724 



HEARING. 



[CH. LIII. 



The following diagram will assist us in understanding how this 

 is brought about. The bones all vibrate as if they were one, the 

 slight movements between the individual bones being inappre- 

 ciable. The utility of there being several bones is seen when the 

 vibrations are excessive ; the small amount of " give " at the 

 articulations is really protective and tends to prevent fractures. 



The handle of the malleus is inserted between the layers of the 

 tympanic membrane ; the processus gracilis (p. </.) has its end A 

 attached to the tympanic wall on the inner aspect of the Glaserian 



fissure ; the end B of the 

 short process (s. p.} of the 

 incus is fastened by a 

 ligament to the opposite 

 wall of the tympanic cavity ; 

 the end D of the long pro- 

 cess of the incus articulates 

 with the stirrup, the base 

 of which is turned towards 

 the reader. The handle 

 vibrates with the mem- 

 brana tympani ; and the 

 vibrations of the whole 

 chain take place round the 

 axis of rotation AB. Every 



time C comes forwards D comes forwards, but by drawing per- 

 pendiculars from C and D to the axis of rotation, it is found that 

 D is about |- of the distance from the axis that C is. So in the 

 transmission of the vibrations from membrane to membrane 

 across the bony chain, the amplitude of the vibration is decreased 

 by about -|, and the force is correspondingly increased. The 

 final movement of the stapes is, however, always very small ; it 

 varies from ^ to less than 10 ooo of a millimetre. 



The action of the tensor tympani by piilling in the handle of 

 the malleus increases the tension of the membrana tympani. It 

 is supplied by the fifth nerve. It is opposed by the strong 

 external ligament of the malleus. The stapedius attached to the 

 neck of the stapes tilts it backwards. It is supplied by the 

 seventh nerve. 



We have still to consider the use of the fenestra rotunda ; this 

 is closed by a membrane, and its action is to act as a vent for 

 the vibrations of the perilymph. The next very simple diagram 

 (fig. 548) will explain how this happens. 



The cochlea is supposed to be uncoiled ; the scala vestibuli 

 leads from the fenestra ovalis, to the other side of which the 



Foot of 

 Stapes 



Fig. 547. Diagrammatic view of ear ossicles. 



