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wards the body, and brings the flexors into action ; where-
as if it be wished to bring the body towards the hand, the
muscles of the fore part of the body would be put into ac-
tion, and this would produce the same effect.
To apply this to the lower jaw ; when we attempt to open
the mouth, while the lower jaw is immoveable, we fix our at-
tention upon the very same muscles (whatever they are)
which we call into action when we depress the lower jaw
and we find that we act with the very same muscles : for our
mind attends to the depressing of the jaw, and not the rais-
ing of the face ; and under such circumstances the mouth is
actually opened. We find then by these means the head is
raised, and the idea that we have of this motion is the same
that we have in the common depression of the jaw ; and we
should not know except from circumstances, that the jaw
was not readily depressed ; and we find at this time too, that
the extensors of the head are not in action : on the con-
trary, when the jaw is fixed in the same situation, if we have
a mind to raise the head, or upper jaw, which of course
must open the mouth ; we fix our attention to the muscles
that move the head backwards, without having the idea of
opening our mouth ; and at this time the extensors of the
head act. This plainly shows that the same muscles which
depress the jaw, when moveable, must raise the head when
the jaw is kept fixed. This is a proof, too, that there are no
other muscles employed in depressing the lower jaw, than
will raise the head under the circumstances mentioned.
This will further appear from the structure of the parts,
wherein four things are to be considered, viz. the articulation
of the jaw ; the articulation of the head with the neck ; the
origin, and the insertion of the digastric muscle.
Suppose the upper jaw to be fixed, and the lower jaw to
be movable on the condyle : if the digastric contracts, its
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