v THE SENSE OF HEARINd 205 



"\alis ; the labyrinthine fluid enclosed within bony walls can 

 move in no other direction than towards the fenestra rotunda 

 with its yielding membrane. The converse process takes place 

 on diminishing the pressure in the auditory passage. 



To determine the amplitude of movement of the different 

 parts of the tympanum when impinged on by sound - waves 

 Bezold (1897) carried out a series of accurate researches, by 

 inserting a small manometer into the labyrinth of the dead 

 subject. With the tympanic cavity open, the maximum move- 

 ment of the stapes in consequence of the oscillations of pressure 

 in the auditory passage averaged 0'04 mm. Of this movement a 

 quarter consists of the incursion, three-quarters of the excursion. 

 The maximal movement of the handle of the malleus is about 

 076, one -third being incursion, two -thirds excursion. The 

 corresponding maximal movement of the tip of the long process 

 of the incus is 0'21 mm., of which one-third is incursion, two- 

 thirds excursion. In the isolated fenestra ovalis the movements 

 of the plate of the stapes, when the tendon of the stapedius 

 muscle has been preserved, are about 0'063 mm., and are fairly 

 equal in both directions. The mobility of the membrane of the 

 fenestra rotunda is about four times greater than that of the 

 plate of the stapes in the fenestra ovalis. 



Bezold observed that in dead subjects the excursions of the 

 chain of ossicles are more ample than the incursions, which, as he 

 himself remarks, is difficult to reconcile with the exact transmission 

 of sounds; and it cannot occur during life under normal conditions. 

 It is caused by the fact that in the dead subject, owing to the 

 decreased tension of the internal muscles of the ear, the articular 

 surfaces between the malleus and the incus are able to glide upon 

 one another, because the spurs do not catch. Normally the incus 

 and stapes are closely pressed together in every position by the 

 tone of the muscles which pull the tympanic membrane and the 

 handle of the malleus inwards. 



IV. It is obvious from their anatomical relations that the 

 internal muscles of the ear, the tensor tympani and stapedius, 

 regulate the position of the ossicles, and thus affect the tension 

 of the tympanic membrane and the pressure of the labyrinthine 

 fluid. 



The musculus malleolaris or tensor tympani (Fig. 79), which 

 was discovered and physiologically interpreted by Eustachius, 

 takes origin in a bony canal above the Eustachian tube, and its 

 tendon curves at a right angle to be inserted on the malleus, a 

 little below its axis of rotation. When the muscle contracts, the 

 manubrium is displaced into the tympanic cavity, and the 

 tympanic membrane and ligaments of the malleus are drawn 

 inwards and tightened, while the perilymph is compressed by the 

 stapes which stretches the membrane of the fenestra ovalis. The 



