994 MUSCLES OF TYMPANUM. [Boon in. 



ends, turns round, almost at right angles to the line of the 

 muscle, over a bony prominence at the end of the groove, and 

 passing athwart the cavity of the tympanum from the median 

 side outwards (Fig. 170 T.T.^) is attached to the upper part of 

 the handle of the malleus. 



lg.s 



ch.t 



FIG. 176. DIAGRAM OF THE OUTER WALL OF THE TYMPANUM AS SEEX FROM 

 THE MESIAL SIDE. Magnified twice. (After Schwalbe.) 



m.t. membrana tympani. mb. handle of M the malleus. I. the incus. E.t. 

 Eustachian tube. T. T. tensor tympani, the tendon of which is seen attached 

 to the upper part of the handle of the malleus. Ig.a. the. anterior and lg.s. the 

 superior ligament of the malleus, ch.t. the chorda tympani nerve traversing the 

 tympanic cavity. 



The effect of the contraction of the muscle is to pull the 

 handle of the malleus and so the tympanic membrane inwards 

 towards the median side. Even in a quiescent state it may be 

 of use in keeping up a certain amount of tension and in pre- 

 venting the tympanic membrane being pushed out too far. 

 When it contracts it certainly renders the tympanic membrane 

 more tense ; hence it has been supposed on the one hand to act 

 as a damper lessening the amount of vibration of the membrane 

 in the case of too powerful sounds, and on the other hand to ac- 

 commodate the apparatus to the sounds falling upon it since the 

 more tense membrane is more readily thrown into vibrations by 

 higher notes and is less sensitive to lower notes. It has been 

 urged that it is readily thrown into contraction at the commence- 

 ment of a sound, especially of a noise, and returns to rest during 

 the continuance of a prolonged musical note, the contraction 

 being a simple contraction or twitch, rather than a continued 

 tetanic contraction ; it is suggested that this may serve to tune 

 the membrane as it were for the sound which follows. Efferent 

 impulses reach it through fibres of the fifth nerve from the otic 

 ganglion, and its activity is regulated by reflex action, vibrations 



