550 THE AUDITORY APPARATUS 



Tympanic Cavity or Middle Ear. The tympanic cavity 

 is a small chamber, filled with air, which is placed between 

 the bottom of the meatus externus and the internal ear or 

 labyrinth. Posteriorly it communicates, by a relatively large 

 orifice, with the tympanic antrum and mastoid air-cells ; 

 whilst anteriorly the auditory tube opens into it and puts it 

 into connection with the cavity of the pharynx. It con- 

 tains the chain of auditory ossicles which crosses from its 

 lateral to its medial wall, and it is lined with delicate mucous 

 membrane. 



The vertical depth and the antero-posterior length of the 

 tympanic cavity are each about half an inch (12.5 mm.). 

 Its width, from side to side, is about 

 a sixth of an inch (4.5 mm.); and, 

 as both its lateral and medial walls 

 bulge into the cavity, its width in 

 the centre is still further reduced. 

 The tympanic cavity consists of (i) 

 an upper part, which extends up- 

 wards beyond the level of the mem- 

 brana tympani, and to which the 

 FIG. 234. Schematic vertical term recessus epitympanicus is ap- 

 section through the Tym- u d and , x th tympanum proper, 

 panum. ( From Testut. ) , . . '. } ' ,. * , 



which lies immediately to the inner 



1. External meatus. j r i. i_ 



2 . Tympanic cavity (the upper Slde of the membrana tympani. 



" 2 " is in the recessus e P i- The tympanic cavity presents for 

 tympanicus). examination a roof and a floor, 



3. Promontory on medial wall. . . 



4. Membrana tympani. With four Walls, VIZ., anterior, pOS- 



terior, lateral, and medial. 



The roof is composed of a thin plate of bone termed the 

 tegmen tympani. This separates it from the middle fossa 

 of the cranium. In chronic inflammatory conditions of the 

 middle ear, an extension of the inflammatory process to the 

 meninges of the brain is always to be feared. 



Theyft?<?r or jugular wall is narrow, and is also formed by 

 a thin osseous lamina, which is interposed between the 

 tympanum and the jugular fossa. It separates the tympanum 

 from the bulb of the internal jugular vein, and an exten- 

 sion of an inflammatory condition of the middle ear, through 

 the bone to the vein, may lead to thrombosis. 



The posterior or mastoid wall presents, in its upper part, 

 the opening or aditus which leads from the recessus epi- 



