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 at the same time as the water-to-land transition of vertebrates, approximately 380 million years ago. The actual coiling or spiral nature of the cochlea occurred to save space inside the skull. The longer the cochlea, the higher is the potential resolution of sound frequencies given the same hearing range. The oldest of the truly coiled mammalian cochleae were approximately 4 mm in length. The earliest evidence available for primates depicts a short cochlea with prominent laminae, suggesting that they had good high-frequency sensitivity as opposed to low-frequency sensitivity. After this, over a period of around 60 million years, evidence suggests