Extraocular Muscles

The movement of each eyeball is controlled by the six extraocular muscles (four rectus and two oblique) innervated by CNN III, IV, and VI. The extraocular muscles are named according to their position on the eyeball (Figure 18-3B and C). All of the muscles except the inferior oblique muscle arise from a common tendinous ring in the posterior part of the orbit. To determine the action of each muscle, it should be noted that the apex of the orbit (where the superior and inferior rectus and oblique muscles originate) is not parallel with the optical axis (when looking directly forward) (Figure 18-3D). Therefore, when the superior and inferior rectus muscles and the oblique muscles contract, there are secondary actions on the eyeball movement, as follows (Figure 18-3E):

Superior rectus muscle (CN III). Elevation of the eye, with adduction and intorsion.

Inferior rectus muscle (CN III). Depression of the eye, with adduction and extorsion.

Lateral rectus muscle (CN VI). Only action is abduction of the eye.

Medial rectus muscle (CN III). Only action is adduction of the eye.

Superior oblique muscle (CN IV). Originates in common with the rectus muscles, courses along the medial wall of the orbit, and then turns and loops through a fibrous sling, called the trochlea, before inserting on the superolateral region of the eyeball. The superior oblique muscle moves the eyeball down and out (depression and abduction), with intorsion.

Inferior oblique muscle (CN III). Originates on the medial wall of the bony orbit and courses laterally and obliquely to insert on the inferolateral surface of the eyeball. The inferior oblique moves the eyeball up and out (elevation and abduction), with extorsion.

Trochlea

Elevation A

Extorsion

Abduction

Z-axis

Intorsion

Abduction

Z-axis

X-axis Y-axis

Depression

Intorsion

X-axis Y-axis

Depression

Superior oblique m (CN IV)

Medial rectus m. (CN III)

Trochlea

Superior rectus m. (CN III)

Lateral rectus m. (CN VI)

Levator palpebrae superioris m. (CN III) (cut)

Common tendinous ring

Superior rectus m. (CN III)

Lateral rectus m. (CN VI)

Levator palpebrae superioris m. (CN III) (cut)

Common tendinous ring

Superior rectus m. (CN III)

Lateral rectus m (CN VI)

Superior rectus m. (CN III)

Superior oblique m. (CN IV)

Lateral rectus m (CN VI)

Medial rectus m. (CN III)

Inferior rectus m. (CN III)

C Inferior oblique m. C (CN III)

Superior oblique m. (CN IV)

Trochlea

Medial rectus m. (CN III)

Inferior rectus m. (CN III)

C Inferior oblique m. C (CN III)

Axis of eyeball

Anatomic actions

Inferior oblique m. Superior rectus m.

Lateral rectus m.-<

Inferior oblique m. Superior rectus m.

Lateral rectus m.-<

Medial rectus m.

Medial rectus m.

E Superior oblique m. Inferior rectus m.

Figure 18-3: A. Movements of the eyeball. Extraocular muscles of the right eye; (B) superior and (C) anterior views. D. Axes of the eyeball and orbit. E. Anatomic actions of the right extraocular muscles.

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