Round window

The round window is one of two membranes that separates the inner ear from the middle ear. (The oval window is the other. The oval window leads to the scala vestibuli, which leads to the helicotrema, which leads to the scala tympani, which leads to the round window.)

The round window is a membrane that is situated below and a little behind the fenestra vestibuli, from which it is separated by a rounded elevation, the promontory.

It is placed at the bottom of a funnel-shaped depression (the round window niche) and, in the macerated bone, leads into the cochlea of the internal ear; in the fresh state it is closed by a membrane, the secondary tympanic membrane, which is a complex saddle point shape. The visible central portion is concave toward the tympanic cavity and convex toward the cochlea but towards the edges, where it is hidden in the round window niche, it curves the other way.

This membrane consists of three layers:
 * an external, or mucous, derived from the mucous lining of the tympanic cavity;
 * an internal, from the lining membrane of the cochlea;
 * and an intermediate, or fibrous layer.

As the oval window membrane moves in, the round window membrane moves out, keeping the pressure equalized. It is smaller than the oval window because as the sound waves are passed through the oval window the same amount has to fit through the round window, however, it is smaller so the waves are packed tighter causing an increase in pressure, allowing the waves to be pushed around the cochlea.