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Portrait of The anatomy of the ear

Science · Medicine & the body

The anatomy of the ear

Organ of Corti described 1851

Medicine — the auditory and vestibular organ

The organ of hearing and of balance, built in three parts. The outer part funnels sound down a canal to the tympanic membrane; the air-filled middle cavity carries the vibration across three tiny bones — the malleus, incus and stapes — to the oval window; and the fluid-filled inner labyrinth converts it into nerve impulses in the spiral cochlea while three semicircular canals report the position and acceleration of the head. The three bones are the smallest in the body, and the innermost of them, the stapes, is the smallest bone a human has.

How to spot them

  • ▸Three tiny bones in the middle cavity: malleus, incus, stapes — hammer, anvil and stirrup
  • ▸The stapes is the smallest bone in the human body; the stapedius is the smallest skeletal muscle
  • ▸A snail-shaped spiral of about two and a half turns — the cochlea — and three semicircular canals for balance
  • ▸The tube named after Bartolomeo Eustachi runs to the nasopharynx and equalises pressure
  • ▸Cranial nerve VIII, the vestibulocochlear nerve, carries both hearing and balance

Key facts

A hammer, an anvil and a stirrup, in that order

The three auditory ossicles: malleus, incus and stapes

The smallest bone in the human body, about three millimetres long, pressing on an oval window

The stapes, the innermost of the three ossicles

A fluid-filled bony spiral of about two and a half turns, tuned high at its base and low at its apex

The cochlea; the tonotopic map along the basilar membrane

Three fluid-filled loops set at right angles to one another, reporting rotation of the head

The semicircular canals, part of the vestibular apparatus

The spiral organ named after the Italian Alfonso Corti, described in 1851, with its inner and outer hair cells (1851)

The organ of Corti, sitting on the basilar membrane

Georg von Békésy's 1961 Nobel Prize for travelling waves along the basilar membrane (1961)

The only Nobel Prize awarded for work on the mechanics of this organ

The tube named after a sixteenth-century Italian anatomist that runs down to the nasopharynx and pops on a plane

The Eustachian or pharyngotympanic tube, which equalises pressure

Endolymph inside the membranous labyrinth, perilymph outside it

The two fluids of the inner labyrinth

Key ideas

  • The middle cavity is an impedance matcher: airborne sound would mostly bounce off fluid, so the three bones and the size ratio between the drum and the oval window concentrate the force about twentyfold.
  • Pitch is coded by place. A travelling wave peaks near the stiff base of the cochlea for high frequencies and near the floppy apex for low ones, so the basilar membrane is a frequency analyser.
  • The same labyrinth does two jobs: the cochlea hears, while the utricle, saccule and semicircular canals sense gravity, linear acceleration and rotation.

Quick-fire facts

  • •Three parts: outer (pinna or auricle and external auditory canal), middle (tympanic cavity), inner (bony and membranous labyrinth)
  • •The three ossicles are the malleus (hammer), incus (anvil) and stapes (stirrup) — the smallest bones in the body
  • •The stapes, about 3 mm long, is the smallest and lightest human bone; the stapedius, which damps it, is the smallest skeletal muscle
  • •The ossicles are the only bones that are full-sized at birth and do not grow afterwards
  • •The tympanic membrane separates outer from middle; the oval and round windows separate middle from inner
  • •The cochlea has about two and a half turns and three fluid channels: scala vestibuli, scala media (cochlear duct) and scala tympani
  • •The receptor cells sit in the organ of Corti, described by Alfonso Corti in 1851, under the tectorial membrane
  • •The membranous labyrinth is filled with endolymph and bathed in perilymph
  • •The utricle and saccule contain otoliths and detect gravity and linear acceleration; the three semicircular canals detect rotation
  • •The vestibulocochlear nerve is the eighth cranial nerve
  • •Georg von Békésy won the 1961 Nobel Prize in Physiology or Medicine for the physical mechanism of stimulation within the cochlea
  • •Otosclerosis fixes the stapes and is treated by stapedectomy; otitis media is infection of the middle cavity
  • •The normal human range of hearing is roughly 20 Hz to 20 kHz, narrowing with age (presbycusis)

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