CHAP. IX.] NERVOUS SYSTEM AND ORGANS OF SENSE. 



297 



The MIDDLE PART OF THE EAR, or TYMPANUM, has been already- 

 described in part in the description of the temporal bone.* It is a 

 chamber entirely enclosed in the temporal bone, but has certain 

 apertures in its walls. It is separated off from the external meatus 

 by the tympanic membrane, but it communicates with the pharynx 

 through the Eustacliian tube. 



The roof of the tympanum is formed by that portion of the 



Inc 



P.Sc 



E.Sc 



Hal 



Fig. 137.— Section of the Middle and Internal Ear of the Cat, much enlarged, showing 

 THE Membranous Labyrinth and its relations (and those op the Bony Labyuinth) 

 TO the Septum of the Bulla, the Eustachian Tqee, and the Tympanic Mem- 

 brane. (Fi'oni a drawing by Mr. Alban Dorau.) 



The auditory ossicles are represented in outline 

 as if transparent. The septum of tlie bulla 

 is partly represented, but is cut away at "" to 

 show its relation to the labyi-inth. The mem- 

 branous labyi'iuth is represented by darker 

 shading. 



Amp, Ampulla. 



C. Cochlea. 



E. c. Opening of Eustachian tube. 



E. sc. External horizontal semi-circular canal. 



Fr. Fenestra rotunda. (It lies almost entirely 

 above the septum of the bulla, and looks 

 into the inner chamber ; but its anterior 

 margiu bulges into the outer chamber.) 



Fo. Fenestra ovalis. 



Inc. Incus. 



Mai. Malleus. 



3Iii. JIanubrium. 



ML Mt. Outline of membrana tympani, witli 



manubrium of malleus touching it. 

 P. Sc. Posterior semi-cu'cular canal. 

 Sh. Septum of buUa. 

 S. .5c. Superior semi-circular canal. 

 .S. Stapes. 



St. Seala tympani of cochlea. 

 Sv. Seala vestibuli of cochlea. 

 *. Cut edge of septum of buUa. 



petrous bone which immediately adjoins the squamosal. Its outer 

 wall has a very large opening closed by the tympanic membrane, 

 and in front of its margin is the inner opening of the fissura Glaseri, 

 before mentioned as transmitting the chorda tympani nerve. 



The tijmjxinic memhranc, which is nearly circular, lies obliquely, 

 its outer surface looking somewhat downwards. It is very thin, and 

 consists of fibrous and elastic tissue, the fibres radiating from about 

 its centre, but there are also circular fibres, especially towards 

 its circumference. 



The inner wall of the tympanum presents several openings and 

 prominences. Towards its upper hinder part is a kidney-shaped or 

 oval foramen called i\\.Q fenestra ovalis. Somewhat in front of the 



See ante, p. 6S. 



