Reception of Sound

In mammals, sound waves are collected by the external, cartilaginous part of the ear called the pinna, then travel through the auditory canal and cause vibration of the thin diaphragm called the tympanum or ear drum, the innermost part of the outer ear (illustrated in the figure below). Interior to the tympanum is the middle ear. The middle ear holds three small bones called the ossicles, which transfer energy from the moving tympanum to the inner ear.

The three ossicles are the malleus (also known as the hammer), the incus (the anvil), and stapes (the stirrup). The aptly named stapes looks very much like a stirrup. The three ossicles are unique to mammals, and each plays a role in hearing. The malleus attaches at three points to the interior surface of the tympanic membrane. The incus attaches the malleus to the stapes.

In humans, the stapes is not long enough to reach the tympanum. If we did not have the malleus and the incus, then the vibrations of the tympanum would never reach the inner ear. These bones also function to collect force and amplify sounds. The ear ossicles are homologous to bones in a fish mouth: the bones that support gills in fish are thought to be adapted for use in the vertebrate ear over evolutionary time. Many animals (frogs, reptiles, and birds, for example) use the stapes of the middle ear to transmit vibrations to the middle ear.

The illustration shows the parts of the human ear. The visible part of the exterior ear is called the pinna. The ear canal extends inward from the pinna to a circular membrane called the tympanum. On the other side of the tympanum is the Eustachian tube. Inside the Eustachian tube the malleus, which touches the inside of the tympanum, is attached to the incus, which is in turn attached to the horseshoe-shaped stapes. The stapes is attached to the round window, a membrane in the snail shell-shaped cochlea. Another window, called the round window, is located in the wide part of the cochlea. Ring-like semicircular canals extend from the cochlea. The cochlear nerve and vestibular nerve both connect to the cochlea.

Sound travels through the outer ear to the middle ear, which is bounded on its exterior by the tympanic membrane. The middle ear contains three bones called ossicles that transfer the sound wave to the oval window, the exterior boundary of the inner ear. The organ of Corti, which is the organ of sound transduction, lies inside the cochlea. (credit: modification of work by Lars Chittka, Axel Brockmann)

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Sensory Systems

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