Walsh & Hoyt: Extraocular Muscles

Each eyeball is rotated by six extraocular muscles: the medial rectus, lateral rectus, superior rectus, inferior rectus, superior oblique, and inferior oblique. These muscles, with the exception of the inferior oblique, take origin from a fibrotendinous ring at the orbital apex called the annulus of Zinn, which surrounds a central opening known as the oculomotor foramen. The oculomotor foramen encircles the optic foramen and the central part of the superior orbital fissure. The optic nerve and the ophthalmic artery pass through the optic foramen, whereas the superior and inferior divisions of the oculomotor nerve, the abducens nerve, and the nasociliary branch of the trigeminal nerve pass through the superior orbital fissure within the annulus of Zinn. The trochlear nerve and the frontal and lacrimal branches of the trigeminal nerve enter the orbit through the superior orbital fissure outside the annulus of Zinn.