Craniofacial Muscles

3 Extraocular Muscle Structure and Function
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Fig. 3.3 The medial orbital walls are parallel to each other, and the lateral wall forms a 45° angle
with the medial wall. If there was no tension on the EOMs the natural direction of gaze would be
at 22.5°, rather than parallel to the medial wall. Thus, tone must be maintained in the medial rectus
muscles bilaterally in primary gaze (see Fig. 3.4d )
the same orbit (and attaching to the same globe) are referred to as agonist/antagonist
pairs. When a person looks to the right in the horizontal plane, the right lateral rectus
and the left medial rectus muscle act in a coordinated fashion; these muscle pairs are
referred to as yoked muscles. The superior and inferior rectus muscles also take their
origin from the same common tendinous ring at the apex of the orbit and insert into the
superior and inferior sides of the globe, anterior to the equator, and play a role in vertical
eye movements. Due to the pyramidal shape of the bony orbit (Fig. 3.3 ), if you draw
a line parallel to the direction of these vertical muscles, it is clear that if one or the other
contracts, the eye would not move directly superiorly or inferiorly. Both also have a
rotational component, the net effect of which is to adduct the eye (move towards the
nose). This secondary action is actually torsional; thus the superior rectus elevates and
intorts the eye, while the inferior rectus depresses and extorts the eye.
The fi nal two EOM in the orbit are the superior and inferior oblique muscles. The
superior oblique muscle takes its origin from the apex of the bony orbit and runs in
the superior part of the orbit along the medial wall. It passes through a cartilaginous
pulley called the trochlea and makes an acute turn posteriorly to insert deep to
the superior rectus muscle on the superior surface of the globe, but posterior to the
equator of the globe. The passage through the trochlea places the effective origin at
the anteromedial orbital wall, and the insertion posterior to the equator results in the
primary action of the superior oblique to be intorsion, which results in abduction of
the eye (rotating away from the nose), and its secondary action is depression. The
inferior oblique takes its origin from the anteromedial inferior orbital wall and
courses posteriorly to insert on the globe posterior to the equator. The direction of
pull of this muscle is parallel to the superior oblique; its primary action is to extort
the eye, which results in abduction of the eye (rotating away from the nose), and its
secondary action is elevation. If you examine the shape of the orbit and the position