4.11.8 The membranous labyrinth and fluids of the inner ear
The component parts of the membranous labyrinth are the three semicircular ducts, each with a sense organ for rotational movement; the cochlear duct, with the sense organ for sound vibrations, and in the vestibule, the utricle and saccule, with their two sense organs for linear movement.
The utricle and saccule connect to the small endolymphatic duct, which goes to a structure not shown here, the endolymphatic sac.
The membranous labyrinth is bounded by a continuous membrane, which forms an essential barrier between the fluid outside it, which is perilymph, and the quite different fluid inside it, endolymph.
In a cross-section, for example of this semicircular canal, this is the bony wall of the semicircular canal, that's part of the bony labyrinth, this is the membranous wall of the semicircular duct, that's part of the membranous labyrinth. The fluid inside the membrane is endolymph; the fluid outside it is perilymph. The difference between the two fluids is essential for the function of the sense organs of hearing and balance.