The third ventricle is one of a system of four communicating cavities within the brain that are continuous with the central canal of the spinal cord.
Four ventricles consist of the two lateral ventricles, the third ventricle and the fourth ventricle.

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 what is called the aqueduct of the midbrain (or the aqueduct of Sylvius).

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