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86 PATHOLOGY OF THE HARD TISSUES OF THE TEETH.
nings of dental caries in certain positions in the enamel to the
exclusion of all others. These are physical conditions controlling
lodgments of debris. They are also physical conditions prevent-
ing the removal of anything which may become attached to the
enamel in these positions.
If we study the relations of the proximal surfaces of the
different teeth to each other, we find that various forms of con-
tact and near approach of these surfaces serve to .shield certain
areas from all friction from mastication and in which the deposit
of a very little gummy material will shield colonies of micro-
organisms from all washings by saliva. This may be studied
in the illustrations. Figures 99, 100, show the buccal and the
occlusal surfaces of the upper bicuspids and molars as they
stand in the arch. The teeth are in the same relation to each
other in each figure. The soft tissue filling the interproximal
spaces has been removed in order that the forms of these may
be better seen. In studying the buccal view, it will be seen that
the points of near approach of surfaces are very narrow and
rounded in form from occlusal to gingival, so that the actual
touch point of unworn teeth is very small, like that of two mar-
bles coming in contact, while, in the view of the occlusal surfaces,
the points of near approach of the teeth to each other are shown
to be very much broader in the bucco-lingual direction.
The decays in the proximal surfaces of the bicuspids and
molars, in Figures 86-91, inclusive, correspond in form with the
areas of near approach of the surfaces of the teeth so closely
and so constantly that we must connect these relations as cause
and effect, controlling both the localization and superficial form
of the beginning and spreading of caries in the enamel. To gain
correct expressions of this, the study must be confined to those
beginnings of decay that have not yet penetrated the enamel,
or in which the enamel rods have not yet fallen out, for, with the
breaking away of the enamel after decay of dentin has begun,
the particular form of the beginning upon the surface of the
enamel is often quickly lost.
A study of the forms of the surfaces making the contact
of different teeth as seen in the occlusal view, will show great
variations in the bucco-lingual length of near approach of these
surfaces. The area of near approach of surfaces is much longer
bucco-lingually between the second bicuspid and the first molar
than that between the two bicuspids. Therefore, a beginning
decay between the second bicuspid and first molar will spread
and become longer bucco-lingually in these than a beginning