Form 2 Scientist : Sensory Organs Part 3

The Sense of Hearing

Structure of the Human Ear

1. The ear is the sensory organs of hearing and balance.
2. The sense of hearing is sensitive to sound stimuli.
3. The human ear can be divided into three main parts : the outer ear, the middle ear and the inner ear.
4. The figure below shows the cross section of the human ear.

• Pinna - collects and directs sound waves (vibrating air) into the auditory canal.
• Auditory canal - directs sound waves to the eardrum
• Eardrum - vibrates at the same frequency as the sound waves that hit it and transfer the vibrations to the ossicles
• Ossicles - amplify the vibrations and transfer them from the eardrum to the oval window
• Oval Window - transfer the vibrations from the ossicles to the inner ear
• Round Window - vibrates in opposition to the oval window lying above it
• Semicircular canals - help the body maintains its balance (not involved in the mechanism of hearing)
• Eustachian tube - helps the balance the air pressure on both sides of the eardrum (not involved in the hearing mechanism).

The hearing mechanism 

The pinna gathers sound waves and directs them to the auditory canal

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The sound waves travel along the auditory canal and reach the eardrum. It vibrates at the same 

frequency as the sound waves.

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The vibrations are transferred to the ossicles. The ossicles amplify the vibrations 20 times

in magnitude before passing them to the oval window.

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Vibrations of the oval window cause the fluid in the cochlea to vibrate.

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The movement of the fluid stimulates the receptors in the cochlea.

Nerve impulse are generated.

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The auditory nerve sends the nerve impulse to the brain.

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The brain interprets the messages as sounds.