abnormal third ventricle morphology dilated third ventricle MP:0000827 dilated ventricle of diencephalon MPheno.ontology dilated ventriculus tertius the luminal space of the third ventricle is increased in volume or area, usually with an increase in contained fluid, in 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 dilated 3rd ventricle third ventricle dilation dilated brain ventricles