The third ventricle is one of the four cerebrospinal fluid-filled ventricles and is the first midline ventricle into which cerebrospinal fluid flows from the lateral ventricles, and from which cerebrospinal fluid flows to the fourth ventricle. Though all four ventricles have a choroid plexus that help create cerebrospinal fluid, the third ventricle is the only one whose posterior wall is comprised of the pineal gland (named for its resemblance to the pine cone). The lateral ventricles drain into the third ventricle via the foramen of Monro. The third ventricle, in turn, drains via the cerebral aqueduct of Sylvius into the fourth ventricle. The thalamus and hypothalamus form the walls of the third ventricle.