460 FORAMINA OF THE TYMPANUM. 



Foramina. The openings in the tympanum are ten in number, Jive 

 large and Jive small; they are 



Large Openings. Small Openings. 



Meatus auditorius, Entrance of the chorda tympani, 



Fenestra ovalis, Exit of the chorda tympani, 



Fenestra rotunda, For the laxator tympani, 



Mastoid cells, For the tensor tympani, 



Eustachian tube. For die stapedius. 



The opening of the meatus auditorius has been already described. 



The Fenestra ovalis (fenestra vestibuli), is a reniform opening, situated 

 at the bottom of a small oval fossa (the pelvis ovalis), in the upper part 

 of the inner wall of the tympanum, directly opposite the meatus. The 

 long diameter of the fenestra is directed horizontally, and its convex 

 borders upwards. It is the opening of communication betw r een the tym- 

 panum and vestibule, and is closed by the foot of the stapes and by the 

 lining membranes of both cavities. 



The Fenestra rotunda (fenestra cochleae) is somewhat triangular in its form, 

 and situated in the inner wall of fhe tympanum, below and rather poste- 

 riorly to the fenestra ovalis, from which it is separated by a bony elevation, 

 called the promontory. It serves to establish a communication between 

 the tympanum and the cochlea. In the fresh subject it is closed by a 



aer membrane (m. tympani secundaria,) as well as by the lining of 

 cavities. 



The Mastoid cells are 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 fibro-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 tympani 

 nerve, and three of the muscles of the tympanum. 



The opening by which the chorda tympani enters the tympanum, is at 

 about the middle of its posterior wall, and near the root of the pyramid 



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, imme- 

 diately 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 wall of 

 the tympanum, immediately behind the fenestra ovalis. 



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

 jection of the aquaeductus Fallopii. 



Beneath the fenestra ovalis and separating it from the fenestra rotunda 

 Is the promontory, a rounded prominence formed by the projection of the 



