CHAP, iv.] 



HEARING. 



195 



very much the direction of that tube. The tendon in which it 

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



Igs. 



Ch.t 



FIG. 174. 



m.t 



DIAGRAM OF THE OUTER WALL OF THE TYMPANUM AS SEEN FROM THE 

 MESIAL SIDE. Magnified twice. (After Schwalbe.) 



.t. membrana tympani. mb. handle of If the Malleus. /. the incus. E.t. Eusta- 

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

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

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

 the tympanic cavity. 



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

 athwart the cavity of the tympanum from the median side 

 outwards (Fig. 168 T. T.) is attached to the upper part of the 

 handle of the malleus. 



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 preventing 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 accommodate 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 commencement 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 



