FORAMINA OP THE TYMPANUM. 511 



spinous process of the sphenoid bone, and passes through an opening 

 in the fissura Glaseri, to be inserted into the long process of the 

 malleus. This is regarded as a ligament by some anatomists. 



The Laxator tympani minor arises from the upper margin of the 

 meatus, and is inserted into the handle of the malleus, near the pro- 

 cessus brevis. This is regarded as a ligament by some anatomists. 



The Stapedius arises from the interior of the pyramid, and escapes 

 from its summit to be inserted into the neck of the stapes. 



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

 large and five 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 the stapedius. 



The opening of the meatus auditorius has been previously 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 directly horizontally, 

 and its convex borders upwards. It is the opening of communication 

 between the tympanum and the 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 the tympanum, below and 

 rather posteriorly to the fenestra ovalis, from which it is separated by 

 a bony elevation, called the promontory. It serves to establish a com- 

 munication between the tympanum and the cochlea. In the fresh 

 subject it is closed by a proper membrane (m. tympani secundaria), as 

 well as by the lining of both cavities. 



The Mastoid celts 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 ob- 

 liquely 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. 



