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FILLING BY CLASSES AND MODIFICATIONS. 185

five —Transverse extension of the decay round one
:
or more angles of the tooth, under the termination of

the enamel.

FILLING BY CLASSES AND MODIFICATIONS.

First Class.—Central cavities of the molars and
bicuspids. These decays always begin in the depres-
sions on the masticatory surfaces, which are vulner-

able points, the enamel-membrane folding together
here, and often being imperfectly united, so that an
opening is left through it to the dentine: besides,
these indentations afford lodgment for foreign sub-
stances, which may be forced into them in the process
of mastication, and there retained, till, becoming
vitiated, they produce decay. Examine carefully the
extent and the nature of the decay, and the form of

the cavity, which, of course greatly varies. In some
cases, the cavity is found with a small diameter and

a comparatively great depth, the diameter at the
orifice being the same as within; in others, with a di-
ameter larger at the orifice—as occurs in cases in which
a considerable portion of the enamel at the depres-
sions on the crown is imperfect. In the majority of
instances, however, the diameter of the cavity is
much larger within than at the orifice. Sometimes
the decay is found to burrow directly beneath the