Thus, the triangular-shaped cochlear duct has: an outer wall against the bony cochlea consisting of thickened, epithelial-lined periosteum (the spiral ligament), a roof (the vestibular membrane), which separates the endolymph in the cochlear duct from the perilymph in the scala vestibuli and consists of a membrane with a connective tissue core lined on either side with epithelium, and a floor, which separates the endolymph in the cochlear duct from the perilymph in the scala tympani and consists of the free edge of the lamina of the modiolus, and a membrane (the basilar membrane) extending from this free edge of the lamina of the modiolus to an extension of the spiral ligament covering the outer wall of the cochlea.