Ears convert pressure waves passing through the air into electrochemical signals that the brain registers as sounds. The ear itself has three main parts called the outer, middle, and inner ear.
The outer ear, also called the pinna, incorporates a flap of skin and cartilage connected to an opening in the head leading to a short cul-de-sac, the external auditory meatus, or canal. The auditory canal is a tunnel about 1 inch (2.5 centimeters) long. It contains a lining of wax and hairs that block invasive insects and bacteria, but let sound waves through to the middle ear.
The air-filled chamber contains structures that transmit sound vibrations from the outer to the inner ear. The middle ear roughly resembles a six-sided chamber joined to the nasal cavity and throat by the Eustachian tube, which opens during yawning or swallowing to equalize air pressure.
The eardrum, or tympanic membrane, which stretches across the outer entrance of the chamber, is a thin, delicate sheet of tissue that vibrates at the frequencies of the sound waves arriving from the outer ear. As it vibrates, it transmits vibrations to three tiny, connected bones (ossicles) that span the chamber. Among the body’s smallest bones, these ossicles comprise the malleus (hammer), incus (anvil), and stapes (stirrup). The malleus and incus hang from the roof of the middle ear and are linked by synovial joints to each other and to the stapes. Ossicles pass on vibrations with diminished range of movement yet greatly increased pressure.
Inner ear: the cochlea
Different regions of the inner ear deal with sound and balance. Hearing depends upon the cochlea—a spiral tube resembling a snail’s shell, filled with fluid and divided lengthwise by the basilar membrane into upper and lower chambers, separated from the middle ear by the oval and round windows, respectively.
It is in the cochlea that vibrations transmitted through the ossicles trigger signals in a nerve communicating with the brain. The stapes vibrates against the oval window, which in turn transmits pressure waves through the fluid of the upper chamber of the cochlea. The round window vibrates freely to equalize pressure between the cochlea and middle ear. Meanwhile, waves set up resonance in the basilar membrane and the attached organ of Corti a tunnel flanked by hair cells that serve as auditory receptors. Disturbance of these cells stimulates fibers of the cochlear nerve. This forms part of the auditory nerve, which transmits signals to the “hearing centers” in the temporal lobes of the brain.
Whether we hear a sound as high or low in pitch depends on the part of the basilar membrane that resonates most strongly. Low-frequency pressure waves are detected where the membrane is broadest, and high-frequency waves have their effect near its narrow end.
Much as both eyes work together to help us judge depth and distance, so both ears help us to determine where a sound comes from. This auditory sense is much less well developed than the corresponding visual sense, however, and the ears themselves are less suited than the eyes to distance judgment and direction finding.
Age, internal injury, or disease may result in hearing loss in one ear or both ears, but there are also a number of ways that deafness can be overcome. Disease that prevents sound vibrations from passing through the middle ear is not sufficient alone to prevent hearing completely because some vibrations find their way to the cochlea by way of the skull bones. Hearing aids set in these bones make use of this. Aids fitted in the outer ear can often help to compensate for damage to the inner ear. And because some acoustic nerve fibers cross from one side of the brain to the other on their way to the tops of the temporal lobes, damage to one temporal lobe need not necessarily cause deafness in the ear on that side.
Inner ear: vestibular system
The inner ear’s vestibular system helps us to keep our balance, even with closed eyes. The system consists of three semicircular tubes, called canals, at right angles to one another, and two sacs (the saccule and utricle), all filled with fluid and located near the cochlea. The canals broaden at one end into flask-shaped chambers (ampullae). Each chamber has a gelatinous capsule containing the hair cells of a receptor organ. The saccule and utricle contain gelatinous masses called static receptors, weighted with crystals called otoliths. When the head moves, fluid flows through the canals and sacs, disturbing the gelatinous masses and hairs, and generating signals in nerve endings near the hairs’ roots.
Different head movements and positions stimulate different groups of nerve endings in the vestibular system. The superior semicircular canal registers nodding, the posterior canal detects tilting, and the lateral (or horizontal) canal responds to turning. Different positions of the head are registered by the saccule and utricle, because these cause different weight distributions of the otoliths, which affect nerve endings in these parts of the vestibular system.
From the vestibular system, signals pass through the vestibular nerve, which merges with the cochlear nerve to form the auditory nerve. From the vestibular nerve, many fibers pass directly to the cerebellum, where they assist limb, eye, and trunk coordination.