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incisors and cuspids, because of the differences in the forms
of proximate surfaces of these teeth, Init the essential char-
acter of the areas are the same in all, so that the}^ are really
much alike. The area of liability to decay upon the prox-
imate surfaces of the bicuspids and molars is to the gingival
of the contact point. Its occlusal border line passes through,
or just gingival of, the contact point horizontally and reaches
slightly out into the buccal and lingual embrasures. It then
rounds to the gingival. The buccal border line passes diag-
onally to the buccal and gingival to the free border of the
gum, and the lingual border line passes diagonally to the
lingual and gingival to the free border of the gum. The
gingival border line follows the free border of the gum from
buccal to lingual, inclosing the area. This area inclosed by
these lines is the proximate area of liability to decay. The
area of tooth surface outside of these boundary lines is im-
mune to decay in all persons who are making free use of
their teeth in mastication. We rarely see decay beginning
upon the mesio-buccal or disto-buccal angles of the teeth,
and it almost never occurs upon the corresponding lingual
angles. The reason is that in the mastication of food it is
forced through the embrasures in such a manner as to sweep
these angles clean. It is for this reason that they are im-
mune.
Stated in shorter terms: TJie proxiuiatc area of liability
to decay is bounded to the occlusal by the proximate contact
point, to the buccal and lingual by the opening of the em-
brasures to the excursions of food during mastication, and to
the gingival by the position of the margin of the healthy gum
septum.
This requires that the operator make a study of each
individual proximate surface involved in decay, determine
the boundaries of its area of liability to decay, and that in
the preparation of the cavity the whole of the area of lia-
bility be included within the cavity outline, together with
such area of ttie occlusal surface in bicuspids and molars
as may be necessary to give convenience in operating and
stability to the filling. If present decay only be removed,
the cavity lines not being extended as described, decay will
recur about the margins of the filling, causing failure. The
provisions for maintaining the health of the gum septum

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