part of brain ventricle/choroid plexus brain ventricle One of the system of communicating cavities in the brain that are continuous with the central canal of the spinal cord, that like it are derived from the medullary canal of the embryo, that are lined with an epithelial ependyma, and that contain a serous fluid. brain ventricles region of ventricular system of brain cerebral ventricle ventricular system of brain ventricle of nervous system