MOVEMENTS OF AUDITORY OSSICLES. 



95 



Movements of the auditory ossicles. The malleus and incus move together 

 round an axis extending backwards from the attachment of the malleus by its anterior 

 ligament to the attachment of the short process of the incus posteriorly. The 

 handle of the malleus follows all the movements of the membrana tympani, and 

 when the membrane is impelled inwards, the incus, moving inwards along with the 

 malleus, pushes the stapes inwards towards the internal ear. In this movement the 

 head of the stapes is slightly raised as well as pressed inwards, and the upper margin 

 of its base moves more than the lower. But the cavity of the inner ear is full of 

 liquid ; and its walls are unyielding, except at the fenestra rotunda ; when, there- 

 fore, the stapes is pushed inwards, the secondary membrane of the tympanum, which 

 blocks up the fenestra rotunda, must be bulged outwards. When the membrana 

 tympani returns to its original condition these movements are reversed. That the 

 movement inwards of the incus must closely accompany that of the malleus, is 

 necessitated by the fact that the lower margin of its articular surface has a well 



Fig. 103. FIGURE TO SHOW THE 



INTERLOCKING OP THE MALLEUS 



AND INCUS. (Helmholtz.) 



The line a I indicates the axis of 

 rotation of the two bones, the line 

 x y z joins this axis with the two 

 ends of the crank lever which these 

 bones form. 



body of incus head of malleus 



tooth of incus 

 interlocking 



with tooth 

 of malleus 



processus 

 yraciliis 



end of long process of incus 



marked projection which 

 catches against the prominent 

 border of the articular surface 

 of the malleus (fig. 102, sp, 

 and fig. 103). If, however, 

 in consequence of increase of 

 tension of the air in the tym- 

 panum, the malleus should be 

 moved too freely outwards, the 

 incus need not follow that 

 movement to its full extent, 

 but may merely glide over the 



smooth adjoining surface of the malleus, and thus the danger that there would 

 otherwise be of forcibly dragging out the stapes from the fenestra ovalis is avoided 

 (Helmholtz). 



The tensor tympani muscle, being attached near the base of the manubrium of 

 the malleus, draws the whole bone and the membrane inwards, tightening the latter. 

 Its action is opposed by the strong external ligament of the malleus. The tensor 

 tympani exerts but little rotating action upon the malleus. The action of the stape- 

 dius muscle is obviously to draw the head of the stapes backwards, in doing which 

 the hinder end of the base of that bone will be pressed against the margin of the 

 fenestra ovalis, while the fore part will be withdrawn from the fenestra. 



The lining membrane of the tympanum. The mucous membrane of the 

 tympanum is continuous with that of the pharynx through the Eustachian tube, and 

 is further prolonged from the tympanum backwards into the mastoid cells. The 

 malleus and incus are invested by the lining membrane of the outer wall of the 

 cavity. The membrane forms also folds extending down from the roof in front of 

 and behind the conjoined heads of the incus and malleus, and another passing down 

 to the chorda tympani nerve or even below it ; these folds wholly or partially 

 separate off small pouch-like portions of the tympanic cavity which will be further 



