Fig. 13. The CS in parasagittal views. A. Osteology. OF, optic foramen; SOF, superior orbital fissure; FR, foramen rotundum; CC, carotid canal; Se, sella turcica; Ac, anterior clinoid process; Pc, posterior clinoid process. Note that the optic canal runs medial to the Ac. B. Venous plexus. The plexus consists of large-caliber fenestrated venous spaces that may communicate with the contralateral CS through foramina in the medial bony wall. II, optic nerve; PG, pituitary gland. C. The carotid siphon and sympathetic plexus. The ophthalmic artery is given off as the first major intracranial branch of the internal carotid artery just as it exits the roof of the CS. Smaller meningeal branches are not shown. The sympathetic nerve supply travels as a neural plexus (not shown) around the carotid artery, entering the orbit with the ophthalmic artery and through the superior orbital fissure. ICA, internal carotid artery (carotid siphon); OA, ophthalmic artery. D. Cranial nerve supply. All cranial nerves except the abducens travel through the CS tightly adherent to the lateral dural wall. The abducens nerve has an unpredictable course through the venous plexus but is usually adherent at least in part to the carotid siphon. Note how the abducens nerve travels vertically over the petrous ridge and becomes tethered by Gruber's ligament just before entering the CS. This tethering predisposes the abducens nerve to deceleration injury in head trauma. The maxillary division is closely associated, but outside, the CS. The parasympathetic nerve supply to the globe travels within the oculomotor nerve. III, oculomotor nerve; IV, trochlear nerve; V-1, ophthalmic division (trigeminal nerve); V-2, maxillary division; V-3, mandibular division; VI, abducens nerve. E. The lateral dural wall. (Modified from Zide BM, Jelks GW: Surgical Anatomy of the Orbit, p 8. New York, Raven Press, 1985)