Suspended within the perilymph but not filling all spaces of the bony labyrinth is the membranous labyrinth, which consists of the semicircular ducts, the cochlear duct, and two sacs (the utricle and the saccule). These membranous spaces are filled with endolymph.
The structures in the internal ear convey information to the brain about balance and hearing:
The cochlear duct is the organ of hearing.