Inner ear: A highly complex structure whose essential component for hearing is the membranous labyrinth, where the fibers of the auditory nerve connect the ear to the brain. The membranous labyrinth is a system of communicating sacs and ducts (tubes) filled with fluid (endolymph), and it is lodged within a cavity called the bony labyrinth. At some points the membranous labyrinth is attached to the bony labyrinth, and at other points the membranous labyrinth is suspended within the bony labyrinth in a fluid called perilymph. The bony labyrinth has three parts: a central cavity called the vestibule; semicircular canals, which open into the vestibule; and a spiraling tube called the cochlea. The membranous labyrinth also has a vestibule, which consists of two sacs (the utriculus and sacculus) that are connected by a narrow tube. The larger of the two sacs, the utriculus, is the principal organ of the vestibular system, which is the system of balance. This system informs a person about the position and movement of the head. The smaller of the two sacs, the sacculus, is also connected by a membranous tube to the cochlea that contains the organ of Corti. The hair cells, which are the special sensory receptors for hearing, are in the organ of Corti.