The third ventricle, or ventriculus tertius, is a narrow cavity located between the two cerebral hemispheres. It is one of four connected fluid-filled cavities which comprise the ventricular system within the cerebrum of human beings. The third ventricle is in the midline, between the left and right lateral ventricles and is filled with cerebrospinal fluid.
The third ventricle is bounded by the thalamus and hypothalamus on both the left and right sides. The third ventricle provides a pathway for cerebrospinal fluid.