The third ventricle is bounded by the thalamus and hypothalamus on both the left and right sides. The lamina terminalis forms the anterior wall.The floor is formed by hypothalamic structures. The roof is formed by the ependyma, lining the undersurface of the tela choroidea of the third ventricle.he third ventricle (ventriculus tertius) is one of four connected fluid-filled cavities comprising the ventricular system within the human brain.