The roof of the third ventricle is formed by a thin layer of ependyma. The lateral walls are formed mainly by the medial surfaces of the 2 thalami.The lower lateral wall and the floor of the ventricle are formed by the hypothalamus and subthalamus.he Third Ventricle. The third ventricle (in blue) is a narrow vertical cleft between the 2 lateral ventricles. It possesses a roof, a floor and four walls: anterior, posterior and two lateral. The roof is formed by the tela choroidea.