The endolymphatic duct is a small epithelial-lined channel, part of the membranous labyrinth that passes through the vestibular aqueduct in the bony labyrinth of the petrous temporal bone. It arises from the utricle and saccule via the utriculosaccule duct and drains endolymph. The distal end is dilated forming the endolymphatic sac, which protrudes beneath the dura of the posterior surface of the petrous temporal bone near the sigmoid sinus. The sac acts as both a reservoir for endolymph and the site for reabsorption into the epidural space. It does not communicate with the perilymphatic duct.