Cerebral ventricle: One of a system of four communicating cavities within the brain that are continuous with the central canal of the spinal cord. They include two lateral ventricles in the cerebral hemispheres, each consisting of a triangular central body and four horns. The lateral ventricles communicate with the third ventricle through an opening called the interventricular foramen. The third ventricle, a median (midline) cavity in the brain, is bounded by the thalamus and hypothalamus on either side. In front, the third ventricle communicates with the lateral ventricles, and in back it communicates with the aqueduct of the midbrain (also known as the aqueduct of Sylvius). The fourth ventricle, which is the lowest of the four ventricles of the brain, extends from the aqueduct of the midbrain to the central canal of the upper end of the spinal cord, with which it communicates, through the two foramina of Luschka and the foramen of Magendie. The ventricles are filled with cerebrospinal fluid.