1328 THE AUDITORY OSSICLES. [BOOK m. 



Careful measurements shew that the whole length of the line 

 from F the fulcrum to P, where the power is applied, is about 



FIG. 173. THE MALLEUS AND INCUS, IN POSITION. (Helmholtz.) 



.. The malleus, c, the head, mbr, the handle, p.f, processes Folianus. T. T. the 



tendon of the tensor tympani. 

 /. The Incus, p'b', 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. 



9'5 mm., while the length from F to W, where the effect is 

 brought to bear, is about 6'3 mm. Hence when 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 tympanic membrane. By the 

 principle of the lever however the amount of pressure exerted by 

 the movement of the stapes, the 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. 



The conduction of sound through the Tympanum. 



816. 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 



