The 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.The floor is made up of the optic chiasm, the tuber cinereum and the infundibulum, the mamilary bodies, the posterior perforated substance and the upermost part of the mesencephalic tegmentum. The anterior wall is the delicate lamina terminalis.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.