abnormal third ventricle morphology enlarged third ventricle MP:0008536 MPheno.ontology enlarged 3rd ventricle enlarged ventriculus tertius increased size of the narrow cleft inferior to the corpus callosum, within the diencephalon, between the paired thalami; its floor is formed by the hypothalamus, its anterior wall by the lamina terminalis, and its roof by ependyma; it communicates with the fourth ventricle by the cerebral aqueduct, and with the lateral ventricles by the interventricular foramina increased third ventricle size increased ventriculus tertius size enlarged brain ventricles