There are six main extraocular muscles, or muscles that move the eyeball: the medial rectus, the lateral rectus, the superior rectus, the inferior rectus, the superior oblique, and the inferior oblique. The medial rectus muscle pulls the eye so that it rotates towards the midline of the body, and the lateral rectus muscle pulls the eye so that it rotates away from the midline of the body. The superior rectus muscle allows the eye to rotate upward and toward the midline, and the inferior rectus muscle allows the eye to rotate downward and toward the midline. The superior oblique muscle allows the eye to rotate downward and away from the midline, and the inferior oblique muscle allows the eye to rotate upward and away from the midline.
Any particular eye movement may use one or more of the muscles listed above. For example, you may note that there is no particular muscle whose task it is to move the eyeball up or down. Instead, the upward and downward motion are controlled by four extraocular muscles - the superior rectus, the inferior rectus, the superior oblique, and the inferior oblique. If the eye is looking straight forward at the beginning of the eye movement, the rectus muscles will supply approximately half of the force needed to move the eye upward or downward, and the oblique muscles of the eye will supply the other half of the force necessary.
Under normal circumstances, the primary muscle that is moving one eye during a given eye movement is yoked to a corresponding muscle of the opposite eye. This forces the two eyes to move together, rather than allowing them to move in different directions.
All of the extraocular muscles, besides for the inferior oblique muscle, meet at the ring of a cone-shaped area called the "annulus of Zinn." The optic nerve runs through the annulus of Zinn, down the middle of the extraocular muscles