This section is from the book "Applied Anatomy: The Construction Of The Human Body", by Gwilym G. Davis. Also available from Amazon: Applied anatomy: The construction of the human body.
The tympanic cavity is flat and narrow and is situated directly behind and also above the membrane. It has a floor and roof, and external and internal walls. It is divided into the portion behind the membrane and the portion above the membrane called the attic. The floor is narrower than the roof and is formed by the tympanic plate, which separates it from the jugular fossa containing the commencement of the internal jugular vein. The bone forming the floor is more difficult for pus to perforate than is that of the roof, so that extension of middle-ear disease is less frequent through it. The roof is comparatively thin and formed of cancellous tissue with a thin and weak outside compact layer; therefore it is a somewhat common site for pus to perforate and thereby obtain access to the middle fossa of the skull. The distance from the floor to the roof is approximately 15 mm. (3/5 in.); half is behind the membrane and the rest forms the attic above.
Fig. 103. - Right temporal bone: - The outer surface has been cut away, exposing the tympanic cavity, its inner wall, the mastoid antrum, Eustachian tube, etc.
The external wall is formed below by the tympanic membrane and above by the bone. As the membrane is the weakest portion of the walls, collections of pus in the middle ear most often find a vent through it. Immediately behind the membrane are the lower portions of the ossicles, and above is the chorda tympani nerve.
The internal wall is formed of bone and is from 2 to 4 mm. (1/12 to 1/6 of an inch) behind the membrane. It is so close that in doing the operation of paracentesis care must be taken not to thrust the knife too deeply. In it are the oval and round windows (Fig. 103).
There is no well-defined anterior or posterior wall. The anterior portion of the cavity is continued forward into the Eustachian tube; the canal for the tensor tympani muscle is immediately above it. Posteriorly the cavity of the attic is continuous through the aditus with the mastoid antrum and the cells beyond. Posterior to the opening of the Eustachian tube is an elevation on the internal wall called the promontory, formed by one of the semicircular canals. Above the promontory is the fenestra ova/is, which lodges the stapes bone and communicates with the vestibule. Below and behind is the fenestra rotunda, closed by a membrane separating the cochlea from the middle ear. Above the fenestra ovalis is a ridge of bone marking the aqueduct of Fallopius, in which runs the facial nerve.
The Eustachian tube passes from the anterior portion of the tympanic cavity downward, forward, and inward to the upper posterior portion of the pharynx about level with the floor of the nose. It is about 3.5 cm. (approximately 1 1/2 in.) in length. The outer third, near the ear, is bony and the inner two-thirds are cartilaginous. The point of junction of the bony and cartilaginous portions is the narrowest portion of the tube and is called the isthmus. The tube is usually closed, but opens in swallowing, yawning, etc., thus admitting air to the tympanic cavity and mastoid cells. Catarrhal affections of the throat readily travel up the tube and set up an inflammation of the middle ear. Swelling of the lining of the tube follows and air no longer passes to the ear. To open the tube two methods are employed - that of Valsalva, and that of Politzer. The former consists in holding the nostrils and mouth shut and attempting to blow, when the action of the throat and palate muscles opens the tube and allows the air to enter. In the method of Politzer, the patient is given a sip of water which he swallows on command. The nozzle of a rubber air-bag is placed in one nostril and the other held shut. As the patient swallows, the air-bag is compressed and the air enters the Eustachian tube. Sometimes this method is varied by asking the patient to say ' 'hock," thus causing the tube to open, when the air-bag is compressed. The calibre of the tube is sometimes so small that probes are passed up it to dilate it. Care is necessary to avoid introducing the probe too far or it will injure the ossicles of the ear. Pus will sometimes discharge through the tube. I have seen pus coming from the middle ear pass down the tube into the inferior meatus and be blown out the anterior nares.
Fig. 104. - View of the tympanic membrane and ossicles of the left ear from within.
Lying in a separate canal immediately above and parallel with the Eustachian tube is the canal for the tensor tympani muscle.
The attic is directly above the tympanic cavity and contains the greater part of the ossicles. Between the two along the inner wall runs a ridge of bone within which is the aquseductus Fallopii, containing the facial nerve. The roof of the attic is called the tegmen. It is a thin shell of bone, varying in thickness, and separates the cavity of the ear from the middle cerebral fossa above. Pus frequently eats its way through at this point and forms a subdural abscess, which by working its way backward involves the lateral (transverse) sinus, causing thrombosis and general septic infection.
The antrum is a little larger than the attic. The two cavities are continuous through the aditus. The roof of the antrum is level with the roof of the attic and its floor is about level with the top of the membrane. It is thus seen to be directly above and posterior to it.