552 THE HUMAN BODY. 



branous labyrinth to the endolymph. These liquids being 

 chiefly water, and practically incompressible, the end of the- 

 stapes could not work in and out at the oval foramen, were- 

 the labyrinth elsewhere completely surrounded by bone: 

 but the membrane covering the round foramen bulges out. 

 when the base of the stapes is pushed in, and vice versa; 

 and so allows of waves being set up in the labyrinthic 

 liquids. These correspond in period and form to those in 

 the auditory meatus; their amplitude is determined by the- 

 extent of the vibrations of the drum-membrane. 



The form of the tympanic membrane causes it to trans- 

 mit to its centre, where the malleus is attached, vibrations-, 

 of its lateral parts in diminished amplitude but increased 

 power; so that the tympanic bones are pushed only a little- 

 way but with considerable force. Its area, too, is about 

 twenty times as great as that of the oval foramen, so that 

 force collected on the larger area is, by pushing the tym- 

 panic bones, all concentrated on the smaller. The ossicles, 

 also form a bent lever (Fig. 144)* of which the fulcrum is at 

 the axial ligament and the effective outer arm of this lever 

 is about half as long again as the inner, and so the move- 

 ments transmitted by the drum-membrane to the handle of 

 the malleus are communicated with diminished range, but. 

 increased power, to the base of the stapes. 



Ordinarily sound-waves re^ch the labyrinth through the 

 tympanum, but they may also be transmitted through the 

 bones of the head; if the handle of a vibrating tuning-fork 

 be placed on the vertex, the sounds heard by the person 

 experimented upon, seem to have their origin inside his own. 

 cranium. Similarly, when a vibrating body is held between 

 the teeth, sound reaches the end organs of the auditory 

 nerve through the skull-bones; and persons who are deaf 

 from disease or injury of the tympanum can thus be made 

 to hear, as with the audiphone. Of course if deafness be 

 due to disease of the proper nervous auditory apparatus no 

 device can make the person hear. 



Function of the Cochlea. We have already seen reason 

 to believe that in the ear there is an apparatus adapted for 

 sympathetic resonance, by which we recognize different: 



*P. 537. 



