From the lateral ventricles, CSF flows through the interventricular foramen (of Monro) to the slitlike third ventricle, which is surrounded by the thalamus and hypothalamus (eFig. 9.15). The third ventricle communicates with the fourth ventricle via the cerebral aqueduct (aqueduct of Sylvius), which courses through the midbrain (eFig. 9.15). Surrounded by the pons and medulla anteriorly and the cerebellum posteriorly, the fourth ventricle sends CSF out of the ventricular system and into the subarachnoid space via the lateral foramina of Luschka and midline foramen of Magendie (eFig. 9.15).