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164 PATHOLOGY OF THE HAED TISSUES OF THE TEETH.
check in the enamel began to show at 125 pounds and the buccal
portion of the enamel split off at 150 pounds.
The disto-lingual cusp of a lower first molar. The
(6.)
mesial portion of the tooth had been destroyed by caries. A
check in the enamel began at 115 pounds, and the cusp crushed
at 150 pounds.
(7.) A sharp disto-buccal cusp of a lower second molar.
A check began at 100 pounds, and a part of the enamel split off
at 140 pounds.
(8.) A lingual cusp of an upper third molar. A check was
seen at 100 pounds, and a part of the enamel chipped away at
140 pounds.
Tests with vulcanized rubber, three thirty-seconds of an
inch thick, between the steel and the cusp of the tooth.
(9.) A fairly sharp buccal cusp of an upper first bicuspid.
No injury at 350 pounds, but the rubber was cut nearly through.
(10.) A sharp mesio-buccal cusp of an upper second molar.
No check appeared up to 325 pounds. At 335 pounds the rubber
suddenly parted, letting the steel come onto the tooth, and the
cusp was split off.
(11.) The cusp of a cuspid. No injury at 350 pounds.
(12.) Edge of a central incisor. No injury up to 240
pounds, at which point the rubber suddenly cut through, allow-
ing the steel to be thrust against the tooth, which was crushed.
These few cases show the strength of the teeth as well as
many additional examples would do. A hard substance, like
tempered steel, bitten upon with full force, is liable to check the
enamel or even split off a portion of a cusp, for the reason that,
with the sharply rounding surface of the cusp the full pressure
is concentrated on a very small area of the enamel. But, when-
ever this is distributed to a larger area of surface, as is done by
a cusp sinking a little into the hard rubber, no harm is done by
even a greater force than is ever used in chewing food. Indeed,
the teeth have sufficient strength above that actually required,
so that, when considerably weakened by caries, they are still
strong enough to withstand the full force of the occlusion on
any substance used as food. These may be injured, however,
by the accident of catching a small pebble or any very hard sub-
stance that might, by accident, be mingled with food, or by biting
upon the harder metals. Also the teeth are sometimes, though
rarely, broken by catching unawares a lead shot or some such
thing in such a way that it wedges between two cusps and splits