OPENINGS OF THE TYMPANUM. 455 



In the fresh subject it is closed by a proper membrane, as well as 

 by the lining of both cavities. 



The Mastoid cells are very numerous, and occupy the whole of 

 the interior of the mastoid process, and part of the petrous bone. 

 They communicate by a large irregular opening with the upper and 

 posterior circumference of the tympanum. 



The Eustachian tube is a canal of communication extending 

 obliquely between the pharynx and the anterior circumference of 

 the tympanum. In structure it is partly fibre-cartilaginous and 

 partly osseous, is broad and expanded at its pharyngeal extremity, 

 and narrow and compressed at the tympanum. 



The smaller openings serve for the transmission of the chorda tym- 

 pani nerve, and "three of the muscles of the tympanum. 



The opening by which the chorda tympani enters the tympanum, 

 is near the root of the pyramid, at about the middle of the posterior 

 wall. 



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, 

 immediately 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 poste- 

 rior wall of the tympanum, immediately behind the fenestra ovalis. 



Directly above the fenestra ovalis is a rounded ridge formed by 

 the projection of the aquceductus Fallopii. 



Beneath the fene'sfra ovalis and separating it from the fenestra 

 rotunda is the promontory,^ rounded prominence formed by the pro- 

 jection of the first turn of the cochlea. It is channeled upon its sur- 

 face 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 

 memlrani 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, 



Apertura chordae (entrance). 





