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134 MECHANICAL DENTISTRY AND METALLURGY.

the basis of the puhey, the wheel, and axle. That of the
inclined plane is the basis of the wedge and screw.
Elasticity, as shown in India-rubber and the spring of
metals, although not classified with the primary forces in
mechanics, plays an important part in the application of
force in regulating teeth. When these laws and their
apphcations are firmly fixed in the mind of the operator,
he can readily take advantage of the one which should
properly be applied, or, when necessary to apply more than
one, can combine them in such a manner as will best ac-
complish the desired result.
Materials Employed.—The materials employed in regu-
lating are platinum, platinized gold, iridio-platinum, gold,
German silver, steel, vulcanized rubber, soft rubber bands,
compressed wood, sea-tangle, and silk, linen, or cotton
ligatures.
The most frequent forms of irregularity are protrusion
of the cuspid teeth, misplaced bicuspids, contraction of arch,
protrusion of the upper jaw, protrusion of the lower jaw,
torsion, and lack of anterior occlusion.
Protrusion of the Cuspid Teeth.—In correcting this
form of irregularity, which is possibly the most frequent
met with, we have to decide from other existing circum-
stances whether the enlargement of the arch is indicated
or the extraction of a tooth posterior to the cuspid. Fre-
quently the former is called for, but there are cases coming
to our hands where the latter, or the simple extraction of a
tooth on one side of the mouth, or both is all that is re-
quired. If the upper arch is large enough, the extraction
of the first or second bicuspid teeth will allow the cuspid
teeth in a young person to move down and back into place
unaided. Where it is desired to hurry the operation, or
where the bone is too hard to permit nature to move the
tooth sufficiently, appliances as are illustrated in Fig", go