The aqueduct of Sylvius.
D002535:Cerebral Aqueduct / Q000033:anatomy & histology / Q000196:embryology; D003251:Constriction, Pathologic / Q000150:complications / Q000209:etiology; D006666:History of Medicine; D006801:Humans; D006849:Hydrocephalus / Q000150:complications / Q000209:etiology