One of the system of communicating cavities in the brain that are continuous with the central canal of the spinal cord, that are lined with an epithelial ependyma, and that contain cerebral-spinal fluid. The system comprises four ventricles, the right and left lateral ventricles (one for each hemisphere), third ventricle, and fourth ventricle. [ http://en.wikipedia.org/wiki/Ventricular_system http://purl.obolibrary.org/obo/UBERON_0004086 ]