The ears contain structures for both the sense of hearing and the sense of balance. The eighth cranial nerve (vestibulocochlear nerve made up of the auditory and vestibular nerves) carries nerve impulses for both hearing and balance from the ear to the brain.
Sound waves cause the tympanic membrane (eardrum) to vibrate. Humans can hear sounds waves with frequencies between 20 and 20,000 Hz. The three bones in the ear (hammer, anvil and stirrup) pass these vibrations on to the cochlea. The cochlea is a snail-shaped, fluid-filled structure in the inner ear. Inside the cochlea is another structure called the organ of Corti. Hair cells are located on the basilar membrane of the cochlea. The cilia (the hair) of the hair cells make contact with another membrane called the tectorial membrane. When the hair cells are excited by vibration, a nerve impulse is generated in the auditory nerve. These impulses are then sent to the brain.
(By the way...the stapes is the smallest bone in the human body. It is only 0.25 to 0.33 cm long [0.10 to 0.13 inches] and weighs only 1.9 to 4.3 milligrams.)
