The Diencephalon (Medulla) The medulla, the most caudal segment of the brain stem, appears as a conical expansion of the spinal cord. Both the pons and the medulla are separated from the overlying cerebellum by the fourth ventricle, and cerebrospinal fluid entering the fourth ventricle from the cerebral aqueduct passes into the cisterna magna, a subarachnoid space surrounding the medulla and the cerebellum, via foramina in the lateral recesses and in the midline of the ventricle.