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196 lUSTUKY OF DEXTAL SUKGERY

being proljal)]y greater than tliat registered by the giiathodynamometer. The
range of power in the jaw muscles of diflerent persons ]>i-oved to be very great,
the table given showing from thirty pounds to one hundred and forty pounds
upon tlie incisor teeth, and from seventy pounds to two hundred and forty
pounds upon the molars. To determine tlie stress necessary in the mastication
of food, esiiecially of meat, Dr. Black devised an instrument which he called
the phagodynamometer, consisting of molar teeth of brass^ cast in molds
made from upj)er and lower natural teeth. The ones rejirescnting the upper
teeth were attached to a slide which acted upon a stiff spring, the movement of
which indicated, by means of a finger rotating on a dial, the number of pounds
pressure exerted vipon the teeth. The opf)osing brass teeth were attached to
a similar slide moved by a powerful lever worked liy liand. When meat was
placed between the brass teeth, pressure upon the handle registered upon the
dial tlie force used. The crushing of meat libres as in chewing was readily
recognized, and the force necessary to accomplisli it registered accurately. Of
course this I'oi-ce would vary \\ ith tlie size of the teetli used, the number covered
by tlie meat at one tiriie, and in a marked degree as tlie cusps were prominent
or much worn ort' and the surfaces Hattened by wear. Tests were also made
of various other articles of food, a few vegetables, but esjjecially hard candies,
and four or iive varieties of nuts. Care was taken to liave the extent of liiting
surface to corresjinnd as nearly as migiit be with that used upon the gnatiio-
dynaiiiometcr so that tlie tests by means of the two instruments miglit be
comjiarcd. It is nut claimed that the actual conditions of force and resistance
existing in the mouth are represented in this way with entire accuracy, but they
are sufficiently so to furnish important information relatixc to the strength
and resistance necessary in filling materials and the manner of seating fillings
so that they may not be dislodged bodily by tlie forces likely -to be put upon
tiiem.
The force i-equired to crush meat fibers between two teeth was found to
vary from tliirty to eiglity pounds, and of beef cut from tlie neck of an old
animal and fried well done, from seventy to ninety pounds, and occasionally
as much as one hundred pounds. The tests indicated that in average persons
the strength of the Jaws is abundant, and more, for the chewing of ordinary
meat. Tests were also made of the force required to lireak a freshly extracted
tooth in wliich it appeared that hard steel applied directly to the point of a
cusp would check the enamel at from one hundred to one hundred and tiiirty-
five iwunds and the enamel \vould split ofp at from one lunidred and forty to
one Inindrcd and sixty-fi\(' pounds or more, but if a ]iiece of vulcanized niliber