(a) Explain briefly how the human ear carries out its function of balancing. (b) (i) make a drawing 8 cm - 10 cm long of the structure of the human ear and ...

(a) How the human ear carries out its function of balancing

Balance is controlled by the inner ear, using the three semicircular canals together with the utricle and saccule.

• The three semicircular canals are set at right angles to one another, so they detect movement of the head in all three planes.
• Each canal is filled with fluid (endolymph) and has a swollen base, the ampulla, which contains sensory hair cells.
• When the head moves or turns in any direction, the endolymph moves and bends the sensory hair cells in the ampulla, setting up nerve impulses.
• The utricle and saccule contain tiny chalk grains (otoliths) that respond to the position of the head with respect to gravity.
• The impulses travel along the auditory (vestibular) nerve to the brain (cerebellum), which interprets them and relays impulses to the body muscles so that posture is adjusted and balance is maintained.

(b)(i) Drawing of the structure of the human ear (8 cm to 10 cm long, fully labelled)

(b)(ii) Structure of the middle ear in humans

The middle ear is an air-filled chamber in the skull.

• It is separated from the outer ear by the ear drum (tympanic membrane / tympanum).
• It contains three small bones, the ear ossicles: the malleus (hammer), the incus (anvil) and the stapes (stirrup), linked in a chain.
• The malleus is attached to the tympanic membrane, each bone is in direct contact with the next, and the stapes fits into the oval window (fenestra ovalis) of the inner ear. In this way the ossicles transmit and amplify the sound vibrations from the ear drum to the oval window.
• The Eustachian tube arises at the base of the middle ear and links it to the back of the throat (pharynx), equalising the air pressure on both sides of the ear drum.