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CARIES OF ENAMEL. 87

decay would do between the two bicuspids. Tbe differences in
this respect among the teeth of the same mouth, and especially
among the teeth of different mouths, are very great.
As this study of forms of contact and near approach
accounts for the differences seen in forms of decay in this
respect, as shown in illustrations given of the bicuspids and
molars, Figures 86-91, inclusive, it also accounts for the differ-
ences shown of the breadth of decay in illustrations made from
cross sections of the teeth, Figures 92-95, as compared with
sections made lengthwise of the tooth. A section made length-
wise of a tooth may show a very narrow injury to the enamel,
while, if the section were made crosswise of the tooth, cutting
through the length of the injury, it would show a very broad
beginning of decay. This will be seen more clearly by a com-
parative examination of sections cut in these different directions
which show the great difference in the form and extent of the
injury to the enamel. This will be appreciated hy comparing
the cross sections, Figures 92-95, with the photographs of split
teeth, Figures 107, 108, 112, 113, 117.
The embrasures, or the openings formed on the buccal and
on the lingual by the rounding of the surfaces of the teeth away
from each other, vary greatly in depth among the different
embrasures of the same mouth, and particularly they vary in
depth among the teeth of different persons, owing to the forms
of the teeth and the form and prominence of the proximal con-
tact points. It must be remembered that in normal conditions
in young people, the interproximal space is filled with gum
tissue to, or very nearly to, the contact point, and that the gum
tissue arches up to this from the buccal and the lingual. In
studying this, it will be found that the beginning of decay is
close to the line of the margin of this arch of gum tissue, and it
does not spread to the gingival unless the gum tissue has been
pushed away by lodgments of food between the teeth. There-
fore, unless lodgments of food have occurred, the forms of
beginning decay will retain that narrowness from occlusal to
gingival that has been shown, and the spreading bucco-lingually
will generally coincide with the length of the close approach of
the surfaces to each other.
If we study the teeth in their occlusion under normal con-
ditions, we will find a provision of very considerable importance
in the relation of the cusps of the upper teeth to the embrasures
between the lower teeth, and, vice versa, the arrangement of
the cusps of the lower teeth with respect to the embrasures in