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HISTOKY OF DENTAL SURGERY 197
were intcrpos^ed, wliicli was slightly (or more) indented by the teeth no injury
was done by a tone of from two hundred and tifty to three hundred and fifty
pounds, "showing jilainly tliat the teeth may be injured by biting upon
the hard metals. ])elibles, or other hard substanres witii the full force of the
muscles, or even with much less than the full force in many cases ; but upon
any substance tliat will be even slightly indented by tlie cusps of the teeth, as
hard rul)l)er, and in this way increase somewhat the area of the enamel upon
which tiie stress is supported, no exertion of the muscles can injure the
tooth."
The iletermination of the amount and direction of the force used in the
sidewise or grinding motions of the jaws j)roved a more difficult problem and
has not yet been satisfactorily worked out, but enough has been learned to
sliow that the grinding motions used in the trituration of starchy foods is
more likely to split a tooth or to break away a cusp from tlie margin of a
filling, than the direct closure of the jaws used in chewing meat.
In his third paper, on filling materials, Dr. Black applies the foregoing
information to the questions relating to the strength necessary in tilling
materials. He says
"If fillings are expected to endure, the material should have sufficient
strength to bear without injury the greatest stress the hunum jaws are able
to bring upon it, and continue to do this year after year. * * *
"The stress in tlie ordinary use of the teeth has been shown to be from
sixty to eigiity pounds upon the area of two molars of medium size. This, if
evenly distributed, would give from seven and a half to ten pounds on a
tilling occupying one-fourth the area of one of tliese teeth. This would be
a filling of ordinary size, Init it frequently liappens that a filling must bear
all of this stress, and occasionally such fillings must bear all of the stress that
the person is capable of exerting. * * * This may be anywhere from
one to two hundred pounds, or even a greater stress in some cases. Oold and
amalgam are the only filling materials in general use that are supposed to be
able to endure this stress continuously."
The third, fourth and part of the fifth in this series of papers describe
Dr. Black's expei-imental work with amalgams, some account of which has
been given in the chapter on amalgam.
The tests of gold showed that pure gold, cut from a cast ingot into pieces
about one-tenth of an inch in diameter and of similar length (either square or
cylindrical) would shorten under a stress of two hundred pounds from about
two to five per cent., varying a good deal in different pieces apparently simi-
HISTOKY OF DENTAL SURGERY 197
were intcrpos^ed, wliicli was slightly (or more) indented by the teeth no injury
was done by a tone of from two hundred and tifty to three hundred and fifty
pounds, "showing jilainly tliat the teeth may be injured by biting upon
the hard metals. ])elibles, or other hard substanres witii the full force of the
muscles, or even with much less than the full force in many cases ; but upon
any substance tliat will be even slightly indented by tlie cusps of the teeth, as
hard rul)l)er, and in this way increase somewhat the area of the enamel upon
which tiie stress is supported, no exertion of the muscles can injure the
tooth."
The iletermination of the amount and direction of the force used in the
sidewise or grinding motions of the jaws j)roved a more difficult problem and
has not yet been satisfactorily worked out, but enough has been learned to
sliow that the grinding motions used in the trituration of starchy foods is
more likely to split a tooth or to break away a cusp from tlie margin of a
filling, than the direct closure of the jaws used in chewing meat.
In his third paper, on filling materials, Dr. Black applies the foregoing
information to the questions relating to the strength necessary in tilling
materials. He says
"If fillings are expected to endure, the material should have sufficient
strength to bear without injury the greatest stress the hunum jaws are able
to bring upon it, and continue to do this year after year. * * *
"The stress in tlie ordinary use of the teeth has been shown to be from
sixty to eigiity pounds upon the area of two molars of medium size. This, if
evenly distributed, would give from seven and a half to ten pounds on a
tilling occupying one-fourth the area of one of tliese teeth. This would be
a filling of ordinary size, Init it frequently liappens that a filling must bear
all of this stress, and occasionally such fillings must bear all of the stress that
the person is capable of exerting. * * * This may be anywhere from
one to two hundred pounds, or even a greater stress in some cases. Oold and
amalgam are the only filling materials in general use that are supposed to be
able to endure this stress continuously."
The third, fourth and part of the fifth in this series of papers describe
Dr. Black's expei-imental work with amalgams, some account of which has
been given in the chapter on amalgam.
The tests of gold showed that pure gold, cut from a cast ingot into pieces
about one-tenth of an inch in diameter and of similar length (either square or
cylindrical) would shorten under a stress of two hundred pounds from about
two to five per cent., varying a good deal in different pieces apparently simi-
