This section is from the book "Animal Physiology: The Structure And Functions Of The Human Body", by John Cleland. Also available from Amazon: Animal Physiology, the Structure and Functions of the Human Body.
The Middle Ear, called also the cavity of the tympanum or drum, is a space of greater vertical height than the canal, and still more extensive from before backwards, but narrow transversely. At its fore part is the opening into the Eustachian tube, a passage about an inch and a half long, leading into the pharynx (p. 89). This tube is small and bounded with bone for a short distance at its tympanic extremity; but in the rest of its extent is cartilaginous, and gradually widens. Its cartilaginous wall is replaced with membrane at its lower part, and. is so related to the levator palati muscle that, in the act of swallowing, that muscle momentarily closes its pharyngeal extremity. It is lined with ciliated epithelium, as is also the tympanum. It allows the passage of air into the tympanum, but is very easily blocked up by the adhesion of its walls, near its fore part; and if, when this occurs, the tympanum have either too much or too little air in it, the effect is a disagreeable sensation and interference with hearing, liable to occur after violently blowing the nose, and producible by holding the nostrils, and making a strong expiration with the mouth shut, so as to force air into the tympanum. When the sensation is produced, it lasts for a variable length of time, according to the extent of contact of the walls of the Eustachian orifice, and the viscidity of the substance which causes them to cohere; and it is often removed by the instinctive repetition of the act of swallowing, and stretching the neck on the affected side, so as to make a greater pull on the floor of the orifice, as the parts fall back into their places on the cessation of the spasm of deglutition.*
* It is only fair to state that the opinion held by the late Mr. Toynbee, that the Eustachian tube is in ordinary circumstances shut, and is momentarily opened in swallowing, is held by many, both in 14 B
Posteriorly, the tympanum communicates with the mastoid cells, a set of small irregular spaces in the mastoid part of the temporal bone, which can scarcely be supposed to be functionally important, as they vary greatly in extent, and have little development in young persons.
On the inner side, the tympanum is bounded by the petrous part of the temporal bone, which contains the internal ear imbedded in it; and here there are two small openings, one above the other, which are important as establishing a communication between the middle and internal ear. The lower of these openings, the fenestra rotunda, about the size of a pin head, is blocked up with a membrane, the secondary membrana tympani; while the upper opening, the fenestra ovalis, somewhat larger, has fitted into it the base of the stapes, one of the small tympanic ossicles.