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32 DIGASTRICUS.
Muscles were to act to bring about this motion of the Jaw, these
parts would be brought forwards, nearer to the straight line
between the Chin and Sternum, which is not the case in this
action ; whereas we find it to be the case in deglutition, in which
these evidently act. By applying our fingers upon the Genio-
Hyoideus, and Mylo-Hyoideus, near the Os Hyoides, between
the two anterior bellies of the Digastric, (not near the Chin,
where the action of these 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 center 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 depressed, in proportion
as the Upper-Jaw is raised ; which is not true in fact. Others
have considered the Condyles of the Occiput as the center of
motion ; and they have conceived the Extensor Muscles of the
Head to be the moving powers. 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 Muscles immediately adapted to them ; and secondly,
when the mind wills any particular action, its power is applied
by instinct to those Muscles only, which are naturally adapted
to that 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 instance fixed,
and the other parts of the body move towards it ; and although
the other parts of the body might be brought towards it by
other Muscles, and would be so, if the mind intended that they
should come towards it, yet these Muscles are not brought into
action. Thus the Flexors of the Arm commonly move the hand
to the body ; but if the hand be fixed, the body is moved by the
32 DIGASTRICUS.
Muscles were to act to bring about this motion of the Jaw, these
parts would be brought forwards, nearer to the straight line
between the Chin and Sternum, which is not the case in this
action ; whereas we find it to be the case in deglutition, in which
these evidently act. By applying our fingers upon the Genio-
Hyoideus, and Mylo-Hyoideus, near the Os Hyoides, between
the two anterior bellies of the Digastric, (not near the Chin,
where the action of these 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 center 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 depressed, in proportion
as the Upper-Jaw is raised ; which is not true in fact. Others
have considered the Condyles of the Occiput as the center of
motion ; and they have conceived the Extensor Muscles of the
Head to be the moving powers. 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 Muscles immediately adapted to them ; and secondly,
when the mind wills any particular action, its power is applied
by instinct to those Muscles only, which are naturally adapted
to that 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 instance fixed,
and the other parts of the body move towards it ; and although
the other parts of the body might be brought towards it by
other Muscles, and would be so, if the mind intended that they
should come towards it, yet these Muscles are not brought into
action. Thus the Flexors of the Arm commonly move the hand
to the body ; but if the hand be fixed, the body is moved by the
