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05 PATHOLOGY OF THE HABD TISSUES OF THE TEETH.
the upper teeth. This will be seen in Figure 101. Beginning
with the third molars, we find that the cusp of the lower third
molar occludes directly under the embrasure between the second
and third molars of the upper jaw and will force food through
that embrasure especially, while the second molar of the upper
jaw has its distal cusp over the embrasure between the second
and third molars of the lower jaw and will force food through
it. This arrangement is repeated among all the molars and
bicuspids. The result is that, in mastication, the food is caused
to run through the embrasures by the crushing action of the
teeth. The effect of this arrangement will perhaps be better
understood by comparison with the occlusal surfaces of the same
teeth as seen in Figures 102, 103, noting carefully the forms of
the individual embrasures and their variations. This abrasive
action by the food is increased by the cheeks and tongue also
pressing upon the food laterally, causing it to be crushed down
over the buccal and lingual surfaces of the teeth, and more par-
ticularly through these embrasures. In fact, this lateral pres-
sure on the food is an endeavor to hold it between the occlusal
surfaces of the teeth. The result is that the food is forced
through the embrasures especially, causing it to rub the angles
of the teeth from their occlusal surfaces to the gum margin.
This abrasive action of the food serves to limit sharply the
distance that attached colonies of microorganisms may spread
toward the buccal or lingual angles of the teeth because of their
removal from the surfaces by the passing of food through these
embrasures during mastication. It also explains another fact
of great importance in the treatment of caries of the teeth by
filling, for it was found by actual count of ten thousand persons
examined that only in about one case per thousand, decay was
found to have spread superficially, or upon the surfaces of the
teeth, across the angles. Therefore, near to, or along these
angles, is the safest place to lay cavity margins.
Caries never regins on the angles of the teeth nor
spreads superficially past these angles in any case in which the
teeth are in normal relations and the person is making active
use of them in mastication ; this may be stated without fear of
successful denial. All of the cases of such spreading that have
come under my observation in twenty-five years of close study
of this point, have been in persons who have practically ceased
to use their teeth in the mastication of food because of some
interference. This has generally been sensitiveness of exposed
pulps in decayed teeth, and at the same time the persons had