|View of the inner wall of the tympanum. (label is 'fen. rotund.' - lower of two black circles.)|
|Interior of right osseous labyrinth. (label is 'cochlear fenestra', at bottom center.)|
|Latin||fenestra cochleae, fenestra rotunda|
|Gray's||subject #232 1051|
The round window is one of two openings that connect the inner ear to 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 situated below and a little behind the oval window, 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, opens into the cochlea of the internal ear; in the fresh state it is closed by a membrane, the secondary tympanic membrane or round window 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.
Both the oval and round windows are about the same size, approximately 2.5 mm2. The entrance to the round window niche can often be much smaller than this.
As the oval window membrane moves in, the round window membrane moves out, this allows movement of the fluid within the cochlea, leading to movement of the cochlear inner hair cells and thus hearing. If the round window were to be absent or rigidly fixed (as can happen in some congenital abnormalities) then the stapes would be pushing incompressible fluid against the unyielding walls of the cochlea. It would therefore not move to any useful degree. The patient would then get a hearing loss of about 60dB. This is, unsurprisingly, the same as for conditions where the stapes itself is fixed, such as otosclerosis.
The round window sometimes fails to develop correctly and causes the hearing loss mentioned above. Unfortunately round window malformations are often associated with other ear malformations and the hearing loss can be much more severe. Some types of ear surgery (now generally abandoned) used to leave the round window open to the outside world and covered over the oval window. Sound pressure therefore hit the round window but was shielded from the oval window. It therefore travelled "backwards" around the cochlea but still gave useful hearing as the hair cells were still deflected in the same way. The round window is often used as an approach for cochlear implant surgery. It has also recently been used as a site to place middle ear implantable hearing aid transducers. This work has been publicised by Prof. Vittorio Colletti in Verona. 
- Diagram (as 'round window') at Southwest Tennessee Community College
- Overview at University of Denver
This article was originally based on an entry from a public domain edition of Gray's Anatomy. As such, some of the information contained herein may be outdated. Please edit the article if this is the case, and feel free to remove this notice when it is no longer relevant.