The essential part of the hearing organ consists of two labyrinthine compartments: the bony labyrinthine and the membranous labyrinth. The bony labyrinth is a complex of three interconnecting cavities or spaces (COCHLEA; VESTIBULAR LABYRINTH; and semicircular canals) in the temporal bone. Within the bony labyrinth lies the membranous labyrinth which is a complex of sacs and tubules (cochlear duct; saccule and utricle; and semicircular ducts) forming a continuous space enclosed by epithelium and connective tissue. These spaces are filled with labyrinthine fluids of various compositions.