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14 OF THE MOTION IN THE JOINT OF THE LOWER-JAW.

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 center of motion lies a little below
the Condyle, in the line between it and the 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 for-
so that
wards, from the cavity to the eminence, and vice versa ;
while one Condyle advances, the other moves backwards, turning
the body of the Jaw from side to side, and thus grinding, be-
tween the Teeth, the morsel separated from the larger mass by
the motion first described, (x) In this case, the center of motion
lies exactly in the middle between the two Condyles. And it is
to be observed, that in these slidings o* the Condyles forwards

exact conformity with the structure of their teeth and the disposition of
the muscles of mastication. In the Ruminantia, again, we find both
glenoid surface and condyle almost flat : the former is bounded behind
by a transverse crest which passes inwards from the zygoma, but has no
corresponding anterior limitation. Such a joint is admirably adapted
for an extensive lateral movement of the jaw in chewing the cud,
whilst the power of gaping
is proportionately limited. (1)]
(.r) [In a paper read before the Royal Society of Edinburgh, Dr John
Smith has advanced the following theory of the mechanism of the
Temporo-Maxillary Joint. He considers that in the movement of
simply opening and shutting the mouth, "the condyles cannot act as a
simple hinge, as they lie, not at right angles to the plane of motion of
the lower jaw, but obliquely to it, each condyle looking inwards and
forwards. Their more perfect action, therefore, cannot occur in this
movement, but seems to belong to that of mastication. The articulating
surface, strictly speaking, on each condyle appears to constitute the
(1) Vide Cuvier, Le?ons d'Anatomie Comparee, Tome iv., P. 1, p. 36,
edit. 1835 ; Todd's Cyclopaedia of Anatomy, Article Tem^ioro-Maxillary
Articulation.