abnormal fourth ventricle morphology dilated fourth ventricle dilated rhombencephalic ventricle fourth ventricle dilation the luminal space of the fourth ventricle is increased in volume or area, usually with an increase in contained fluid, in the irregularly shaped cavity in the rhombencephalon, between the medulla oblongata, the pons, and the isthmus in front, and the cerebellum behind; it 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 MP:0000829 dilated ventriculus quartus dilated IVth ventricle dilated 4th ventricle dilated ventricle of rhombencephalon MPheno.ontology dilated brain ventricles