The fourth ventricle is one of the cavities of the ventricular system which lies at base of the cerebrum. It is filled with cerebrospinal fluid and extends from the aqueduct of Sylvius to the obex. In cross-section of the human brain, the fourth ventricle is diamond-shaped.
The roof of the fourth ventricle is formed by the cerebellum (superior and inferior medullary vela), the floor by the rhomboid fossa, and the side "walls" formed by the cerebellar peduncles. Among the prominent features of the floor of the fourth ventricle are the: facial colliculus, which is formed by the internal part of the facial nerve as it loops around the abducens nucleus in the lower pons; the sulcus limitans, which represents the border between the alar plate and the basal plate of the developing neural tube; and the obex, which represents the caudal tip of the fourth ventricle and is also a marker for the level of the foramen magnum of the skull and therefore is a marker for the imaginary dividing line between the medulla and spinal cord.