CHAP, ix.] NERVOUS SYSTEM AND OEGANS 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 Eustachian tube. 



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



Inc 



S.Sc 



Fig. 137. SECTION OF THE MIDDLE AND INTERNAL EAR OF THE CAT, MUCH ENLARGED, SHOWING 

 THE MEMBRANOUS LABYRINTH AND ITS RELATIONS (AND THOSE OF THE BONY LABYRINTH) 

 TO THE SEPTUM OF THE BULLA, THE EUSTACHIAN TUBE, AND THE TYMPANIC MEM- 

 BRANE. (From a drawing by Mr. Alban Doran.) 



The auditory ossicles are represented in outline 

 as if transparent. The septum of the bulla 

 is partly represented, but is cut away at * to 

 show its relation to the labyrinth. The mem- 

 branous labyrinth is represented by darker 

 shading. 



Amp. Ampulla. 



6'. 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 

 margin bulges into the outer chamber.) 



Fo. Fenestra ovalis. 



Inc. Incus. 



Mai. Malleus. 



Mn. Manubrium. 



Mt. Mt. Outline of membrana tympani, with 



manubrium of malleus touching it. 

 P. Sc. Posterior semi-circular canal. 

 Sb. Septum of bulla. 

 S. Sc. Superior semi-circular canal. 

 S. Stapes. 



St. Scala tympani of cochlea. 

 Sv. Scala vestibuli of cochlea. 

 *. Cut edge of septum of bulla. 



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 tympanic membrane, 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 the fencstra ovalis. Somewhat in front of the 



* See ante, p. 68. 



