In the middle ear, the three smallest bones in the human body​—the malleus, incus, and stapes, commonly called hammer, anvil, and stirrup—​form a bridge linking the eardrum with the oval window, the portal to the inner ear.

Middle Ear​—A Mechanic’s Dream

The job of the middle ear is to transform the acoustical vibration of the sound wave into mechanical vibration and pass it on to the inner ear. What takes place in this pea-​sized chamber is truly a mechanic’s dream.

Contrary to the notion that loud sounds cause significant movement of the eardrum, sound waves actually do so by only microscopic amounts. Such minuscule movement is hardly enough to cause the fluid-filled inner ear to react. The way this obstacle is overcome demonstrates once again the ingenious design of the ear.

The linkage of the three little bones of the middle ear is not only sensitive but also efficient. Functioning as a lever system, it magnifies any incoming forces by about 30 percent. Furthermore, the eardrum is about 20 times larger in area than the footplate of the stirrup. Thus, the force exerted on the eardrum is concentrated on a much smaller area at the oval window. These two factors together amplify the pressure at the vibrating eardrum to 25 to 30 times as much at the oval window, just enough to set the fluid in the cochlea in motion.

Do you find that a head cold sometimes affects your hearing? This is because proper operation of the eardrum requires that the pressure on either side of it be equal. Normally this is maintained by a small vent, called the Eustachian tube, that connects the middle ear with the back of the nasal passage. This tube opens every time we swallow and relieves any pressure build-​up in the middle ear.

MIDDLE EAR

Hammer

Anvil

Stirrup

Eustachian tube