Final answer:
The cochlea is the part of the inner ear that is set into vibration by the third ossicle, known as the stapes or stirrup. The three auditory ossicles transfer sound vibrations from the eardrum through to the cochlea, where they are transformed into electrical signals interpreted by the brain.
Step-by-step explanation:
The membrane vibrated by the third bone of the ossicles initiates the vibration of the cochlea. The auditory ossicles consist of three bones: the malleus (hammer), the incus (anvil), and the stapes (stirrup), which are the smallest bones in the human body and unique to mammals. These three bones transfer the vibrations from the eardrum to the fluid-filled cochlea. Specifically, the hammer vibrates the anvil, which in turn vibrates the stirrup. The stirrup then presses against the oval window, another membrane, initiating the vibration of the cochlea. When the vibrations reach the cochlea, they cause fluid inside it to move, stimulating tiny hair cells that convert these mechanical vibrations into electrical signals for the brain to interpret as sound.