Page 39 - My FlipBook

37

two bellies may occasion a mistake,) we find these muscles
quite flaccid ; which is not the case in deglutition, nor in
speaking, in which they certainly do act ; nor do we find the
muscles under the os-hyoides at all affected, as they are in
the motion of the larynx.
It has been observed, that when we open the mouth,
while we keep the lower jaw fixed, the fore part of the head
or face is necessarily raised. Authors have been at a good
deal of pains to explain this. Some of them considered the
condyles of the jaw, as the centre of motion ; but if this
were the case, that part of the head, where it articulates with
the spine, and, of consequence, the whole body must be de-
pressed in proportion as the upper jaw is raised ; which is
not true in fact. Others have considered the condyles of
the occiput as the centre of motion ; and they have conceiv-
ed the extensor muscles of the head to be the moving pow-
ers. The muscles which move the head in this case, are
pointed out by two circumstances, which attend all muscular
motion : in the first place, all actions of our body have mus-
cles immediately adapted to them ; and secondly when the
mind wills no particular action, its power is applied by in-
stinct to those muscles only which are naturally adapted to
the motion, and further, the mind being accustomed to see the
part move which is naturally the most moveable, attends to
its motion in the volition, although it be in that instant fixed,
and the other parts of the body move towards it ; and al-
though the other parts of the body might be brought towards
it by other muscles, and would be so if the mind intended
they should come towards it, yet these muscles are not
brought into action. Thus the flexors of the arm common-
ly move the hand to the body, but if the hand be fixed, the
body is moved by the same muscles to the hand. In this
case, however, the mind wills the motion of the hand to-