ANATOMY IN A NUTSHELL. 573 



The floor of the tympanum is formed by the thin plate of bone which sepa- 

 rates the tympanum from the jugular fossa. 



The anterior Avail of the tympanum is deficient above where the Eustachian 

 tube opens into it. This wall separates the tympanum from the carotid canal 

 and has opening of canal for Tensor tympani muscle above, and below the open- 

 ing of the Eustachian tube. These canals are separated from one another by 

 the processus cochleariformis. 



The posterior wall of the tympanum separates the tympanum from the mas- 

 toid cells and communicates wtih the mastoid antrum by one large and several 

 small openings. 



The outer wall of the tympanum is formed by the membrana tympani and 

 by the squamous portion of the temporal bone above. It has the following 

 points for consideration: 



1. The iter chordae postertus, which is close to the posterior edge of 

 the drum head. The chorda tympani nerve which is a branch of the seventh 

 cranial nerve enters the tympanum through this opening. 



2. The iter chordae anterius, which is just above the drum head, 

 is the opening by which the chorda tympani nerves leaves the tympanum. 



3. The Glaseriax fissure, which opens above and in front of the head, 

 lodges the long process of the malleus, the Laxator tympani muscle. and the 

 tympanic branch of the internal maxillary artery. 



The inner wall of the tympanum is the outer wall of the labyrinth and 

 presents several points for consideration. 



1. The fenestra ovalis leads to the vestibule and is closed by a mem- 

 brane which is attached to the base of the stapes. 



2. The fenestra rotunda is below the fenestra ovalis and leads into 

 the scala tympani of the cochlea. It is closed by the membrana tympani secund- 

 daria . 



3. The promontary is an elevation formed by the first turn of the cochlea. 

 It is covered by the tympanic plexus and is located between the two fenestras 

 and in front of them. 



-i. The pyramid is behind the fenestra ovalis and from its summit passes 

 the tendon of the Stapedius muscle, and a branch of the seventh nerve pierces 

 the pyramid to supply the Stapedius. 



5. The ridge of the aqueductus Eallopii. which is above the fenestra 

 ovalis and covers the seventh nerve in its passage through the tympanum. 



Blood Supply. — Tympanic branch of the internal maxillary artery, tym- 

 panic branch of internal carotid artery, stylo-mastoid branch of the posterior 

 auricular artery, petrosal branch of the middle meningeal artery, a branch 

 of ascending pharyngeal artery which enters through the Eustachian tube, tym- 

 panic branch of vidian artery. The veins of the middle ear follow the corres- 

 ponding arteries and empty into the temporo-maxillary vein, the superior petros- 

 al sinus, the lateral sinus, the internal jugular vein and pharyngeal veins. 



The lymphatics of the middle ear end in the posterior auricular and caro- 

 tid nodes. 



Nerve Supply. — Tympanic plexus which supplies the mucous membrane 



