528 



^STHESIOLOGY. 



latter opening leads into the cochlea, and in the recent 

 subject is closed by the lining membrane of the cavity ; 

 between the two fenestra; is the projection termed the promon- 

 tory. At the posterior part of the tympanum, numerous open- 

 ings lead into the mastoid cells, which are small, irregular, 

 freely-communicating cavities in the auditory bulb of the temporal 

 bone. Behind the fenestra ovalis is a conical eminence, the 

 'pyramid, in the apex of which is a groove for the tendon of the 

 stapedius muscle. The anterior part of the tympanum narrow^s 

 towards the opening of two canals, separated from each other 

 by a lamina of bone, the processus cochlear if or mis ; the upper 

 and smaller of these canals contains the tensor tympani muscle, 

 and the other leads into and forms 

 the bony portion of the Eustachian 

 tube, a tube partly osseous and 

 partly cartilaginous, which extends 

 from the tympanum to the guttural 

 pouch, and is the passage through 

 which air is supplied to the middle ear. 

 The Glaserian fissure is immediately 

 in front of the ring to which the 

 membrana tympani is inserted. 



The bones of the middle ear are 

 united and kept in their positions by 

 muscles and ligaments. The muscles 

 are the stapedius, which has its origin 

 in the interior of the pyramid, and is 

 inserted into the neck of the stapes ; 

 the tensor tympjarvi, which arises from 

 the cartilaginous portion of the Eusta- 

 chian tube, and surface of the sphenoid, and passing through a 

 small canal, becomes inserted into the handle of the malleus ; the 

 laxator tympani, which arises from the sphenoid, and passing 

 through an opening in the Glaserian fissure, is inserted into the 

 neck of the malleus. The ligaments are the suspensory, which 

 attaches the head of the malleus to the upper wall of the 

 tympanum ; the posterior, which attaches the short crus of the 

 incus to the margin of the mastoid cells ; and the annular, 

 which attaches the foot of the stapes to the membrane closing 

 the fenestra ovalis. 



The tympanum is lined by a vascular mucous membrane, 



JFlG. 202. 

 External wall of the tympauum— 

 viewed from within, a, Meatus audi- 

 torius externus ; h. Tympanic mem- 

 braue ; c, Malleus ; d d, Mastoiil cells ; 

 e c, Outer boundary, or ring of tym- 

 panum ; /, Orifice for Eustachian 

 tube. 



