[TA] an enlarged portion of the subarachnoid space located immediately superior to the tectum of the mesencephalon; may extend forward between the corpus callosum and thalamus as the cistern of the velum interpositum; contains caudal portions of the internal cerebral veins as they join to form the great cerebral vein (of Galen), distal parts of the quadrigeminal artery, the P4 segment of the posterior cerebral artery, and the exit of the trochlear nerve. SYN: cisterna quadrigeminalis [TA], cistern of great cerebral vein*, cisterna venae magnae cerebri*, Bichat canal, cisternal quadrigeminalis, superior cistern.