The Middle-ear Cavity

The Middle-ear Cavity: – The structures of the middle ear are situated in a part of the temporal bone of the skull known as the mastoid process.

Figure 5: middle ear cavity

The tympanic membrane is attached to an opening in the bony walls making up the outer-ear canal leading to the middle ear. The bony middle-ear walls form a cavity known as the middle ear or tympanic cavity. In contrast to the bony ear canal ,which is covered by skin, the walls of the middle-ear cavity are covered with mucous membrane.. Attached to the walls of the middle ear are ligaments and other attachments that hold the ossicles in place. The tympanic ring is a cartilaginous ring in the most lateral part of the middle ear that holds the tympanic membrane in place. The superior portion of the middle-ear cavity is called the attic. The attic is a bony shelf, above which is the brain. Therefore, the middle ear is separated from the brain only by this bone .The bony walls of the attic are filled with holes or cells, known as the mastoid cells. The medial wall of the middle-ear cavity contains the oval window, as described above, and has two other prominent features. The first is another window or hole in the bony wall; this is round and is called the round window. This window is covered with a membrane called the round window membrane and it functions to relieve pressure from the fluid of the inner ear when the footplate of the stapes sets up waves in this fluid.This will be discussed further below in the section on inner-ear functioning. The other prominent structure is called the promontory, a part of the medial bony wall that sticks into the middle ear. The promontory is a portion of one of the channels of the inner ear called the scala media, which is discussed further below in the section on inner-ear structures. (Speaks, C.E., et al 1999)

Floor: The most prominent feature of the floor of the middle-ear cavity is an opening that leads to a long tube connecting themiddle ear with the back of the throat near the adenoids. This tube is called the auditory or Eustachian tube and is a critical part of the middle-ear cavity. As a cavity, there is no normal opening between themiddle ear and the outer world.However the middle-ear cavity is full of air and the mucous membrane cellular structures lining the middle ear require oxygen.Thus, the cells of this mucous membrane use up air in the middle-ear space and this would eventually cause a vacuum if the air were not replaced. The Eustachian tube is the structure that allows replacement of the air that has been exhausted. It is normally closed in children, from toddler age to adult.Movement of the muscles attached to the lateral part of the Eustachian tube in the area of the throat makes the tube open and allows air to rush into the middle-ear cavity. This effect is often noticed when there is a change in air pressure such as when going up high into the mountains or flying in an aeroplane. Passengers in cars and planes usually swallow, yawn or do something else to cause themuscles to open their Eustachian tubes allowing an equalisation of pressure between the middle ear and the outer world. (Clark, J.G, et al 2002)