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70 THE TECHNICAL PROCEDURES IN FILLING TEETH.

Labial cavities in the incisors and cuspids.— These
cavities occur in surfaces of the enamel that are smooth and per-
fect. There is no pit, groove or fissure which serves as a start-
ing point for the carious process. In this they are totally
different in their conditions and surroundings from the cavities
which begin in structural imperfections of the surfaces of the
teeth, as pits and fissures. In the beginning of pit and fissure
cavities, lodgments of d6bris and fermentable material is con-
fined to the pit itself ; the surface of the tooth immediately sur-
rounding the pit or fissure is so situated in relation to the uses of
the teeth that it is constantly kept clean by the friction of masti-
cation. Decay never begins upon clean surfaces. Therefore,
there is no disposition for decay to spread upon the surface of
the enamel, or to make a new beginning in the immediate
neighborhood of it spreads beneath the
the pit or fissure
;
enamel only.
In all smooth-surface cavities the case is reversed. There is
no pit or fissure that holds debris, but the position and relation
of the surrounding parts of the surface to the first starti-ng point
of decay are such that it is not kept well cleaned by the fric-
tion of mastication and the motions of the tongue, lips and
cheeks. Therefore, it is much of the time in an unclean condi-
tion. This area of uncleanness will vary indefinitely, so that the
cases met with in practice vary indefinitely. The first beginning
of decay in these surfaces is central to the area of habitual
uncleanness, or better, central to the greatest constancy of the
uncleanness. As this is close to the gum margin, the first begin-
ning of cavities in labial surfaces is usually close to the gum
margin. But as the area of uncleanness is spread upon the sur-
face, so has the beginning of decay a tendency to spread upon
the surface. For this reason we find in labial and buccal cavities
broad, shallow injuries to the enamel, very frequently, and many
instances, when seen early, in which the enamel is broken
through at a number of points near together, or spread in a line
near the gum margin. An examination of these cases show
clearly the tendency for the decay to begin in the enamel in a
constantly widening area. Therefore, in preparing this class of
cavities for filling, the principle is established of so extending the
cavity outline as to include the area of the surface, which, under
the conditions presented, is especially liable to decay in the