752 PHYSIOLOGY. 



The cavity of the tympanum forms a dilatation added to the 

 auditory canal. It has an internal wall, an external wall, and the 

 Eustachian tube. The mastoid cells communicate by a large orifice 

 with the upper, back part of the tympanum. They are lined 

 throughout with a delicate mucous membrane. 



The external wall is occupied in its greatest extent by an open- 

 ing which is nearly circular and closed by the membrana tympani. 

 The latter is semitransparent, concave externally and convex inter- 

 nally. To its inner surface is attached the malleus, one of the three 

 car ossicles. 



Fig. 309. Left Tympanum and Auditory Ossicles. (LANDOIS.) 



A.G., External meatus. M, Membrana tympani, which is attached to the 

 handle of the malleus (M) and near its short process (p). Ji, Head of the 

 malleus, a, Incus. K, Its short process, with its ligament. I, Long process. 

 8, Stapes. 



The internal wall is convex and has in its central portion a 

 tirbercle known as the promontory. Its base corresponds to the 

 origin of the cochlea. The most prominent of the grooves upon its 

 surface marks the position of the nerve of Jacobson,. 



Above the promonotory is found the oval window. Its shape is 

 really reniform; it leads to the vestibule. 



The round window is situated just beneath the oval window. It 

 is closed by a membrane. 



The ossicles, which form an articulated chain, reach from the 



