Chapter 15
The Special Senses
547
15
the eyeball
(Figure 15.3)
. Tey allow the eyes to follow a mov-
ing object, and provide external “guy-wires” that help maintain
the shape of the eyeball and hold it in the orbit.
Te four
rectus muscles
originate from the
common tendi-
nous ring
(or annular ring) at the back of the orbit and run
straight to their insertion on the eyeball. Teir locations and
the movements that they promote are clearly indicated by their
names:
superior, inferior, lateral
, and
medial rectus muscles
.
Te actions of the two
oblique muscles
are less easy to deduce
because they take rather strange paths through the orbit. Tey
move the eye in the vertical plane when the eyeball is already
turned medially by the rectus muscles. Te
superior oblique
muscle
originates in common with the rectus muscles, runs
along the medial wall of the orbit, and then makes a right-angle
turn and passes through a fibrocartilaginous loop called the
trochlea
(trok
9
le-ah; “pulley”) suspended from the frontal bone
before inserting on the superolateral aspect of the eyeball. It
rotates the eye downward and somewhat laterally.
Inferior
rectus
muscle
Inferior
oblique
muscle
Superior oblique muscle
Common
tendinous ring
Trochlea
Superior oblique tendon
Superior rectus muscle
Lateral rectus muscle
(a) Lateral view of the right eye
(b) Superior view of the right eye
Axis of
rotation
of eye
Medial
rectus muscle
Inferior
rectus muscle
Lateral
rectus muscle
(c) Summary of muscle actions and innervating cranial nerves
Lateral rectus
Medial rectus
Superior rectus
Inferior rectus
Inferior oblique
Superior oblique
Moves eye laterally
Moves eye medially
Elevates eye and turns it medially
Depresses eye and turns it medially
Elevates eye and turns it laterally
Depresses eye and turns it laterally
VI (abducens)
III (oculomotor)
III (oculomotor)
III (oculomotor)
III (oculomotor)
IV (trochlear)
Muscle
Action
Controlling cranial nerve
Figure 15.3
Extrinsic eye muscles.
Te
inferior oblique muscle
originates from the medial
orbit surface and runs laterally and obliquely to insert on the
inferolateral eye surface. It rotates the eye up and laterally.
Te four rectus muscles would seem to provide all the eye
movements we require—medial, lateral, superior, and inferior—
so why the two oblique muscles? Te simplest way to answer
this question is to point out that the superior and inferior recti
cannot elevate or depress the eye
without also turning it medially
because they approach the eye from a posteromedial direction.
For an eye to be
directly
elevated or depressed, the lateral pull of
the oblique muscles is necessary to cancel the medial pull of the
superior and inferior recti.
Except for the lateral rectus and superior oblique muscles,
which are innervated respectively by the
abducens
and
trochlear
nerves
, the
oculomotor nerves
serve all extrinsic eye muscles.
Figure 15.3c summarizes the actions and nerve supply of these
muscles. ±able 13.2 (pp. 495–497) illustrates the courses of the
associated cranial nerves.
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