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38 THE TECHNICAX, PEOCEDXJEES IN FILLING TEETH.

himself, or he may have the instnmient maker do it for him. The
very small worn burs had as well be thrown away. All of the
larger ones can l)e made as good as new several times by grind-
ing. The finishing files may be reground many times and still
do good work. There is perhaps no instrument more seriously
injured by becoming dulled than the finishing files. Therefore,
frequent shari^ening is very essential to their usefulness. Any
small bur that has been once used on enamel, for instance, in
cutting a slot into the occlusal surface of a molar from a prox-
imal cavity for the formation of a step, should at once be dis-
carded. The edges will have become so injured that it will be
unfit for further use. Such a bur may be used many times for
a similar amount of cutting in dentin without serious injury.

Studies of Insteument Fokms and Theie Foemxjlas, with Beiep
Outline of Theie Uses.

In this, only the University set of instruments will
Note :
be used, but, by changing the formulas to agree with another set
chosen, the i^lan here given may be used in a similar way.

oedinaeies.
The set of ordinaries has been so called because it includes
the hatchet and hoe forms that have been most used by dentists
in the past. Instruments similar to them, but less perfectly
assorted, are found in every dentist's operating case. The
smaller sizes of this set are used but little except in the prepara-
tion of cavities in the incisor teeth, and mostly in the final shap-
ing of the walls and angles of these cavities after they have been
opened by other instruments.
The hoe 12-5-6 used as a chisel is the most useful instrument
in opening cavities in the incisors, especiallj^ for cutting away the
labial enamel margins until the j^roper form has been reached.
The hatchet 12-5-6 joerforms the same office for the gingival wall.
Very often, in the more delicate cutting, the middle sizes, i. e.,
hoe 8-3-6 and hatchet 8-3-6, will serve better. In many cases it
will be found more convenient to use the same sizes in the angle
of 12 centigrades, or the hoe 12-5-12 and hatchet 12-5-12, etc., or
the sizes 8-3. The smallest size, or 6-2-6, -12 and -23, are used
only in shaping the internal parts of cavities in the incisors and
cuspids, and especially in squaring out the axial line angles of
these cavities so that they are sharp and definite. Those with
the angle of 23 centigrades are convenient for reaching certain