4.1.8 What is the Inner Ear?

The inner ear is a maze of bony passages collectively known as the labyrinth, which houses the organs of hearing and balance. It includes the cochlea, a spiral shaped bony chamber containing the hearing organ, the “organ of corti”.  It also includes the semi-circular canals which control the sense of balance.

The whole labyrinth is about a quarter of an inch across

The cochlea and the semi circular canals form a continuous chamber made out of solid bone and lined with membranous ducts. These chambers and passages contain the unique fluids of the inner ear.

The bony canals are filled with perilymph fluid, while the membranous ducts are filled with endolymph fluid.

The cochlea is divided into three chambers which run through the length of its spiral shape.

A membranous tube called the scala media (or the cochlear duct) forms the central chamber of the cochlea. Inside this tube is the hearing organ which is made up of the sensory hair cells called stereocilia. On either side of the scala media are two larger, bony canals called the scala vestibuli and the scala tympani

Sound waves coming from the middle ear enter the cochlea through the oval window under the footplate of the stirrup. From there they run up around the spiral through the scala vestibuli and then back down through the scala tympani.

At the bottom of the scala tympani is another membranous window called the round window. This allows for the release of pressure from strong vibrations running through the cochlea.

The central core of the cochlea, called the modiolus, contains the auditory nerve, the cochlear artery and cochlear vein.

Many branches of the nerve are extended into each of the hair cells that line the cochlea, so that the signals from every hair cell are transmitted to the nerve.

The vestibular portion of the ear consists of three semi circular canals and two chambers which specialize in the sense of balance and movement (the vestibular sense.)