I, The opening along the choroidal fissure has been extended posteriorly by opening the tenia fornix along the edge of the body and crus of the fornix. The upper part of the quadrigeminal cistern, where the internal cerebral veins converge on the vein of Galen, has been exposed. The medial posterior choroidal arteries course with the internal cerebral veins. J, The opening of the choroidal fissure has been extended downward along the choroidal fissure to the central part of the quadrigeminal cistern, exposing the basal and internal cerebral veins, pineal, and superior colliculus. Branches of the medial posterior choroidal arteries course beside the pineal. K, Enlarged view. The tip of the pineal projects posteriorly above the superior colliculus and between the terminal part of the internal cerebral veins. L, The dissection has been extended forward along the choroidal fissure toward the temporal horn by dividing the tenia on the edge of the fimbria of the fornix to expose the basal vein, posterior cerebral arteries, and trochlear nerve in the posterior part of the ambient cistern below the thalamus. M, The choroidal fissure in the temporal horn has been opened by dividing the tenia fimbria. The choroid plexus attachment to the thalamus has not been disturbed. The posterior cerebral artery and basal vein course through the ambient cistern on the medial side of the temporal portion of the choroidal fissure. N, The exposure has been extended through the amygdala anterior to the choroidal fissure to expose the oculomotor nerve and origin of the posterior cerebral artery. The posterior cerebral artery passes above the oculomotor nerve. (Images courtesy of AL Rhoton, Jr.)