The tympanic membrane is a circular disc made of thin skin, 8-9mm in diameter, and shaped as an inward-looking cone. The middle ear itself lies inside the tympanic membrane and is an air-filled space containing three ossicles (malleus, incus, and stapes) that connect the tympanic membrane to the inner ear.
The malleus has a handle and a head and the handle is located between the layers of the tympanic membrane. The head of the malleus osculates an area of the middle ear called the tympanic cavity and hinges on the rather bulky body of the incus. From the incus stems a filament which turns to the lower middle ear and finally connects to the head of the stapes. The two limbs of the stapes are connected at the base, which is located in a small orifice, the oval window. Just below the oval window, there is another small orifice which leads to the inner ear, which is called the round window. A thin film seals this hole and when the stirrup base moves in-and-out, then the film of the round window moves out-and-in due to the presence of fluid in the inner ear which transmits the changes in pressure.
The facial nerve passes through the middle ear, on its way from the brain, through the cranium and to the facial muscles that control facial expression.
The middle ear also has two small muscles. One of these, the tensor tympani muscle, is located at the front and its function is not clear; it is possible that its aim is simply to make chewing and swallowing less noisy. The second, the stapedius muscle, is located at the back of the middle ear and responds to loud sounds by contracting and tightening the auditory ossicles’ chain, possibly reducing the transmission of prolonged and potentially damaging sounds to the inner ear.
