The middle ear’s three tiny bones act as a mechanical impedance transformer

Evidence

Eighteen hundred years ago, natural philosophers basically knew that sound entered through the eardrum on its journey to the brain via the auditory nerve. It wasn’t until 1543 that the “hammer” and “anvil” (the malleus and the incus) were described by Andreas Vesalius, a Belgian anatomist and physician (see figure 21.1). These are two of the three smallest bones in the human body: the ossicles, responsible for transmitting sounds coming from the eardrum to the cochlea. The “stirrup,” or stapes, was discovered shortly thereafter. The malleus and incus weigh in at only 25 milligrams each; the stapes, only 3 milligrams. Together with the tympanum, the job of these three bones is to provide a mechanical impedance transformer, improving impedance matching by a lever action, mediating the huge change between the low impedance of air outside the tympanum to the high impedance of the oval window. — Eric J. Heller, Why You Hear What You Hear- An…, 1940

Categorization

• DEFINE: Clarifies what the ossicles are and their impedance-transformer function.