370 INTERNAL EAR OR LABYRINTH. 



thought to resemble a ligament, arises within the hollow 

 of the pyramid, and is inserted into the neck of the stapes. 



Function. By pressing the base of the stapes against 

 the fenestra ovalis ; and thus drawing the bony chain in- 

 ward, it has been supposed to act as a tensor upon the 

 membrana tympani. 



Lining Membrane of Tympanum. It has already been 

 stated that this membrane is mucous, and continuous with 

 that of the pharynx and Eustachian tube ; but in the tym- 

 panum it is much more delicate, and closely united to the 

 periosteum, so as to be considered a fibro-mucous mem- 

 brane. It covers all the inner walls of the tympanum is 

 reflected round the muscles, nerves, and bones lines the 

 tympanic surfaces of the membrana tympani, membrana 

 vestibuli, membrana rotunda, and the space between the 

 crura of the stapes, and is continued into the mastoid cells. 



INTERNAL EAR OR LABYRINTH. 



The internal ear or labyrinth, so called from the intricacy 

 of its internal arrangement, is situated on the inner side 

 of the tympanum, and deep within the petrous portion of 

 the temporal bone. This is the essential part of the organ 

 of hearing, and consists of the vestibule, cochlea, and semi- 

 circular canals. 



The vestibule occupies the centre of the labyrinth, being 

 situated between the tympanum and meatus auditorius in- 

 ternus, having the cochlea in front, and the semicircular 

 canals behind. It has three dilatations, called cornua or 

 ventricles ; one superior, one anterior, and the third posterior. 

 The superior receives two openings of the semicircular 

 canals, the posterior the other openings of the canals, while 

 the anterior opens into the scala of the cochlea. The inner 

 wall of the vestibule is cribriform and corresponds to the 

 base of the meatus internus, transmitting some fine ves- 

 sels, and fibrillee of the auditory nerve. On the posterior 

 wall there is a foramen called the aqueduct of the vestibule, 

 which opens on the posterior surface of the petrous bone. 

 behind the meatus internus. 



