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SECTION IV.

OF THE MOTION IN THE JOINT OF THE LOWER JAW.'

The lower jaw from the manner of its articulation, is sus-
ceptible of a great many motions. The whole jaw may be
brought horizontally forwards by the condyles sliding from
the cavity towards the eminences on each side. This mo-
tion is performed chiefly when the teeth of the lower jaw
are brought directly under those of the upper, in order
to bite, or hold any thing very fast between them. Or,
the condyles only may be brought forwards, while the
rest of the jaw is tilted backwards, as in the case when the
mouth is open ; for on that occasion the angle of the jaw is
tilted backwards, and the chin moves downwards and a little
backwards also. In this last motion, the condyle turns its
face a little forwards ; and the centre of motion lies a little
below the condyle, in the line between it and the lower
angle of the jaw. By such an advancement of the condyles
forwards, together with the rotation mentioned, the aperture
of the mouth may be considerably enlarged ; a circumstance
necessary on many obvious occasions.
The condyles may also slide alternately backwards and
forwards, from the cavity to the eminence, and vice versa
so that while one condyle advances the other moves back-
wards, turning the body of the jaw from side to side, and thus
grinding, between the teeth, the morsel separated from the
larger mass by the motion first described. In this case the
centre of motion lies exactly in the middle, between the two
condyles. And it is to be observed, that in these slidings of

Hunter, 12—14.