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44 THE TECHNICAXi PEOCEDTJEES IN FILLING TEETH.

few for almost all of their work. An orderly arrangement such
as the formula plan gives will enable them to think easily in
several ranges of forms and to adapt a greater variety of forms
to the varying needs. This will contribute both to accuracy and
to speed in operating. This is worth many hundred times what
it costs in effort to accomplish it.

Use of the Dental Engine.
ILLUSTRATION: FIGURE 25.
The effort to use some kind of mechanical device for the
turning of burs and drills in the excavation of cavities and in
finishing fillings was begun many years ago. As early as 1840
several persons had made devices for this purpose, worked by
a lever in the handle operated by a finger or thumb. None of
these became diffused through the profession, but were indi-
vidual instruments, usually rudely made by the operators who
used them. Toward the close of the sixth decade of the last
.century, a small pneumatic engine was attached to a sleeved
mandrel carrying a bur or drill, which was worked by a bellows-
like air pumjD connected with the instrument by a rubber tube.
This pump was i^ut in action by the foot of the operator. It
proved practicable, and was the first device for this purpose
regularly manufactured for the use of dentists. Within ten years
many different forms of dental engines were offered the pro-
fession. These soon became crystallized into a few favorite
forms and their use at the operating chair rapidly became
general.
The engine is used for certain parts of the work of exca-
vating cavities, for trimming fillings to form after they have
been inserted, and for a large amount of the work of polishing.
For these several purposes, engines should be equiijped with
certain sizes and forms of excavating burs, a few sizes and forms
of finishing burs, stones for grinding and sandpaper disks of
different grades of fineness, rubber disks for carrying polishing
powders, etc. In excavating, the bur is nowadays indispensable,
and yet but a small part of the excavating should be done with
burs. The tendency among students, as well as practitioners,
is to continually use the bur too much and to use it in improper
places.
The forms of bur most useful are what are known as the
inverted cone bur and the fissure bur. Round burs are not often
used to advantage, and yet there are certain dofinite purposes
requiring round burs.

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