THE TYMPANUM. 81 



everywhere smooth, but marked in many situations with depressions, some deeper 

 others shallower, which are lined by a prolongation of the mucous membrane of the 

 cavity and also contain air. These depressions are known as the tympanic cells. A 

 roof and floor, an outer and inner wall, and an anterior and posterior boundary are 

 commonly described. 



The roof of the tympanum is formed by a thin plate of bone (tec/men tympani), 

 which may be easily broken through so as to obtain a view of the tympanic cavity 

 from above ; it is situated on the upper anterior surface of the petrous portion of 

 the temporal bone near the angle of union with the squamous portion, the petro- 

 squamous fissure passing just external to it. The tegmen tympani also roofs over 

 the canal of the Eustachian tube and tensor tympani muscle. It is not unfrequently 

 partly deficient (compare Vol. II., p. 42). The floor is narrow, in consequence 

 of the outer and inner boundaries being inclined towards each other. It passes 

 without any sharp demarcation into the anterior and posterior boundaries. It 

 is separated by a thin plate of bone, which in rare cases is found to be incomplete, 

 from the lateral part of the jugular fossa, and exhibits a small aperture through 

 which the tympanic nerve reaches the inner wall. 



The outer wall is formed, to a small extent, by bone, but mainly by the 

 membrane (uwmbrana lympani) already mentioned as closing the inner end of the 



Fig. 90. VlKW OK THK LKKT .MK.MI5KANA TYMI'AM 

 AXD AUDITORY OSSICLES FROM THE INNER SIDE, 

 AND SOMEWHAT FROM ABOVE (E. A. S. ). f 



?/i, malleus ; /, incus ; *, stapes ; py, pyramid 

 from which the tendon of the stapedius muscle is 

 seen emerging ; t t, tendon of the tensor tympani 

 cut short near its insertion ; I a, anterior ligament 

 of the malleus : the processus gracilis is concealed 

 by the lower fibres of this ligament ; I s, superior 

 ligament of the malleus ; I. i, ligament of the incus ; 

 ch, chorda tympani nerve passing across the outer 

 wall of the tympanum. 



external auditory meatus. Immediately 



in front of the ring of bone into which 



the membrana tympani is inserted, is the 



inner extremity of the fissure of Glaser 



(petro -tympanic fissure). Close to the 



inner end of this fissure is the opening 



of a small canal (iter chordcv anterius), 



through which the chorda tympani nerve 



passes out from the tympanum. The margin of the tympanic ring is inter- 



rupted above by a notch (incisura Ririni) which is bounded in front and behind 



by prominent angles, the anterior and posterior tympanic spines. The outer wall 



of the recessus epitympanicus is formed by the thickened part of the squamosal 



which lies behind the root of the zygoma : it may contain, according to Kirchner, 



cells communicating with those of the mastoid process. This recess overhangs the 



inner end of the external auditory meatus. 



The membrana tympani is an ellipsoidal disc, the longer axis of which i 

 directed from behind and above, forwards and downwards, and is about 10 mm. in 

 length : the shorter axis being about mm. It is nearly as large at birth as in the 

 adult. It is inserted into the groove already noticed at the end of the meatus 

 externus, and so obliquely that the membrane inclines towards the anterior and lower 

 part of the canal at an angle of about 55 (fig. 102). It is said to be more vertical in 

 musicians and more horizontal in the congenitally deaf. In the foetus and even 

 at or near birth it is as already stated nearly horizontal. The handle of the malleus. 



VOL. 111.. J'T. 



