The membranous labyrinth is a system of vesicles and canals that is surrounded on all sides by a hard, bony capsule. The cavities in the bone have the same shapes as the mem-branous structures, and their cast (C) pro-vides a crude representation of the mem-branous labyrinth. We therefore distinguish between an osseous (bony) labyrinth and a membranous labyrinth. The osseous laby-rinthcontains a clear, aqueous fluid, the perilymph (light greenish-blue), in which themembranous labyrinth is suspended. The perilymphatic space communicates with the subarachnoid space via the perilym-phatic duct (A1) at the posterior edge of thepetrous bone. The membranous labyrinth contains the endolymph (dark greenish-blue) which is a viscous fluid.
The vestibular window (AC2) is closed by the stapes and leads into the middle part of the osseous labyrinth, the vestibule of the ear (AC3). The vestibule communicates anteri-orly with the bony cochlea (C4) and at its posterior wall with the bony semicircularcanals (C5).
The vestibule contains two membranous parts, the saccule (AB6) and the utricle (AB7). Both structures contain sensory epithelium in a circumscribed part of the wall (blue), the macula of the saccule (AB8) and the mac-ula of the utricle (AB9), and are intercon-nected by the utriculosaccular duct (AB10). The latter gives off the slender endolym-phatic duct (A11) which runs to the poste-rior surface of the petrous bone and ends beneath the dura mater as a flattened ves-icle, the endolymphatic sac (A12). The unit-ing duct (AB13) forms a connection betweenthe saccule and the membranous cochlear duct.
The osseous cochlea (C4) has about two and a half turns. The spiral canal of thecochlea (C14) contains the membranous cochlear duct (AB15) which starts with ablind end, the vestibular cecum (B16), and ends in the tip of the cochlea, or cupula (C17), with the cupular cecum (B18). The perilymphatic spaces are above and belowthe cochlear duct, or scala media; the scalavestibuli(AB19) lies above it and opens intothe vestibule, and the scala tympani (AB20) lies beneath it and is closed by the cochlearwindow (A–C21).
The three bony semicircular canals (C5) emanating from the vestibule contain the membranous semicircular ducts (A22), which are connected to the utricle. They are surrounded by perilymph and attached to the walls of the perilymphatic space by con-nective-tissue fibers. The three semicircular ducts are arranged perpendicularly to each other. The convexity of the anterior semi-circular duct (B23) is oriented toward thesurface of the petrous pyramid, the posteriorsemicircular duct (B24) runs parallel to theposterior surface of the petrous bone, and the lateral semicircular duct (B25) runs hori-zontally.
Each semicircular duct has a dilatation at its transition to the utricle, the membranousampulla (B26), which corresponds to an os-seous ampulla in the bony canal. The ante-rior and the posterior semicircular ducts join to form the common membranous crus(AB27). Each ampulla contains sensory epithelium, the ampullary crest.
The courses taken by the semicircular ducts do not correspond to the axes of the body. The anterior and posterior semicircular ducts diverge from the median and frontal planes by 45!; the lateral semicircular duct is tilted in posterocaudal direction by 30!towards the horizontal plane.