How the ear works
"What matters deafness of the ears when the mind hears? The one true deafness, the incurable deafness, is that of the mind." Victor Hugo
The ears are very important and very complex organs. Apart from their obvious function in hearing, they are also a very important part of the balance system. Each ear has more than 17,000 tiny hairs, which are a vital part of the hearing process, and the smallest three bones in the body.
The ears have three sections – the outer ear, the middle ear, and the inner ear. Each of the three sections are described in detail below.
The Outer Ear
The outer ear is made up of two parts, the pinna, the part you can see on the side of your head, and the ear canal – the passage sound travels along. The pinna helps to collect sounds and direct them along the ear canal. The canal is about 2.5cm or 1 inch long and lined with skin. Near to the outside the canal has hairs and glands that produce wax that keep it clean and help prevent infection. The hairs, called cilia move the wax outwards and that is why wax comes out of the ear.
At the other end is the tympanic membrane, the eardrum, which covers the end of the canal. Sound is small fluctuations in air pressure, and sound reaching the eardrum makes it vibrate.
The Middle Ear
The middle ear is an air filled space deep within the skull behind the eye socket about 1cm across. For the ear drum to be sensitive to sound, the air pressure in the middle ear must be the same pressure as the air in the outer ear. The middle ear is connected to the nose and throat by the Eustachian tube, which equalizes the pressure between the two.
Between the middle ear and the inner ear is a wall of bone that has two small openings in it, the oval window and the round window, sealed by a membrane.
Stretching across the middle ear in a chain are three tiny bones, the smallest in the body, collectively called the ossicles. They are the malleus, incus, and stapes. They are also known as the hammer, anvil and stirrup because of their shapes.
The malleus is attached to the eardrum. The incus stretches between the malleus and the stapes. The end of the stapes fits into the oval window. When sound enters the ear, the eardrum vibrates and the vibrations are passed across the middle ear by the ossicles to the inner ear.
The Inner Ear
The inner ear has two parts, the cochlea and the vestibular apparatus, which are both embedded in bone.
The Cochlea
The cochlea is the hearing part of the inner ear. It is a spiral tube about 3.5cm long that coils around, and is called the cochlea because it looks like a shell. The spiral contains two fluid–filled chambers. The outer chamber starts at the oval window, goes to the end of the cochlea and then doubles back to the round window. Vibrations caused by sound pass from the stapes through the oval window and into the fluid in the outer chamber.
The inner central chamber of the cochlea contains the Organ of Corti, which has thousands of small ‘hairs’ along the length of the cochlea. Each hair is connected to nerve fibres that join other similar fibres to form the auditory nerve which goes from the cochlea to the brain.
Sound waves entering the fluid in the cochlea move the hairs, which send electrical messages to the auditory nerve. Different frequencies of sound are picked up by different hair cells. The auditory nerve passes the electrical impulses to the brain, which recognises them as different sounds.
The vestibular apparatus
The second part of the inner ear is the vestibular apparatus. The vestibular apparatus is made up of the saccule, utricle and semicircular canals, and plays an important role in balance. The saccule, utricle and semicircular canals aren't used for hearing. They are part of the balance system. They're filled with fluid called endolymph and have hair cells, similar to those in the cochlea. They are connected to the vestibular nerve and electrical signals from the stereocilia, caused by fluid moving them when you move, are sent along the vestibular nerve to the brain, where they are processed, giving your brain information about the position of your head.
Some problems that affect hearing can also affect the delicate stereocilia, and some deaf people also have balance problems.
The ear is a very complicated organ and there is much that can go wrong. Our article, "When hearing goes wrong" looks at some common causes of deafness.
