The Ear: Structure and Function

The ear is the sense organ responsible for two quite distinct senses — hearing (detecting sound) and equilibrium/balance (detecting the position and movement of the head) — and it is structurally divided into three main parts: the outer ear, the middle ear and the inner ear.

The outer ear consists of the pinna, the visible fleshy flap that helps collect sound waves from the surroundings and funnel them inward, and the external auditory canal, a short tube leading from the pinna to the tympanic membrane (eardrum), a thin, taut membrane that vibrates in response to the sound waves reaching it. The middle ear is a small, air-filled cavity lying just beyond the eardrum, and it contains a chain of three tiny bones (ossicles) — the malleus, the incus and the stapes — which mechanically link the eardrum to a second, smaller membrane called the oval window; because the ossicles act as a lever system concentrating the vibration from the relatively large eardrum onto the much smaller oval window, they amplify the sound vibrations considerably as they pass through. The middle ear cavity is connected to the back of the throat (pharynx) by the Eustachian tube, whose function is to equalise air pressure between the middle ear and the outside atmosphere (for example, when swallowing or yawning), which keeps the eardrum free to vibrate normally; without this equalisation, pressure differences across the eardrum would distort hearing and cause discomfort.

The inner ear is a complex, fluid-filled system embedded in bone and contains the actual sensory apparatus for both hearing and balance. For hearing, it contains the coiled, snail-shell-shaped cochlea, which is filled internally with fluids (perilymph and endolymph); vibrations transmitted through the oval window set this fluid in motion, and the resulting movement stimulates specialised hair cells within the organ of Corti (which runs along the length of the cochlea), converting the mechanical vibration into a nerve impulse that travels along the auditory (cochlear) nerve to the auditory area of the cerebral cortex, where it is perceived as sound. …