Page 293 - My FlipBook

THE FORCE USED IN MASTICATION. 163
to my attention in which I was satisfied that the full capability
of the muscles had been registered. The most notable of these
was a young man, who, for some years, had made a business of
crushing glass and other hard, brittle substances with his teeth,
as a show, for a living. He was a slender fellow with no con-
siderable muscular strength. He could register 215 pounds and
would stop at that every day very precisely in each trial for a
week. He claimed stoutly that he used all the muscular power
he was capable of without the slightest pain about the teeth. I
am inclined to believe his statement. He was much chagrined
to find others registering more pounds than he, "a professional
hard biter," could do.
The strength of the teeth is ample for all the stress that
is brought against them, provided very hard substances are
excluded, such as the harder metals, pebbles and similar things.
This is made clear by trials of the strength of freshly extracted
teeth. These were cut squarely off at the junction of the middle
and gingival third of the crown, so that the occlusal portion
would stand solidly on a flat piece of steel, and arranged in
a registering dynamometer. Figures 164, 165. Then a steel
point with a squared end was applied directly to the cusp and
pressure slowly turned on. Another test was made in the same
way, except that a slip of hard vulcanized rubber three thirty-
seconds of an inch thick was interposed between the steel point
and the cusp of the tooth.

Tests with hard steel directly applied.
(1.) Mesio-buccal cusp of an upper second molar ; a fairly
sharp cusp. A portion of the enamel of the buccal portion of
the cusp split away at 125 pounds.
(2.) A rather obtuse lingual cusp of the same tooth. The
enamel checked over a small area, and the tooth not otherwise
broken with a stress of 350 pounds.
(3.) Disto-buccal cusp of an upper second molar, rather
sharp. The enamel began to check at 135 pounds, and a part of
the cusp split off at 165 pounds.
(4.) A well-rounded lingual cusp of an upper first bicuspid.
A check in the enamel began to show at 100 pounds, but no
further break occurred with 350 pounds. On removing the tooth,
the enamel was found disintegrated over a small area where
the steel was applied.
(5.) A sharp buccal cusp of an upper first bicuspid. A