The bony walls of the vestibule and of the semicircular canals inclose and protect a membranous apparatus of the same form, which is separated from them by a space filled with a limpid fluid called perilymph or liquor Cotunnii. The membranous labyrinth is therefore smaller in proportion than the osseous; the difference in size is about one-half. Its cavities contain a fluid analogous to the perilymph, and which De Blainville has compared to the vitreous humor of the eye; they also contain semi-transparent tubes and membranous sacs, the appearance of which is closely analogous to that of the retina. The membranous vestibule is composed of two distinct parts, the saccule and utricle, in which there exists a calcareous dust, which appears to represent in man and the mammifera the auditory stones or otoliths of fishes.