5io THE AUDITORY APPARATUS 



passes forwards, close to the tympanic membrane and above 

 the tendon of the tensor tympani, must also be preserved, if 

 possible. 



MIDDLE EAR, TYMPANIC ANTRUM, AND AUDITORY TUBE 



Cavum Tympani or Middle Ear. The tympanic cavity 

 is a small chamber, containing air, which is placed between 

 the bottom of the meatus externus and the internal ear or 

 labyrinth (Fig. 209). Posteriorly, it communicates, by a 

 relatively large orifice, called the aditus, with the tympanic 

 antrum and the mastoid air-cells ; whilst, anteriorly, the 

 auditory tube opens into it and puts it into connection with 

 the cavity of the pharynx. It contains 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 12.5 mm. (half an inch]. Its 

 width, from side to side, is about 

 4.5 mm. (a sixth of an inch); 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 

 upwards beyond the level of the 

 membrana tympani, and to which 

 FIG. 208. Schematic vertical the term recessus epitympanicus is 



. 



the tympanum proper, which lies im- 



. mediate 'y to the medial side of the 



is in the recessus epi- membrana tympani. The tympanic 

 tympanicus). cavity presents for examination a 



3. Promontory on medial wall. r j n -i r n 



4 . Membrana tympani. TOOf and a floor, and four Walls, VIZ., 



anterior, posterior, lateral, and medial. 



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

 tegmen tympani, which separates the cavity from the middle 

 fossa of the cranium. In chronic inflammatory conditions 

 of the middle ear, an extension of the inflammatory process, 

 through the tegmen, to the meninges of the brain is always to 

 be feared. 



The floor or jugular wall is narrow, and it also is formed 

 by a thin osseous lamina, which is interposed between the 



