abnormal fourth ventricle morphology absent fourth ventricle 2014-03-19T14:24:13Z MP:0012678 MPheno.ontology absence of the irregularly shaped cavity in the rhombencephalon, normally located between the medulla oblongata, the pons, and the isthmus in front, and the cerebellum behind; the fourth ventricle is continuous with the central canal of the cord below and with the cerebral aqueduct above, and through its lateral and median apertures it communicates with the subarachnoid space absent 4th ventricle absent IVth ventricle absent rhombencephalic ventricle absent ventricle of rhombencephalon absent ventriculus quartus csmith