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OF THE MOTION IN THE JOINT OF THE LOWER-JAW. I •">
and backwards, the moveable cartilages do not accompany the
Condyles in the whole extent of their motion ; but only so far
as to adapt their surfaces to the different inequalities of the
Temporal Bone : for as these cartilages are hollow on their lower
surfaces where they receive the Condyle, and on their opposite
upper surfaces are convex where they lie in the cavity ; but for-
wards, at the root of the eminence, that upper surface is a little
hollowed ; if they accompanied the Condyles through the whole
extent of their motion, the eminences would be applied to the
eminences, the cavities would not be filled up, and the whole
articulation would be rendered very insecure.
This account of the motion of the Lower-Jaw, and its car-
tilages, clearly demonstrates the principal use of these cartilages
namely, the security of the articulation ; the surfaces of the
cartilage accommodating themselves to the different inequalities,
in the various and free motions of this joint. This cartilage is
also very serviceable for preventing the parts from being hurt
by the friction ; a circumstance necessary to be guarded against,
thread, or rather part of the thread, of a conical screw passing over an
axis lying at or about right angles to the plane of motion in simple
opening and closing of the jaws. This spiral course of the articular
surface is perhaps best seen in some of the large Carnivora, such as the
lion, but is also obvious in a well-developed human condyle.
" The action of this conical screw or tap within the glenoid cavity, con-
sidered as the conical die, takes place with accuracy only when one
joint alone acts with the condyle within the glenoid cavity—the other
condyle being beyond it, and gliding on the surface of the zygoma, as
during mastication. The food is in this process crushed between the
molar teeth of that side whose condyle remains within the glenoid
cavity, this condyle screwing the jaw back, so to speak, to its natural
position at each closure of the teeth.
"By this construction a great amount of friction is avoided; what
would otherwise be a rubbing being thus converted into a rolling motion
between the condyloid and glenoid surfaces ; while, by one or other con-
dyle always remaining in the glenoid cavity during mastication, greater
steadiness and security is afforded to the joint." (1)
It need scarcely be remarked here, that however the above explanation
(1) Edinb. riiilosopli. Journal, N.S., vol. vi
OF THE MOTION IN THE JOINT OF THE LOWER-JAW. I •">
and backwards, the moveable cartilages do not accompany the
Condyles in the whole extent of their motion ; but only so far
as to adapt their surfaces to the different inequalities of the
Temporal Bone : for as these cartilages are hollow on their lower
surfaces where they receive the Condyle, and on their opposite
upper surfaces are convex where they lie in the cavity ; but for-
wards, at the root of the eminence, that upper surface is a little
hollowed ; if they accompanied the Condyles through the whole
extent of their motion, the eminences would be applied to the
eminences, the cavities would not be filled up, and the whole
articulation would be rendered very insecure.
This account of the motion of the Lower-Jaw, and its car-
tilages, clearly demonstrates the principal use of these cartilages
namely, the security of the articulation ; the surfaces of the
cartilage accommodating themselves to the different inequalities,
in the various and free motions of this joint. This cartilage is
also very serviceable for preventing the parts from being hurt
by the friction ; a circumstance necessary to be guarded against,
thread, or rather part of the thread, of a conical screw passing over an
axis lying at or about right angles to the plane of motion in simple
opening and closing of the jaws. This spiral course of the articular
surface is perhaps best seen in some of the large Carnivora, such as the
lion, but is also obvious in a well-developed human condyle.
" The action of this conical screw or tap within the glenoid cavity, con-
sidered as the conical die, takes place with accuracy only when one
joint alone acts with the condyle within the glenoid cavity—the other
condyle being beyond it, and gliding on the surface of the zygoma, as
during mastication. The food is in this process crushed between the
molar teeth of that side whose condyle remains within the glenoid
cavity, this condyle screwing the jaw back, so to speak, to its natural
position at each closure of the teeth.
"By this construction a great amount of friction is avoided; what
would otherwise be a rubbing being thus converted into a rolling motion
between the condyloid and glenoid surfaces ; while, by one or other con-
dyle always remaining in the glenoid cavity during mastication, greater
steadiness and security is afforded to the joint." (1)
It need scarcely be remarked here, that however the above explanation
(1) Edinb. riiilosopli. Journal, N.S., vol. vi
