The supraclinoid portion of the artery gives rise to the posterior communicating, anterior choroidal, middle cerebral, and anterior cerebral arteries. The supra-clinoid portion of the artery is divided into three segments based on the site of origin of these branches: an ophthalmic segment (C4-Op.) that extends from the origin of the ophthalmic artery (not shown because the ICA was divided above the level of origin of the ophthalmic artery) to the origin of the PComA; a communicating segment (C4-Co.) that extends from the origin of the PComA to the origin of the AChA; and a choroidal segment (C4-Ch.) that extends from the origin of the AChA to the level of the bifurcation of the ICA into the anterior cerebral and middle cerebral arteries. The ophthalmic segment sends perforating branches to the optic nerves, optic chiasm, and the tuber cinereum. The superior hypophyseal arteries pass to the infundibulum of the hypophysis. The communicating segment sends one perforating branch on each side to the optic tracts and the region around the mamillary bodies. The perforating arteries are as large as the adjacent AChA and PComA. The choroidal segment sends its perforating branches into the anterior perforated substance. The posterior cerebral arteries arise from the basilar artery and pass laterally around the cerebral peduncles. The temporal lobe is lateral to the carotid artery. The frontal lobes, gyrus rectus, and olfactory nerves are above the optic nerves. The thalamoperforating arteries pass posteriorly between the oculomotor nerves. (Images courtesy of AL Rhoton, Jr.)