512 FORAMINA OF THE TYMPANUM. 



The opening of exit for the chorda tympani is at the fissura Glaseri 

 in the anterior wall of the tympanum. 



The opening for the laxator tympani muscle is also situated in the 

 fissura Glaseri, in the anterior wall of the tympanum. 



The opening for the tensor tympani muscle is in the inner wall, im- 

 mediately above the opening of the Eustachian tube. 



The opening for the stapedius muscle is at the apex of a conical bony 

 eminence, called the pyramid, which is situated on the posterior watt 

 of the tympanum, immediately behind the fenestra ovalis. 



Directly above the fenestra ovalis is a rounded ridge formed by the 

 projection of the aquaductus Fallopii. 



Beneath the fenestra ovalis and separating it from the fenestra ro- 

 tunda is the promontory, a rounded prominence formed by the projec- 

 tion of the first turn of the cochlea. It is channeled upon its surface 

 by three small grooves, which lodge the three tympanic branches of 

 Jacobson's nerve. 



The Foramina and processes of the tympanum may be arranged, 

 according to their situation, into four groups. 



1. In the External wall is the meatus auditorius, closed by the 

 membrana tympani. 



2. In the Inner wall., from above downwards, are the 



Opening for the tensor tympani, 



Ridge of the aquaeductus Fallopii, 



Fenestra ovalis, 



Promontory, 



Grooves for Jacobson's nerve, 



Fenestra rotunda. 



3. In the Posterior wall are the 



Opening of the mastoid cells 



Pyramid, 



Opening for the stapedius, 



Opening for Jacobson's nerve, 



Apertura chordae (entrance). 



4. In the Anterior wall are the 



Eustachian tube, 



Fissura Glaseri, 



Opening for the laxator tpmpani, 



Apertura chordae (exit). 



The tympanum is lined by a vascular mucous membrane, which in- 

 vests the ossicula and chorda tympani, and forms the internal layer of 

 the membrana tympani. From the tympanum it is reflected into the 



