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CUTTING INSTEUMENTS. 41

formula names. They are illustrated in Figures 20, 21, and those
included in the University set are shown in Figures 22, 23.
The enamel hatchets form one group of three pairs, each
pair different in size and length of blade, and in the University
set they are all of one angle. They are enamel hatchets 20-9-12,
15-8-12 and 10-6-12, each pair beveled rights and lefts, direct cut-
ting instruments. Their use has been particularly mentioned.
The spoons are also three pairs of true lateral cutting rights
and lefts of the same formulas as the enamel hatchets. They are
spoons 20-9-12, 15-8-12 and 10-6-12. The sizes of the blades of
both these sets of pairs ought to be readily remembered, and the
class and formula names spoken on sight after a few trials,
coupled with measurements on different days.
The gingival margin trimmers are incomi:)lete in the range
of sizes, in that no 10-6-12 is used. But the formulas 20(95)-9-12
and 15 (95) -8-12 are the same as hatchets and spoons, except for
the addition of the angle of the cutting-edge. (See directions
for measurement of angles of cutting edges on instrument
gauge.) In all the other instruments the cutting-edge is at right
angles to the length of the blade ; but in these a different angle
of the edge with the blade is introduced. They are made for the
one purpose of beveling the gingival cavo-surface angle of
proximal cavities in the bicuspids and molars. For this purpose
the blades of one pair are cut so that the edge is at an angle of
95 centigrades with its shaft, fitting it for the beveling of the
gingival cavo-surface angle of a distal cavity in a bicuspid or
molar, the one cutting to the right and the other to the left. The
other pair of instrmuents have the cutting edges cut to 80 centi-
grades with the shafts, fitting them for the similar beveling of
the gingival cavo-surface angle of a cavity in the mesial surface
of a bicuspid or molar. On account of this peculiarity of form,
two pairs of each size of blade are necessary : one pair for distal
cavities and one pair for mesial cavities.
The pairs of gingival margin trimmers 15(95)-8-12, and
15(80)-8-12, are for use in bicuspid cavities which are often too
narrow mesio-distally to admit the width of the pair 20(95)-9-12,
or 20(80)-9-12. The approach of the 20(80)-9-12 to the gingival
wall of a mesial cavity is shown in Figure 195.
In the binangle chisels we use the 6 centigrades angle only,
as in the range, binangle chisel 20-9-6, 15-8-6 and 10-6-6, or we
name them simply as chisel 20-9-6, etc. The straight chisels we
designate the range as chisel 20, chisel 15 and chisel 10, using
only the width term.