direct cutting instruments in which one is suited for
shaving down one wall of a cavity and the other suitable for the
opposite wall. (See enamel hatchets. Figure 22.) Such instru-
ments are made heavier and stronger in their shanks and blades
than others for the heaviest usage in chipping enamel and shav-
ing enamel or dentin walls. The edges are of a shorter bevel,
to enable them to better stand the strain of this heavy cutting
of hard enamel. Yet, they will cut dentin equally well.
Contra-angles. Contra-angles are formed for the purpose
of placing the cutting edge of an instrument with a long blade
in the line of the axis of the shaft, or so nearly so that the
divergence will not be material in its (>ffect in the working bal-
ance of the instrument, rules for which have been given. See
Figures 15, 16. This is an important factor in instrument con-
struction that has been much ignored by ])ersous designing
instrument forms. For this reason, persons selecting instru-
ments should examine carefully to see that the working point
is sufficiently near the liiio of the central axis of the shaft for