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OF THE MOTION OF THE JAW, ETC. 109
mentioned action. When the true grinding motion is to be
performed, a greater degree of this last motion takes place ;
that is, the Condyle of the opposite side is brought farther
forwards, and the Condyle of the same side is drawn farther
back into the cavity of the Temporal Bone, and the Jaw is a
little depressed. This is only preparatory for the effect to be
produced ; for the moving back of the first mentioned Condyle
into the socket, is what produces the effect in mastication.
The lateral Teeth in both Jaws are adapted to this oblique
motion ; in the Lower they are turned a little inwards, that
they may act more in the direction of their axis ; and here
the Alveolar Process is strongest upon the outside, being there
supported by the ridge at the root of the Coronoid Process.
In the Upper-Jaw the obliquity of the Teeth is the reverse,
that is, they are turned outwards, for the same reason ; and
.
the longest fang of the Grinders is upon the inside, where the
socket is strengthened by the bony partition between the antrum
and nose. Hence it is, that the Teeth of the Lower-Jaw have
their outer edges worn down first ; and, vice versa, in the
Upper-Jaw.
GENERAL COMPARISONS BETWEEN THE MOTION OF
THE JAW IN YOUNG AND IN OLD PEOPLE.
In children who have not yet Teeth, there does not
seem to be a sliding motion in the Lower-Jaw. The
articular eminence of the Temporal Bone is not yet formed,
and the cavity is not larger than the Condyle; there-
fore the centre of motion in such, must be in the Condyle.
In old people, who have lost their Teeth, the centre of motion
appears to be in the Condyles, and the motion of their Jaw to
be only depression and elevation. They never depress the Jaw-
sufficiently to bring the Condyle forwards on the eminence,
because in them the mouth is sufficiently opened when the Jaw-
is in its natural position.
Hence it is that in old people, the gums of the two Jaws do
not meet in the fore-part of the mouth, and they cannot bite
o