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SECTION X.
OF THE ACTION OF THE TEETH, ARISING FROM THE MOTION
OF THE LOWER JAW.*

" The lower jaw may be said to be the only one that has
an * motion in mastication ; for the upper jaw can only move
with the other parts of the head. That the upper jaw and
head should be raised in the common act of opening the
mouth, or chewing, would seem, at first sight, improbable ;
and from an attentive view of the mechanism of the joints,
and muscles of those parts, from experiment and observa-
tion, we find that they do not sensibly move. We shall
only mention one experiment in proof of this, which seems
conclusive. Let a man place himself near some fixed point,
and look over it to another distant and immovable object,
when he is eating. If his head should rise in the least de-
gree, he would see more of the distant object over the near-
est fixed point, which in fact he does not. The nearer the
fixed point is, and the more distant the object, the experi-
ment will be more accurate and convincing. The result of
the experiment will be the same, if the nearest point has the
same motion with the head ; as when he looks from under
the edge of a hat, or any thing else put upon his head, at
some distant fixed object. We may conclude then that the
motion is entirely in the lower jaw ; and as we have already
described both the articulation and the motion of the bone,
we shall now explain the action of mastication, and at the
same time consider the use of each class of teeth.
With regard to the action of the teeth of both jaws, in
* Natural History of the Human Teeth, by John Hunter, London 1803.
Pages 68 to 71.