How Does the Ear Work?
ByThe ears are responsible for hearing and balance. Each ear has an outer, a middle and an inner part. All three are involved in hearing, but only the inner part with balance.
The outer ear (pinna, meaning ‘wing’ in Latin) funnels sound down the ear canal. This ends at the ear-drum, which separates the outer ear from the middle ear. The lining of the canal produces wax which has a cleansing action and contains natural antiseptics.
The air-filled cavity of the middle ear is connected to the back of the throat by the Eustachian tube. This tube drains fluid from the middle ear to the throat. It also allows the equalization of air pressure between the middle ear and the air outside, so preventing acute earache with changes of atmospheric pressure – for instance when going up or down in a lift, or when flying. If the tube is blocked, such pressure changes can be painful.
The middle ear cavity is also connected to the air cells of the mastoid bone, which is why untreated middle ear infections can spread and cause mastoiditis.
The middle ear is connected to the inner ear by the oval window. The inner ear, or labyrinth, which contains the spiral, snail-shaped hearing organ, the cochlea, is embedded in bone and filled with a fluid called endolymph. Also in the inner ear are gravity-sensitive cells and three semi-circular canals at right angles to each other. The fluid inside these canals moves with the body and sends messages to the brain about its state of balance.
The ear is like a microphone. It receives sound waves, converts them into electrical impulses and sends messages to the brain. The sound waves collected by the outer ear make the ear-drum vibrate. These vibrations are passed along a chain of three tiny bones – the ossicles – in the middle ear. The bones, known as the hammer (malleus), the anvil (incus) and the stirrup (stapes), are jointed together and their lever action magnifies the vibrations. The hammer is attached to the ear-drum and the ‘footplate’ of the stirrup fits into the oval window. Vibrations pass through the oval window into the inner ear, where special cells in the cochlea sense the vibrations as pressure changes and relay them as electrical impulses along the auditory nerve to the brain. The brain cuts out unnecessary sounds and focusses on those the child wishes to hear. Sound vibrations finally leave the inner ear via the round window.
When things go wrong
Earache is a common sign that something is wrong with the ear. Pain can arise from the outer ear (perhaps from a boil or a foreign body), the middle ear or the ear-drum. It can also be ‘referred’ from areas outside the ear which share a common nerve supply with the outer ear.
However, the commonest cause of earache is inflammation of the middle ear (otitis media) which occurs with many upper respiratory tract infections, such as colds and flu, and with measles, sore throats, whooping cough and infected tonsils and adenoids. Infection spreads from the throat up the Eustachian tube and makes its lining swell. This blocks the tube, causing the absorption of the air normally present in the middle ear, and prevents fluid (formed as a result of infection spreading to the middle ear) from draining out. The child’s hearing seems muffled and her own voice sounds different to her.
If there is a bacterial infection, pus forms in the middle ear and its pressure makes the inflamed ear-drum bulge outwards into the outer ear canal. The ear-drum is relatively thick in babies which explains why even a small amount of fluid or pus in the middle ear can cause such pain. If untreated, the eardrum can eventually perforate, relieving the pain, but causing a discharge from the ear.
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