CHAP, iv.] HEARING. 989 



the tympanic membrane is driven inwards, the corresponding 

 inward movement of the stapes in the fenestra is as far as 

 extent is concerned only about two-thirds of that of the tym- 

 panic membrane. By the principle of the lever, however, the 

 amount of pressure exerted by the movement of the stapes, the 



p 



^i 



X 



FIG. 175. THE MALLEUS AND INCUS, IN POSITION. (Helmholtz.) 



M. The malleus, c, the head, m&r, the handle, p.f, processes Folianus. 

 T. T, the tendon of the tensor tyinpani. 



/. The incus. p'&', the short process, p'l'. the long process, t. tooth locking 

 with the malleus. 



The line XX represents the lever formed by the two ossicles, with, F, the 

 fulcrum at the attachment of the short process of the incus, P, the point where 

 the power is applied at the end of the handle of the malleus, W, the point where 

 the effect is produced at the os orbiculare of the incus. 



force of the movement, is one and a half times greater than 

 the force expended in producing the movement of the tympanic 

 membrane. The arrangement of the lever of ossicles therefore 

 is such as to convert a relatively large movement into a smaller 

 movement of greater intensity; the benefit of such a conversion 

 is obvious. 



614. The conduction of sound from the external air to the 

 labyrinth takes place by means of the tympanic membrane and 

 the chain of ossicles acting as a lever in the manner just described. 



Stretched membranes have the property of being readily 

 thrown into vibrations by aerial waves of sound, and of trans- 

 mitting the vibrations to bodies in contact with themselves. 

 The tympanic membrane is a stretched membrane which, by its 

 size, nature and conformation is specially adapted to take up 

 and transmit a great variety of vibrations. Sound is a vibration 

 of the particles of matter, a series of movements of the particles 

 from and to a fixed point. In air and other gases the move- 

 ments of the particles lead to alternating condensation and 

 rarefaction of the medium, the sound is propagated as waves of 

 alternating condensation and rarefaction, which since the to- 

 and-fro movement of the particles is in the same direction as 



