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210 PATHOLOGY OP THE HAED TISSUES OF THE TEETH.
surfaces of these teeth, but the essential characters of the areas
are the same in all, so that they are really much alike. The con-
ditions which give opportunity for these decays are to the gingi-
val of the contact point.
The proximal area of liability to decay is bounded to the
occlusal, or incisal, by the proximal contact point, to the buccal,
or labial, and lingual by the opening of the embrasures to the
excursions of food during mastication, and to the gingival by the
position of the margin of the healthy gum septum.
In a strict account of the examination of ten thousand per-
sons who applied for treatment in the clinic in Northwestern
University Dental School, only nine persons were found in whose
teeth superficial spreading of decay of the enamel had passed
across the angles of the teeth from one surface to another, or
had extended from two surfaces and met across the angles. All
of these were cases of unusual severity of caries that had been
neglected until all effort to chew food had been practically aban-
doned by the patients. This marks the angles of the teeth, or an
approach to them, as the line of safety along which to lay cavity
margins in the preparation of cavities in proximal or buccal
surfaces. It also points out that the angle of the tooth should
not be passed in the extension of cavities unless some special
condition presents, such as the extension of caries in dentin,
requiring it.
The best management of these cavities requires that the
operator make a study of each individual proximal surface
involved, determine the boundaries of its area of liability to
decay, and that in the preparation of the cavity, the whole of the
area of liability be included within the cavity outline, together
with such area of the occlusal surface in bicuspids and molars
as may be necessary to give convenience in operating and stabil-
ity 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 should not be over-
looked. Caries never makes a beginning on a portion of the
tooth surface covered by a healthy gum septum or free gum
margin. All such portions are strictly immune. A margin of a
filling so laid that it will be continuously protected by healthy
gum tissue is as safe as if laid upon a surface fully exposed to
the friction of mastication. Therefore, the preservation of the
health of the septum of gum which normally fills the interproxi-
mal space to the contact point is one of the important factors in