A, Inferior view. The internal carotid artery gives rise to the ophthalmic, posterior communicating, anterior choroidal, anterior cerebral, and the middle cerebral arteries. The supraclinoid portion of the ICA is divided into three segments based on the origin of these branches: an ophthalmic segment (C4-Op., blue) that extends from the origin of the ophthalmic artery to the origin of the PComA; a communicating segment (C4-Co., light green) that extends from the origin of the PComA to the origin of the AChA; and a choroidal segment (C4-Ch., dark green) that extends from the origin of the AChA to the bifurcation of the ICA into the anterior and middle cerebral arteries. The perforating branches arising from the ophthalmic segment extend to the optic nerve, optic chiasm and the optic tracts, and the floor of the third ventricle around the infundibulum and tuber cinereum. The superior hypophyseal arteries arise from the ophthalmic segment and extend to the infundibulum of the pituitary gland. The branches arising from the communicating segment reach the optic tracts, floor of the third ventricle, and the area around the mamillary bodies. The perforating branches of the choroidal segment pass upward and enter the anterior perforated substance. The posterior cerebral arteries arise from the basilar artery and pass backward below the optic tracts. The ACA and AComA course above the optic chiasm and pass between the frontal lobes. The olfactory nerves are lateral to the gyrus rectus. B, Anterior view. The left optic nerve has been divided near its entrance into the optic canal and elevated to give a clearer view of the perforating branches. The ophthalmic artery arises above the cavernous sinus. The carotid artery courses through the cavernous sinus and then laterally and produces a prominence in the wall of the sphenoid sinus before giving rise to the ophthalmic artery. The oculomotor, trochlear, abducens, and the ophthalmic, maxillary, and mandibular divisions of the trigeminal nerve pass lateral to the sphenoid sinus in the walls of the cavernous sinus. The superior hypophyseal arteries arise from the ophthalmic segment. (Images of AL Rhoton, Jr.)