156 . SPECIAL SENSES. 



thin plate of bone. The floor is bony, and is much narrower 

 than the roof. The inner wall, separating the tympanic 

 cavity from the labyrinth, is irregular, presenting several 

 small elevations and foramina. The fenestra ovalis, an ovoid 

 opening near its upper portion, leads to the cavity of the 

 vestibule. This is closed, in the natural state, by the base of 

 the stapes and its annular ligament. Below, is a smaller, 



FIG. 11. 



Bones of the tympanum of the riprht side (from Arnold). ? A, malleus; 1, its head; 2, the 

 handle : 3. long, or slender process ; 4, short process ; B, incus ; 1, its body ; 2, the long 

 process with the orbicular process ; 3. short, or posterior process ; 4, articular surface re- 

 ceiving the head of the malleus: C. stapes; 1, head; 2, posterior crus; 3. anterior crus; 4, 

 base ; C*, base of the stapes ; D, the three bones in their natural connection as seen from 

 the outside; a, malleus; 6, incus; c, stapes. (QuAix, Elements of Anatomy. London, 

 1S67, vol. ii., p. 74S.) 



ovoid opening, the fenestra rotunda, which leads to -the 

 cochlea. This is closed, in the natural state, by a membrane, 

 called the secondary membrana tympani. In addition, the 

 posterior wall presents several small foramina leading to the 

 mastoid cells, which are lined by a continuation of the mu- 

 cous membrane of the tympanic cavity. The tympanic cav- 

 ity also presents an opening leading to the Eustachian tube, 

 and a small foramen, which gives passage to the tendon of 

 the stapedius muscle. The Eustachian tube extends from 

 the upper part of the pharynx to the tympanum. 



