A, lateral view of the right internal carotid artery. B, superior view of the internal carotid arteries, with the right optic nerve and right half of the optic chiasm reflected forward to expose the origin of the ophthalmic artery. The intracavernous portion of both carotid arteries and the course of the left ophthalmic artery are shown by dotted lines. The aneurysms arise on curves in the artery at the site of origin of its branches. The aneurysms point in the direction (arrows) of the maximal hemodynamic force immediately proximal to the aneurysm site and in the direction the blood would have gone if there were no curve at the aneurysm site. The aneurysm sites on the internal carotid artery are usually located immediately distal to the origins of its branches. Aneurysms arising at the origin of the ophthalmic artery point upward into the optic nerve. Aneurysms arising at the origin of the superior hypophyseal artery point medially under the optic chiasm. Aneurysms arising near the origin of the posterior communicating artery point posteriorly toward the oculomotor nerve and are usually located superolateral to the posterior communicating artery. Aneurysms arising near the origin of the anterior choroidal artery point posterolaterally and are usually located immediately superior to the origin of the anterior choroidal artery. Aneurysms arising at the carotid bifurcation into the anterior and middle cerebral arteries point upward lateral to the optic chiasm toward the anterior perforated substance. (Images courtesy of AL Rhoton, Jr.)