Left lateral view of orbital structures at the orbital apex and superior orbital fissure. Bone has been selectively removed to demonstrate relationships at these two sites. The optic nerve as a developmental tract of the brain is enclosed by meninges (optic nerve sheath) as it passes through the optic canal. The annulus of Zinn (common ring tendon) is a thickening of fibrous tissue that surrounds the optic nerve and bridges across the superior orbital fissure. It serves as the proximal attachment site for the rectus muscles (superior, medial, inferior, lateral) of the eye. The oculomotor nerve (including its superior and inferior divisions), abducens nerve, optic nerve, and nasociliary branch of the ophthalmic nerve pass through the annulus. The superior division of the oculomotor nerve innervates the superior rectus and levator palpebrae superioris muscles. The inferior division innervates the medial and inferior rectus and inferior oblique muscles and gives rise to the parasympathetic root to the ciliary ganglion for innervation of the ciliary and sphincter pupillae muscles (smooth muscle). The trochlear nerve innervates the superior oblique muscle while the abducens nerve provides motor innervation to the lateral rectus muscle. The ophthalmic nerve gives rise to three branches in the orbit: frontal, lacrimal, and nasociliary. The nasociliary nerve crosses over the optic nerve from lateral to medial and gives rise to long ciliary nerves and a sensory root to the ciliary ganglion. A sympathetic root from the internal carotid nerve plexus also enters the ciliary ganglion and supplies the dilator pupillae muscle. (Image courtesy of AL Rhoton, Jr.)