incus

Meaning: The incus, or anvil, is the middle of the three ossicles, the tiny bones of the middle ear. Sound vibrates the tympanic membrane, the malleus (hammer) passes the vibration to the incus, and the incus passes it to the stapes (stirrup). The stapes pushes on the oval window of the cochlea, moving the fluid inside. The eardrum is much larger than the oval window and the ossicles act as a lever, so pressure is amplified.

On the MCAT: Expect questions that ask you to order the path of sound: pinna, auditory canal, tympanic membrane, malleus, incus, stapes, oval window, cochlea, hair cells, auditory nerve. A passage may describe damage to one structure and ask whether the problem is conductive (outer or middle ear) or sensorineural (cochlea or nerve). Ossicle problems typically produce conductive loss.

Common confusion: Students think the ossicles convert sound into neural signals. They only transmit and amplify mechanical vibration. Transduction happens when hair cells on the basilar membrane in the cochlea bend. Remember the order with M-I-S: malleus, incus, stapes, from the eardrum inward.