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30 MECHANICAL DENTISTRY AND METALLURGY.

in its motion is removed, and the jaw swings beyond its
normal position a distance which varies much in different
cases, and usually increases with the time the patient has
been without teeth. The motion of the chin is forward
and upward, as illustrated by dotted lines in Fig. ii. The
condyloid process of the lower jaw hinges in to the glenoid
cavity of the temporal bone at A. The line B is drawn
from the joint to the chin, and the dotted line C is the
curve on which the lower jaw swings when in motion.
When the teeth have been lost the lower jaw is free to
move farther, and the line B swings forward and upward
to the dotted line E, carrying the chin upward and forward
tow^ard the nose, on the dotted line C, raising it a distance
represented by the horizontal parallel lines F, and forward
the distance rej)resented by the parallel lines G. The lips
have been held forward in their normal pose by the teeth.
With the loss of the teeth they lose this support and drop
back to the dotted lines /, I, dragging with them the sur-
rounding soft parts and not infrequently pull down the
point of the nose. This occurs most frequently in persons
who have been without teeth for some years. With the
loss of teeth the tongue undergoes a change of form.
Being purely muscular, and capable of assuming a great
variety of forms, and having been confined upon either
side by the teeth, it spreads laterally, as if in endeavor
to fill the space. It soon becomes much broader than be-
fore, so that more of its bulk comes forward into the upper
part of the oral cavity; and the floor of the mouth and
submental tissues are raised at the point H.
This, with the dropping back of the lower lips, gives
the point of the chin a peculiarly sharp appearance. The
changes in appearance which the face undergoes is, in
some degree, represented in Fig. 12. These changes are,
however, as variable as persons, and we refer to the manner