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PIT AND PISSUKE CAVITIES 49

eoiniiioii practice of using dentate fissure burs for this work is con-
sidered as brutal to the patient and is a thief of the operator's
time. A No. % or 1 round bur is noAv used in the engine and ap-
plied to the dentine. By swaying the hand piece to and fro the
dentine is cut away from beneath the enamel walls. The bur should
be frequently removed to allow of cooling as heat readily develops
and is a great and frequent source of pain to the patient.
.The Use of the Chisel is next advised for the removal of the
overhanging enamel wall; first, because this is the easiest and
speediest means of its accomplishment, and second, because this is
the only means of securing the cleavage of the enamel, giving the
operator the opportunity to judge the amount of resistance to
stress in the several localities, and to learn of the direction of the
enamel rods. Many times a chisel-edged hatchet will be most ad-
vantageous, one Avhich has a chisel edge upon the sides of the
blade as well as the cutting edge. The size should be governed by
the size of the opening secured, but in every case as large an in-
strument as the orifice will admit should be used. This process
should be repeated with bur for cutting dentine and chisel or
hatchet for cleaving enamel until the desired cavity outline is ob-
tained.
Resistance Form. The operator should include all fissure and
sulcate grooves. Cross all grooves and ridges at as near a right
angle as possible. Avoid eminences of primary calcification. Lay
the outline as much as possible along the sloping sides of the tri-
angles and ridges, as these are the most favored localities for a
cavity margin, for on these sloping surfaces we find the greatest
amount of friction during the process of mastication, due to the
excursions of food, and they are the least exposed to direct stress,
as the blows are of glancing nature.
Retention Form. Here is a good rule to follow in cavities of
Class One. When the depth of the cavity is equal to or greater
than the width, parallel walls are suflficient. But Avhen the width
exceeds the depth the external walls should meet the internal wall
at a slightly acute angle. These angles are best made acute by the
use of a chisel-edged hatchet or hoe, having corners that are slight-
ly acute. "With a planing motion they should be made to travel
parallel with the base line angles. This will, at the same time,
flatten the seat or pulpal wall. The extreme ends of long arms in
a filling, such as results from following a slender fissure, must be
made retentive.