874 



ORGANS OF SPECIAL SENSE 



ring (tig. 498a). Above and in front where the ring is wanting there is a slight recess 

 known as the notch of Rivinus, and here the membrane is extended outwards, to 

 \mite with the groovelcss margin of the tympanum to form what is known as the 

 Rivinian segment. 



The shape of the membrane varies with tliat of the inner end of tlie meatus. 

 Its long axis is from above downwards and forwards. The membrane, broadly 

 viewed, is concave externally, and convex towards the cavity of the tympanum. 

 This is mainly due to the traction of the handle of the malleus, Avhich descends 

 obliquely downwards and backwards between its two inner layers to a ])oint a little 

 ))elow the centre of the membrane. This curvature is deepest at the umbo, opposite 

 the flattened extremity of the handle. In front of and below the umbo, extending 

 towards the periphery, the membrane is slightly convex externally. 



At the anterior superior pole, the short process of the malleus is plainly visible 

 as a rounded point projecting outwards. A fold of the membrane passes backwards, 

 and another forwards from this spot, and above these folds may, under favorable 

 conditions, be seen two short, tightly stretched striae, which, taking origin from the 

 corners of the notch of Rivinus above, converge as they descend to meet at the 

 short process. The membrane between these stria? and the edge of the notch is 

 thin and flaccid, and appears above the short process as a small pit-like depression 



Fig. 497. — External View of the Left Membrana Tympani. (Enlarged from life.) 



Shrapnell's membrane Posterior fold of membrana tympani 



Anterior fold of membrana 

 tympani 



SHOST PROCESS OF MALLEUS 



UMBO, OR LOWER EXTREMITY OF 

 HANDLE OF MALLEUS 



Cone of light 



LONG PROCESS OF INCUS 



Posterior portion of 

 membrana tympani 



(membrana Shrapnelli). An anastomosis between the tympanic vessels and those 

 of the external meatus occurs at this si)ot. 



Viewed from within, the head of the malleus is seen above the membrane 

 articulating with the incus, its curved handle passing downwards and slightly back- 

 wards between its layers, and strongly projecting from the inner surface. The 

 chorda tympani nerve may be seen crossing the neck of the malleus forwards to 

 the CJlaserian fissm-e. Folds of the membrane occur in front of and behind the 

 malleus, bounding what are known as the anterior and posterior ])ouches of the 

 membrana tympani. The free margin of the posterior ])ouch is commonly well 

 marked. Passing slightly downwards and outwards to the base of the handle of 

 the malleus, is the strongly marked fold containing the tendon of the tensor 

 tympani muscle. 



Structure, — The membrana tympani consists of three layers — as])ecial fibrous 

 layer, with a cuticular covering externally continuous with that which lines the 

 external auditory meatus; and a mucous lining internally derived from the tym- 

 })anic cavity. 



The fibrous layer consists of two lanielhe closely connected — an external, in 

 which the fibres radiate from the handle of the malleus; and an internal, the 

 fibres of which are arranged concentrically, and are especially strong at the 

 circumference. 



