21 Control of Eye Movements
Six Extraocular Muscles Move the Eye in the Orbit
We need to move each eye in various combinations of six directions. Four of them are obvious—medially (adduction), laterally (abduction), up (elevation), and down (depression). The two others are torsional movements, the kind you would make to keep an eye level as you tilt your head to one side or the other. Intorsion rotates the top of the eye closer to the nose and extorsion rotates it away. Movements in these six directions are accomplished by six small extraocular muscles, but the correspondence between movements and individual muscles is not always direct (Table 21-1).
Table 21-1 Extraocular muscles, eye movements, and cranial nerves
| Movement | Principal Muscle | Other Contributors |
|---|---|---|
| Abduction | Lateral rectus (VI) | Inferior oblique (III) Superior oblique (IV) |
| Adduction | Medial rectus (III) | Inferior rectus (III) Superior rectus (III) |
| Depression | Inferior rectus (III) | Superior oblique (IV) |
| Elevation | Superior rectus (III) | Inferior oblique (III) |
| Extorsion | Inferior oblique (III) | Inferior rectus (III) |
| Intorsion | Superior oblique (III) | Superior rectus (III) |
The Superior and Inferior Recti and the Obliques Have More Complex Actions
The four remaining muscles—the superior rectus, inferior rectus, superior oblique, and inferior oblique—do not lie entirely in the same plane as one of the directions of eye movement, so their actions are more complex. For example, the eye (when looking at something far away) points straight ahead in the orbit, but the axis of the orbit itself—the direction in which the superior and inferior recti pull—points not only backward but also toward the nose (Fig. 21-1). The result is that contraction of the superior rectus mainly causes elevation, but also pulls the top of the eye toward the nose (i.e., intorsion and adduction). Similarly, the inferior rectus mainly causes depression, but also causes extorsion and adduction. The superior and inferior obliques mainly cause intorsion and extorsion, respectively. However, because they insert behind the middle of the eye and pull partially anteriorly, they too cause movement in additional directions (see Table 21-1).
