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30 DIGASTRICUS.
principally to depress it ; but according as one acts a little more
forcibly than the other, it thereby gives the Jaw a small
rotation ; and becomes, in that respect, a kind of antagonist to
the Pterygoidaeus Externus. Besides depressing the Lower-Jaw.
when we examine the dead body, they would appear to raise the
Larynx. But although they have this effect, a proper attention
to what happens in the living body, will probably shew, that
their principal action is to depress the Lower-Jaw, and that they
are the Muscles which are commonly employed for this purpose.
Let a Finger be placed on the upper part of the Sterno-
Mastoidgeus Muscle, just behind the posterior edge of the
Mastoid Process, about its middle, touching that edge a little
with the finger ; then depress the Lower-Jaw, and the posterior
head of the Digastric will be felt to swell very considerably,
and so as to point out the direction of the Muscle. In this
there can be no deception ; for there is no other Muscle in this
part that has the same direction ; and those who are of opinion
that the Digastric does not depress the Lower-Jaw, will more
readily allow this, when they are told, that we find the same
head of the Muscle act in deglutition ; but not with a force
equal to that which it exerts in depressing the Lower-Jaw. (h)
Further, if the Sterno-Hyoidei, Sterno-Thyroidei, and Costo-
Hyoidei, acting at the same time with the Mylo-Hyoidei, and
Genio-Hyoidei, assisted in depressing the Jaw, the Os Hyoides,
and Thyroide Cartilage, would probably be depressed, as the

(h) [Hunter here probably refers to a paper by Alexander Monro
primus on the Articulation Muscles and Luxation of the lower jaw,
which appeared in the first volume of the Edinburgh Medical Essays.
Monro opposes the commonly-received opinion as to the function of the
digastric by the following arguments : That the shortness of the bellies
of the digastric does not admit so large a contraction as is often required
;
—that " the proportional force of the digastric to that of the elevators
of the jaw is considerably less than is seen in other parts of the body
where antagonist muscles are, which proportional force of these muscles
is on some occasions greatly lessened by the angle of insertion of these
digastric muscles into the jaw, decreasing as the mouth is opened." He
denies the existence of any mechanism akin to a pulley, such as is seen
in the larger oblique muscle of the eye. He describes the connection
of the digastric with the os hyoides as taking place simply by a firm
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