The answer is a cerebral aqueduct. The cerebral aqueduct is situated inside the mesencephalon and interfaces the third ventricle in the diencephalon to the fourth ventricle in the locale of the mesencephalon and metencephalon. It contains the cerebrospinal liquid (CSF). The cerebral reservoir conduit (of Sylvius) is the structure inside the brainstem that associates the third ventricle to the fourth.