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Ear

The ears are the organs of hearing and of balance. The ear consists of three parts - the outer, middle and inner ear. The outer and middle parts are concerned with hearing, and parts of the inner ear are concerned with balance as well as hearing.

The outer ear consists of the part we can see, i.e. the ear flap (pinna) guarding the ear canal, which links the outer ear with the middle ear. The ear canal is also protected by tiny hairs, sweat glands and oil glands which produce wax to stop particles of dust and dirt from getting in. The ear canal is about 2.5cm long.

At the end of the ear canal is a six-sided box which is the middle ear. Four sides of the box are made of bone but the fifth side opens into the Eustachian tube and the sixth side, which is the one facing into the ear canal, is covered by the thin, transparent membrane that forms the eardrum. Inside the box are the three tiniest bones in the human body, commonly called after their respective shapes the hammer, the anvil and the stirrup, but more correctly known as the malleus, incus and stapes. The stapes is the smallest bone in the body. Sound waves are collected by the outer ear, passed down the ear canal to vibrate the eardrum and then into the middle ear where the sounds are amplified by the tiny bones.

The Eustachian tube connects the middle ear to the back of the nose. In essence, sound consists of small fluctuations in air pressure and this tube enables the same pressure to be maintained in the middle ear as in the outside atmosphere, so that the middle ear can pick up the sound waves. If the outside pressure alters more quickly than the middle ear can adjust to, such as when we are flying or diving, the ear will hurt. The familiar pop of the ears in these circumstances is the pressure adjusting by air suddenly moving through the eustachian tube. The connection of the tube to the throat means that phlegm and mucus sometimes travel along it and cause middle-ear infections.

The inner ear is filled with fluid and contains the cochlea (named because it is shaped like a cockle or snail shell) which is the part where hearing occurs. Sound passes from the middle ear through a fluid-filled chamber and into the cochlea where, in a tiny hair-lined section called the organ of Corti, it is converted into nerve impulses. These nerve impulses are then transmitted to the brain by the auditory nerve which registers them as sounds.

The inner ear is also the organ of balance. Above the cochlea are three semicircular canals set at different angles (the vestibular apparatus). These are filled with fluid which moves as the head moves. Highly sensitive hairs pick up the movement and send impulses to the brain indicating the position of the head and body.