Muscle Structure of the Eye

The eye is controlled by six different muscles that attach in specific areas. These muscles contract to pull the eye in different directions, and are innervated (i.e., controlled) by several different cranial nerves. Additionally, internally the eye has several muscles that work to change the amount of light allowed into the eye as well as to focus on items held up close.

Names of Muscles

• The human eye has six different muscles attached to it that allow for movement. The muscles are: lateral rectus, medial rectus, superior rectus, inferior rectus, superior oblique and inferior oblique.

Lateral and Medial Rectus Muscles

• The lateral rectus abducts (turns the eye out) and the medial rectus adducts (turns the eye in). These muscles are yoked for the two eyes, meaning that when the lateral rectus contracts to turn one eye out the medial rectus of the opposite eye contracts to turn that eye in, ensuring that both eyes remain pointed in the same direction.

Superior and Inferior Muscles

• The superior and inferior rectus muscles primarily elevate and depress the eye, respectively, and the superior and inferior obliques primarily intort (turn the 12 o'clock position of the eye in toward the nose) and extort (turn the 12 o'clock position away from the nose), respectively.

Innervation of Eye Muscles

• Cranial nerve III innervates the superior, medial, and inferior rectus muscles as well as the inferior oblique muscle. Cranial nerve IV innervates the superior oblique muscle. Cranial nerve VI innervates the lateral rectus muscle.

Internal Musculature of the Eye

• Inside the eye are three involuntary muscles. The iris contains two of these muscles: the iris dilator muscle makes the pupil bigger to allow more light into the eye, and the iris sphincter muscle makes the pupil smaller to decrease the amount of light entering the eye. The ciliary body muscle contracts to change the shape of the lens and allow focusing for objects that are held closely. As a person ages, this ability of the lens to change shape is lost, resulting in presbyopia (a form of farsightedness). At this point, reading glasses are needed in order to see up close.