| Round window | |
|---|---|
 Middle ear, with round window at right. | |
 Interior of right osseous labyrinth (label iscochlear fenestra, at bottom center) | |
| Details | |
| Identifiers | |
| Latin | fenestra cochleae, fenestra rotunda |
| MeSH | D012405 |
| TA98 | A15.3.02.015 |
| TA2 | 6904 |
| FMA | 56932 |
| Anatomical terminology | |
Theround window is one of the two openings from themiddle ear into theinner ear. It is sealed by the secondary tympanic membrane (round window membrane), which vibrates with opposite phase to vibrations entering the inner ear through theoval window. It allows fluid in thecochlea to move, which in turn ensures that hair cells of thebasilar membrane will be stimulated and that audition will occur.
The round window is situated below (inferior to) and a little behind (posterior to) the oval window, from which it is separated by a rounded elevation, thepromontory.
It is located at the bottom of a funnel-shaped depression (the round window niche) and, in themacerated bone, opens into thecochlea of theinternal ear; in the fresh state it is closed by a membrane, thesecondary tympanic membrane (Latin:membrana tympani secundaria, ormembrana fenestrae cochleae) or round window membrane, which is a complexsaddle point shape. The visible central portion is concave (curved inwards) toward the tympanic cavity and convex (curved outwards) 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:
The membrane vibrates with opposite phase to vibrations entering the cochlea through theoval window as the fluid in the cochlea is displaced when pressed by thestapes at the oval window. This ensures that hair cells of thebasilar membrane will be stimulated and that audition will occur.
Both the oval and round windows are about the same size, approximately 2.5 square millimetres (0.0039 sq in). The entrance to the round window niche is often much smaller than this.
Thestapes bone transmits movement to the oval window. As the stapes footplate moves into the oval window, the round window membrane moves out, and this allows movement of the fluid within the cochlea, leading to movement of the cochlear innerhair cells and thus hearing. If the round window were to be absent or rigidly fixed (as can happen in some congenital abnormalities), the stapes footplate would be pushing incompressible fluid against the unyielding walls of the cochlea. It would therefore not move to any useful degree leading to a hearing loss of about 60dB. This is, unsurprisingly, the same as for conditions where the stapes itself is fixed, such asotosclerosis.
The round window is located within the mesotympanum, at the posterior extremity of the basal turn of thecochlea. Theoval windows is also located within the mesotympanum, opening at the inferior and lateral part of thevestibule.[1] Both can be seen readily onCT.
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 forcochlear implant surgery. It has also recently been used as a site to placemiddle ear implantable hearing aid transducers. This work has been publicised by Prof. Vittorio Colletti inVerona.[2]
This article incorporates text in thepublic domain frompage 1051 of the 20th edition ofGray's Anatomy(1918)
