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46 THE TECHNICAL, PBOCEDUBES IN FILLING TEETH.

draw it to tlie surface of the enamel ; engage it again in tlie same
way and repeat the motion. With this movement, nsing a small
bur, a groove is readily cut through the enamel into the center
of the occlusal surface. In this cutting, the line of the mesial
(or distal) groove should be followed, because this is the weakest
part of the enamel. If the bur is too large, it will not cut so
well. It is only by concentrating the force on a small bur that
the enamel can be cut to any advantage, and even then the cut
should be made from within outward. After this first cut has
been made, the groove or channel formed should be broadened
by chipiDing away the enamel with chisels, or the enamel hatchets,
and the inverted cone bur again used to undermine the enamel
upon either side, which is again chipped away. In this way a
seat or occlusal step of any required extent is readily formed,
the pulpal wall of which will have sharp and definite angles with
the surrounding walls.
Grooves that need to be cut out, merely for the purpose
of finding a position for finishing the filling, i. e., when there
is solid dentin beneath, are to be cut in the same manner with
the inverted cone or fissure bur. In none of these cases should
any attempt be made to cut the enamel from without inward
with the bur. Cutting the enamel with the bur dulls the blades
very quickly. After the bur has been used a few times, it should
be discarded and a new one employed. The dull bur may be
sharpened by the instrument maker.
In squaring out angles of cavities, the inverted cone bur
may often be used to advantage in such positions as are readily
accessible, as in the occlusal, labial and buccal cavities. Usually
this is done by flattening the pulpal wall in occlusal cavities,
or the axial wall in buccal or labial cavities. In order to accom-
plish this with the bur, the approach must be such that the
square end of the bur may be placed in the plane of the pulpal
or axial wall to be formed, or, in other words, the axis of the
hand-piece must be at right angles to the pulpal or axial wall
to be formed. Then the side or periphery of the bur is engaged
in the deeper part of the rounded pulpal or axial wall, and made
to cut toward one of the surrounding walls. Tliis is then
repeated in another direction, and the bur is finally carried in
a similar way around the whole circumference of the cavity in
such a way that its square end leaves the pulpal or axial wall
flat, and the line angles with the surrounding walls are made
sharp and definite.
In making extensions for prevention in any of the axial