748 HEARING [CH. LIV. 



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 concentrates its power upon 

 an area (the membrane of the oval window) only one-twentieth of 

 its size. The final movement of the stapes is, however, always very 

 small ; it varies from T V to less than 10 ^ 00 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. 550) will explain the use of 

 the fenestra rotunda. 



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

 from the fenestra ovalis, to the other side of which the stapes is 



F.Oualls 

 Stapes 



Scala Vestibuli (Perilymph) 



Scala Tympani (Perilymph) 



Helicotrema 



F. Rotunda 



Fio. 550. Diagram to illustrate the use of the fenestra rotunda. 



attached ; the scala tympani leads to the fenestra rotunda ; the two 

 scalse communicate at the helicotrema, and are separated from the 

 canal of the cochlea by the basilar membrane, and the membrane of 

 Keissner. C.E. is the canalis reuniens leading to the saccule. The 

 cochlea is filled with incompressible fluid in an inexpansible bony 

 case, except where the windows are closed by membranes. Hence 

 every time the membrane of the oval window is bulged in by the 

 stirrup, the membrane of the round window is simultaneously bulged 

 out to the same extent, and vice versd. These changes of pressure 

 are transmitted from one scala to the other directly through the 

 cochlear canal, setting it into vibration, and through the helicotrema. 

 The range of hearing extends over 10 or 11 octaves; the lowest 

 audible tone having about 20, the highest about 25,000, vibrations 

 per second. The range varies in different people, and diminishes 

 from childhood onwards. The upper limit of hearing may be tested 

 by minute forks, metal rods, or by Galton's whistle. Many animals 

 appear to be able to detect high tones which lie beyond the human 

 limit. The lower limit may be determined by very large forks, or by 

 employing very low difference-tones. 



