Aqueduct of Sylvius: A canal that communicates between the third and fourth ventricles in a system of four communicating cavities within the brain that are continuous with the central canal of the spinal cord.

The four ventricles consist of the two lateral ventricles, the third ventricle and the fourth ventricle:

• Lateral ventricles: 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 what is called the interventricular foramen (opening).
• The third ventricle is a median (midline) cavity in the brain that is bounded by the thalamus and hypothalamus on either side. Anteriorly (in front) the third ventricle communicates with the lateral ventricles and posteriorly (in back) the third ventricle communicates with the aqueduct of Sylvius (also called the aqueduct of the midbrain).
• The fourth ventricle is the most inferior (lowest) 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 (opening) of Magendie.

The ventricles are filled with cerebrospinal fluid, which is formed by structures called choroid plexuses located in the walls and roofs of the ventricles.