Cochlea to structures in the brainstem for further processing. The "stapes" (stirrup) ossicle bone of the middle ear transmits vibrations to the "fenestra ovalis" (oval window) on the outside of the cochlea, which vibrates the perilymph in the "vestibular duct" (upper chamber of the cochlea). The ossicles are essential for efficient coupling of sound waves into the cochlea, since the cochlea environment is a fluid–membrane system, and it takes more pressure to move sound through fluid–membrane waves than it does through air; a pressure increase is achieved by the area ratio of the tympanic membrane to the oval window, resulting in
Evolution of the cochlea cells and their afferent innervating fibers can be very sharply tuned to frequency. In birds and crocodilians, the similarity of the structure of the basilar papilla betrays their close evolutionary relationship. The basilar papilla is up to about 10mm long and contains up to 16500 hair cells. While most birds have an upper hearing limit of only about 6 kHz, the barn owl can hear up to 12 kHz and thus close to the human upper limit. Egg-laying mammals, the monotremes (spiny anteater and platypus), do not have a spiral cochlea, but one shaped more like a banana, up to