796 HEARING [CH. LVI. 



stapes is attached). Thus the vibrations are communicated to 

 the fluid of the internal ear, which is situated on the other side of 

 the oval window. 



The accompanying 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 inappreci- 

 able. 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.g.) has its end A 



attached to the tympanic wall on 

 the inner aspect of the Glaserian 

 fissure ; the end B of the short pro- 

 cess (s.p.) of the incus is fastened 

 by a ligament to the opposite wall 

 of the tympanic cavity; the end 

 D of the long process of the incus 

 articulates with the stirrup, the 

 -Foot of base of which is turned towards 

 Stapes the reader. The handle vibrates 

 with the membrana tympani ; and 

 the vibrations of the whole chain 

 C take place round the axis of rota- 



FIG. 496. Diagrammatic view of ear ossicles. tlOU AB. Every time C COmeS 



forwards D comes forwards; but 



by drawing perpendiculars from C and D to the axis of rotation, it is 

 found that I) is about f 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 J, and the force is correspondingly increased. This increase of 

 power is augmented by the fact that the tympanic membrane concen- 

 trates its power upon an area (the membrane of the oval window) only 

 one-twentieth of its size. The final movement of the stapes is, how- 

 ever, always very small; it varies from ^ to less than 10 ; OQ of a 

 millimetre. 



The action of the tensor tympani, by pulling 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 and diminishes the intra-tympanic air- 

 pressure. It is supplied by the seventh nerve. 



The next very simple diagram (fig. 497) will explain the use of 

 tihefenestra rotunda. 



The cochlea is supposed to be uncoiled ; the scala vestibuli leads 



