In the cochlea, sound waves are converted into an electrical signal at the organ of Corti. The organ of Corti rests upon the basilar membrane in the scala media, which is filled with endolymph (eFig. 9.80). In addition to contributing to the formation of the scala media with the vestibular membrane, the basilar membrane separates the cochlea into the scala vestibuli and scala tympani, which are filled with perilymph and are continuous with one another at the helicotrema (eFig. 9.80).