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 forces for the larger cochlear labyrinth may have started after the basal primate node. Mammals are the subject of a substantial amount of research not only because of the potential knowledge to be gained regarding humans, but also because of their rich and abundant representation in the fossil record. The spiral shape of the cochlea evolved later on in the evolutionary pathway of mammals than previously believed, just before the therians split into the two lineages marsupials and placentals, about 120 million years ago. Parallel to the evolution of the cochlea, prestins show an increased rate of evolution in therian