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THE EAR

Ear and
other anatomical
models available
online via this link

The ear has two functions, but the second is less obvious:

1) detection of sounds - hearing.

2) detection of movement, and the position, of the head - balance.

Although we use the word "ear" for the projecting part (pinna), this should be called the outer ear because there are other, more functional, parts or regions - middle ear and inner ear - protected in the skull. Let us examine the role played by each part.

Hearing

Outer ear
Sound is a compression wave resulting from energy being passed on from vibrating objects around us, and as such it is caused by air molecules moving back and forth at different frequencies, according to the notes of the sound being listened to. It is thus a fundamentally different type of waveform than light, for instance. These sound waves are concentrated by the funnel-like pinnae and pass into the ear canal.

The pinnae are especially noticeable in certain animals, e.g. rabbit, donkey, etc., and in these cases they can also be swivelled around so as to detect sounds from different directions. The only group of animals which possess pinnae are mammals .

Middle ear

At the end of the ear canal is a thin sheet of skin called the ear drum . Here the compression waves are converted into vibrations . The ear drum therefore vibrates in time with the sound, and these vibrations are passed on to 3 small bones called ossicles. Because of their shapes, these are called hammer (malleus), anvil (incus) and stirrup (stapes). This part of the ear is air-filled but almost sealed, and there is a tube ("Eustachian tube") which connects with the throat, to equalise the internal pressure with the outside.

Inner ear

Vibrations in the ear ossicles are passed through another flexible "oval window" into a fluid-filled section, and the resulting changes in fluid pressure are channelled into a coiled structure called the cochlea.

According to the frequency of the note, sensory hairs in different parts of the cochlea are stimulated. These produce impulses which are passed up the auditory nerve to the brain, which then interprets them as the sensation of sound.

Use the mouse, or tap, to label this diagram of the ear, including the outer, middle, and inner ear sections.

What do you think might result if the middle ear was not at the same pressure as the outside?
> ears "bang"/ hearing becomes muffled - eardrum bulges one way

How do we usually get over this?
> swallow/suck sweets/blow nose etc.

Most hospitals have E.N.T. departments, amongst others.
What does E.N.T. stand for, and why are they lumped together?
> Ear Nose and Throat > all "connected"

Balance

Your brain uses information from your ears as well as your eyes in order to monitor your body's movement and position.

Semicircular canals, and other parts of the inner ear, also contain sensory cells which respond to movement of the fluid they contain. According to the plane of movement, one or more sets of cells (on each side of the head) will send impulses to the brain, which reacts by controlling the appropriate muscles to "keep balance".

The principle on which the semicircular canals operate is liquid inertia. The fluid inside the canals tends not to move when the head does, so the sensory hairs in the swellings on the canals are stimulated by the relative movement.

How does this explain giddiness when we spin round and round?
> the fluid keeps on moving (even after spinning stops), giving a false impression of movement
How do ballet dancers get round the problem in a pirouette?
> "spotting" with the eyes to give a point of visual reference

Ear and other anatomical models and charts available by following this link.

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