It is 1 5 cm long and 1 2 mm in diameter. A removal of the right cerebellar hemisphere showing the fourth ventricle right superolateral view b the brainstem after removal of the cerebellum showing the floor of the fourth ventricle superior view c removal of the right half of the brainstem showing the roof of the fourth ventricle right inferolateral view. The cerebral aqueduct is a narrow canal in the midline connecting the third and fourth ventricle.