Cochlea a protein motor called prestin on their outer membranes; it generates additional movement that couples back to the fluid–membrane wave. This "active amplifier" is essential in the ear's ability to amplify weak sounds. The active amplifier also leads to the phenomenon of soundwave vibrations being emitted from the cochlea back into the ear canal through the middle ear (otoacoustic emissions). Otoacoustic emissions are due to a wave exiting the cochlea via the oval window, and propagating back through the middle ear to the eardrum, and out the ear canal, where it can be picked up by a microphone. Otoacoustic emissions
Stirrup remained in use until the late Heian period when a new stirrup was developed. The "fukuro abumi" or "musashi abumi" had a base that extended the full length of the rider's foot and the right and left sides of the toe cup were removed. The open sides were designed to prevent the rider from catching a foot in the stirrup and being dragged. The military version of this open-sided stirrup ("shitanaga abumi") was in use by the middle Heian period. It was thinner, had a deeper toe pocket and an even longer and flatter foot shelf. This stirrup stayed in