Brain ventricle: One of the communicating cavities within the brain. There are four ventricles: two lateral ventricles, the third ventricle, and the fourth ventricle. The lateral ventricles are in the cerebral hemispheres. Each lateral ventricle consists of a triangular central body and four horns. The lateral ventricles communicate with the third ventricle through the interventricular foramen (opening). The third ventricle is a median (midline) cavity in the brain, 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 (the aqueduct of Sylvius). The fourth ventricle is the most inferior of the four ventricles of the brain. It extends from the aqueduct of the midbrain to the central canal of the upper end of the spinal cord, with which it communicates by the two foramina (openings) of Luschka and the foramen of Magendie. The ventricles are filled with cerebrospinal fluid, which is formed by structures, called choroid plexuses, that are located in the walls and roofs of the ventricles.