The muscles of the eye include the lateral, medial, inferior and superior rectus, the inferior oblique and the superior oblique. Within the bone structure supporting the eye, all but one of these muscles — the inferior oblique — form a cone, with its point directly behind the eyeball. This point is called the annulus of Zinn, and it is the point through which the optic nerve runs into the eye. When receiving signals from the brain through the optic nerve, the muscles of the eye — also known as the extraocular muscles — provide the eye's movements.
The medial rectus muscle supports the side of the eyeball closest to the nose and is responsible for eye movements toward the bridge of the nose or the nose. The lateral rectus muscle is across from the medial rectus. It hangs horizontally and supports the side of the eyeball farthest away from the nose, providing eye movements away from the nose.
At the top of the eye is the superior rectus, and it is primarily responsible for moving the eye upward. It also helps move the eye in a downward direction and can rotate the eye toward the top of the nose. The inferior rectus forms the bottom of the cone of muscles and is located at the bottom of the eye. Its primary purpose is to move the eye upward, but it can also move the eye away from the nose and rotate it inward.
The superior oblique differs from the other four muscles that form the cone. Before attaching to the eye, it passes through the trochlea, a tendon that serves as a pulley and is attached to the upper nasal structure next to the eye. Its primary purpose is to rotate the top of the eye toward the nose, but it can also assist with moving the eye downward and outward. The inferior oblique does not connect to the point of the cone, or annulus of Zinn, but instead runs underneath the eye and attaches the upper nasal structure to the outside of the eye. It can rotate the eye away from the nose as well as move the eye outward and upward.
The muscles of the eye are said to be "yoked" together to provide simultaneous and synchronized movements in both eyes. For example, the right lateral rectus works simultaneously with the left medial rectus to allow a person to look to the right. Also, although the muscles are designed to allow a person to converge his or her eyes inward, or become cross-eyed, a person cannot voluntarily diverge his or her eyes. The muscles of the eye often can be repaired through surgery if they fail to work together properly.