COCHLEAR MICROPHONICS 281 



seconds later the tympanic tensor contracted, overpowering the stape- 

 dius contraction and forcing the footplate inward. This subjected the 

 interscalar fluid to a steep pressure gradient at the footplate. The 

 stapedius then relaxed, permitting the tympanic tensor to push the foot- 

 plate further inward. The tympanic tensor then relaxed, and finally 

 the elastic recoil of the supporting structure shifted the footplate back 

 into its normal position. 



If Wiggers' interpretation of the electrical response data is correct, it 

 should be borne out by the structural characteristic of the stapes and its 

 possible degrees of freedom of motion under the action of the interaural 

 muscles as shown in Fig. VII-15. 



I. The stapedius tensor contracts. The footplate moves outward, 

 and as a result a small increase in negative potential is developed at the 

 apex of the cochlea. 



Mechanically, when the stapedius tensor contracts, the heel (posterior 

 margin) of the footplate is pushed in, and the toe (anterior margin), 

 suspended by longer ligaments than those attached to the heel, is pulled 

 out further. Since the structure of the arch is not symmetrical, the 

 stapedius contraction forces the superior margin out and the inferior 

 margin in. This diagonal thrust produces an effective outward excur- 

 sion of the footplate. All these movements are shown diagrammatically 

 in Fig. VII-15. 



II. The tympanic tensor contracts and overpowers the sustained 

 stapedius contraction. The footplate is forced inward. A positive 

 potential is generated. 



Mechanically, when the tympanic tensor contracts and overpowers 

 the stapedius contraction, it pushes the footplate in. The heel, set for 

 maximum displacement, cannot move further in, but the toe can be 

 pushed in with the accompanying production of a positive electrical 

 potential at the cochlear apex. 



III. The high positive potential rapidly drops to zero after the foot- 

 plate has attained its initial inward displacement. The fluid in the 

 cochlea must adjust itself via the helicotrema, reducing the electrical 

 potential to zero. 



IV. The stapedius tensor relaxes. The tympanic tensor continues to 

 contract and thrusts the anterior edge of the footplate as far inward as 

 the length of the annular ligaments permit, thus creating a second 

 positive potential at the apex. 



After the tension of the stapedius tensor has fallen to zero, the toe 

 of the footplate is displaced farther inward until it reaches its elastically 

 limiting position. This excursion is not so great as the previous positive 

 one and will, therefore, produce a smaller positive potential. 



