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108 THE TECHNICAL PROCEDURES IN FILLING TEETH.

of persons are tabulated. The angles are almost always immune
to decay, but a few cases will be found in which decay has crossed
these angles, or in which decays from the proximal and buccal
surfaces have joined. Cases will rarely be found in which decay
has surrounded the crown of the tooth completely. Therefore
decays beginning in the smooth enamel have a constant tendency
to spread on the surface in a direction aromid the crown of the
tooth, but very seldom cross an angle.
This is illustrated in diagram Figure 116. In this figure
the lowest line, the dotted line, represents the gingival line at
which the gimis are attached to the tooth. The next dark line,
which is continuous from tooth to tooth, represents the position
of the free margin of the gum, and the width of the space between
this and the dotted line represents the depth of the free portion
of the gum, the gingivae, and the distance to which this overlaps
the enamel. The double line which cuts through the darkened
areas and rises toward the occlusal as it passes to the proximal
surfaces, represents a saw-cut along the center of the most sus-
ceptible area to the beginning of decay as it passes around the
crowns of the teeth. Figure 117 represents the same teeth after
the occlusal portions of the crowns have been removed. The line
of greatest susceptibility will be seen to be near, but not quite
touching the free border of the gum in every part, following the
arching of the gum septum between the teeth. Figure 118 shows
the portion of this line that is most highly susceptible on the
proximal portion, while Figure 119 shows the same on the buccal
surface. If a similar line were drawn about the crowns of the
incisors and cuspids, it would similarly represent the areas of
greatest susce])tibility as they exist on the smooth surfaces of
these teeth. Of this line the central part of the proximal portion
is the most susceptible to the beginning of caries. The tendency
is to spread buccally and lingually. In persons who are making
the normal use of their teeth in chewing food, this seldom, if ever,
s})reads across the angles of the tooth to the buccal or lingual
surfaces. The extent of spreading to the buccal and lingual is
very variable, and is controlled in large degree by the shape of
the ])roximal surfaces. When the teeth are well rounded, the
points of proximal contact narrow, the emlirasures to th(( Iniccal
and to tlie lingual deep, food runs through Ihom well in chewing,
and keeps the surfaces of the teetli forming these embrasures,
i. e., the angles of the teeth, well cleaned. In this case the area
of decay will be narrow bucco-lingually. In the reverse condi-
tions, in which the ))roximal contacts are broad, the surfaces of