632 



THE EAR. 



a thin semitransparent membraue — membrana tympani (fig. 445, 1), 

 which closes the inner end of the external auditory meatus. Imme- 

 diately in front of the ring of bone into which the membrana tympani 

 is inserted, is the inner extremity of the fissure of Glaser, which gives 

 passage to the laxator tympani muscle, and attachment to the processus 

 gracilis of the malleus. Close to the back of this fissure is the opening 

 of a small canal (named by Cruveilhier the canal of Huguier), through 

 which the chorda tympani nerve usually escapes from the cavity of the 

 tympanum and the skull. 



Fig. 445. — Right Membrana 

 Tympani as seen from the 

 outer and inner side re- 

 spectively. 



A, the outer surface ; C, the 

 inner ; in the latter the small 

 bones are seen adherent to the 

 nieiubrane ami adjacent 2>arts 

 of the temporal bone ; in A, 

 the shaded part indicates the 

 small bones as partially seen 

 through the membrane ; 1, mem- 

 brana tympani ; 2, malleus ; 3, 

 stapes ; 4, incus. 



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

 which is directed from behind and above, forwards and downwards, and 

 is about A\ lines in length : the shorter being about 4 lines. 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°. The handle of the 

 malleus (fig. 445, 2), one of the small bones of the tympanum, descends 

 between the middle and inner layers of the membrana tympani to a little 

 below the centre, where it is firmly fixed ; and, as the direction of this pro- 

 cess of the bone is somewhat inwards, the outer surface of the membrane 

 is thereby rendered somewhat conical (see fig. 440), being depressed 

 towards the centre. 



Although very thin, the membrana tympani is composed of three 

 distinct structures. A prolongation of the skin of the external meatus 

 forms the outer layer ; the mucous membrane lining the cavity of the 

 tympanum furnishes an inner layer ; and between these two is the 

 proper substance of the membrane, mainly composed of fibrous tissue. 

 The greater number of the fibres radiate from near the centre at the 

 attachment of the handle of the malleus (fig. 445), but there are 

 also circular fibres which are situated within or more interiorly than 

 the radial, and close to the circumference, and form a dense, almost 

 ligamentous ring. Owing to the presence of the circular fibres, the 

 radial fibres are not straight, but are slightly bowed outwards, so 

 that between the most depressed part and the attached border the 

 membrane is slightly convex outwardly. 



This is particularly well marked at the upper and anterior part, where 

 the fibres stretch across the mouth of a small notch in the bony ring to 

 which the membrane is attached (notch of Rivinus). The notch is occu- 

 pied by a lax part of the membrane {memlrana flaccida, Shrapnell), 

 consisting of loose connective tissue, with vessels and nerves covered 



