Fourth ventricle: One cavity in a system of four communicating cavities within the brain, which are continuous with the central canal of the spinal cord. The fourth ventricle is the most inferior (lowest) of these. It extends from the aqueduct of the midbrain to the central canal of the upper end of the spinal cord, with which it communicates via the foramina of Luschka and the foramen of Magendie. It is filled with cerebrospinal fluid that is formed by structures called choroid plexuses located in the walls and roofs of the ventricle.
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