a tertio ad quartum ventriculum iter aquaeductrs cerebri aquaeductus cerebra aquaeductus cerebrimesocelia aquaeductus mesencephali aquaeductus mesencephalon aqueduct of mesencephalon aqueduct of midbrain aqueduct of sylvius aqueductus sylyii cerebral aqueduct iter a tertio ad quartum ventriculum iter of sylvius mesencephalic aqueduct midventricle ventricular aqueduct