A Review of Extraocular Muscles

Strabismus, or an eye that is turned, is often caused by a weakness in one of the six extraocular muscles that allow the eye to move in many directions. These muscles are attached to the eye itself and surround it...this is the reason contact lenses can't get "lost" behind the eye. Each muscle has a specific function, and this is a review of these functions.
There are two types of intraocular muscles. The rectus muscles move the eye in straight, primary motions or gazes...up, down, left or right. The oblique muscles rotate the eye as well as help move the eye in each of the four primary gazes. Some of the muscles have secondary and tertiary actions as well, and are innervated by different cranial nerves.
The medial rectus muscle is responsible for lateral movement of the eye, inward toward the nose. This is its only action. It is innervated by the third cranial nerve. The lateral rectus muscle is responsible for moving the eye laterally as well, but in an outward direction. It is innervated by the fourth cranial nerve.
As an example, when using the eyes to look straight off to the right, the right eye is using the lateral rectus muscle as it moves outward while at the same time the left eye is using the medial rectus muscle as it moves inward. Both eyes are moving in the same direction but using different muscles to do so. A weakness in one of these muscles will cause an eye to stay turned in, or out.
The superior rectus muscle is attached to the top of the eyeball, and its primary function is to allow the eye to elevate, or look up. It is innervated by the third cranial nerve. The superior rectus muscle has both a secondary and a tertiary function...rotating the eye inward and moving the eye inward.
Remember that inward eye movement is primarily controlled by the medial rectus muscle. The inferior rectus muscle is attached at the bottom of the eyeball, and its primary function is to allow the eye to depress, or look downward. It is innervated by the third cranial nerve as well. The inferior rectus muscle has both a secondary and tertiary function as well...it rotates the eye outward and moves the eye outward, though the primary muscle responsible for this action is the lateral oblique.
The superior oblique muscle, attached at the top of the eyeball, is innervated by the fourth cranial nerve and its primary function is to rotate the eyeinward, toward the nose. The secondary function is moving downward; the tertiary function is moving outward. Remember that the downward motion is primarily controlled by the inferior rectus and the outward motion is primarily controlled by the lateral rectus.
The inferior oblique muscle, attached at the bottom of the eyeball, is innervated by the third cranial nerve and its primary function is to rotate the eye outward. The secondary function is moving up; the tertiary function is moving in. Remember that the upward gaze is primarily controlled by the superior rectus muscle, and the inward gaze is primarily controlled by the medial rectus muscle.