Hearing:
Pinna collects and amplifies sound waves which then pass along the auditory canal to the eardrum.
i. Sound waves strike the eardrum and cause vibrations in its thin stretched membrane. The eustachian tube equalizes air pressure on either side of eardrum which allows a free vibration.
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ii. The vibrations reach ear ossicles in the middle ear. Ear ossicles transmit vibrations from the eardrum to the denser fluid in the inner ear.
iii. The lever-like action of malleus and incus magnifies the vibrations of the stapes.
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iv. The vibrating stapes transmits vibrations to the membrane of the oval window.
v. Vibrations from oval window get transmitted to cochlea. This leads to vibration in the fluid of cochlear canals.
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vi. Vibration of fluid in cochlear canals triggers movement of sensory hair cells of organ of Corti in cochlea.
vi. Movement of sensory hair cells is converted to a nerve impulse.
vii. Nerve signal is transmitted to brain via auditory nerve and this result in hearing.
Balancing:
The sensory hair cells in semi-circular canals are concerned with dynamic balance, i.e. while the body is in motion. Similarly, sensory cells in utriculus and sacculus are concerned with static balance with respect to the centre of gravity.
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Movement of fluid inside semi-circular canals triggers sensory hair cells of ampulla. This sensation passes to nerve cells and then to brain. The three canals are at right angles to each other so that the brain can detect even slightest tilting of the body in any direction.