Page 341 - My FlipBook

THE KEY. 337

angles with its vertical axis, but curves down to the
point, almost or quite as low as the base of the bolster.
When properly constructed, the hook embraces the
tooth at the neck on one side, and the bolster rests a

little below this on the other. When the instrument
is applied to a tooth, the center of the shaft is the
axis of motion ; but, as force is applied to the instru-
ment, this axis is transferred from the shaft to the
base of the bolster, which is the center of motion the
moment it is fixed on the gum and alveolus, and the
shaft describes an arc about it. Now, as a result of
this motion and arrangement, the line of force is at
an angle of from forty to sixty degrees with the axis

of the tooth ; and hence it is at this angle that the
tooth must be extracted, if at all. The axis of power
exerted on the tooth by the instrument, is in a line
from the point of the hook to its attachment to the
shaft; and the line of this force has its termination
below the neck of the tooth on one side, and just
above the crown on the opposite side. The angle
formed by the line of power with the axis of the

tooth, is different in the different relative positions of
the key to the tooth. If the instrument is applied to
an inferior molar, with the bolster on the inside, the
angle of the line of force with the axis of the tooth is
about forty degrees; but, if placed on the outside of
the jaw, as recommended by some, the angle con-