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DIUASTRICUS. 33
same Muscles to the hand. In this case, however, the mind
wills the motion of the hand towards the body, and brings
the Flexors into action ; whereas if it wished to bring the
body towards the hand, the Muscles of the forepart of the
body would be put into action, 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
attention 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 raising 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 really depressed ; and we
find at this time too, that the Extensors of the head are not in
action. On the contrary, 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 em-
ployed in depressing the Lower-Jaw, than what 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 Digastric Muscle.
Suppose A, the Upper-Jaw, to be fixed, and the Lower-
Jaw B, to be moveable on the Condyle C : if the Digastric
contracts, its origin E, and insertion F will approach towards