The Human Body

The Middle Ear

The eardrum separates the outer ear from the middle ear. A narrow passageway called the eustachian tube connects the middle ear to the throat and the back of the nose. The eustachian tube helps keep the eardrum intact by equalizing the pressure between the middle and outer ear. For example, if a person travels from sea level to a mountaintop, where air pressure is lower, the eardrums may cause pain because the air pressure in the middle ear becomes greater than the air pressure in the outer ear. When the person yawns or swallows, the eustachian tube opens, and some of the air in the middle ear passes into the throat, adjusting the pressure in the middle ear to match the pressure in the outer ear. This equalizing of pressure on both sides of the eardrum prevents it from rupturing.

The middle ear is a narrow, air-filled chamber that extends vertically for about 15 mm (about 0.6 in) and for nearly the same distance horizontally. Inside this chamber is a linked chain of three ossicles, or very small bones. Both the Latin and common names of these bones are derived from their shapes. They are called the malleus, or hammer; the incus, or anvil; and the stapes, or stirrup, which is the tiniest bone in the body, being smaller than a grain of rice.

The hammer is partly embedded in the eardrum, and the stirrup fits into the oval window, a membrane that fronts the inner ear. Vibrations of the eardrum move the hammer. The motion of the hammer moves the anvil, which in turn moves the stirrup. As sound vibrations pass from the relatively large area of the eardrum through the chain of bones, which have a smaller area, their force is concentrated. This concentration amplifies, or increases, the sound just before it passes through the oval window and into the inner ear. When loud noises produce violent vibrations, two small muscles, called the tensor tympani and the stapedius, contract and limit the movement of the ossicles, thus protecting the middle and inner ear from damage.