a slitlike, fluid-filled space of variable width between the left and right transparent septa, which occurs in fewer than 10% of human brains and may communicate with the third ventricle. SYN: cavum septum pellucidum [TA], Duncan ventricle, fifth ventricle, pseudocele, pseudoventricle, sylvian ventricle, ventricle of Sylvius, ventriculus quintus, Vieussens ventricle, Wenzel ventricle.