726 HEARING. [OH. LIII. 



vibrates in response to a sound but at different rates for different 

 sounds, so in this theory it is supposed that the basilar membrane 

 vibrates as a whole, the hair-cells on it are affected, the nerve 

 impulse travels to the brain, and the analysis of the sound occurs 

 there. In other words, the basilar membrane acts very much like 

 the membrana tympani. " It is the internal drum-head, repeating 

 the complex vibrations of the membrana tympani, and vibrating 

 in its entire area to all sounds although more in some parts than 

 in others giving what we may designate as acoustic pressure 

 patterns between the membrana tectoria and the subjacent field 

 of hair-cells. In place of an analysis by sympathetic vibration 

 of particular radial fibres, it may be imagined that varying com- 

 binations of sound give varying pressure patterns, comparable 

 to the varying retinal images of external objects." (Waller.) 



The Range of hearing is more extensive than that of voice. 

 Sounds can be heard that are produced by 30 vibrations per 

 second, up to those caused by 30,000 to 40,000 vibrations per 

 second ; and in this range as many as 6,000 variations of pitch 

 can be perceived, or about twice as many as the pairs of arches 

 of Corti. Two sounds can be recognised as distinct if the interval 

 between them is less than o'i second, a fact that shows us the 

 perfection of the damping as well as of the vibrating mechanism. 



The distinction between musical notes is not equally obvious 

 to all observers. People differ a good deal in the musical element 

 in their nature. But in all there is a limit" to the perception of 

 high-pitched notes. In Galton's whistle, one has an instrument 

 by which the rate of vibration of the air which produces the 

 sound can be increased ; it gets shriller and shriller, and at last 

 when the vibration frequency exceeds 30,000 or 40,000, the 

 sound becomes inaudible. Probably many animals, however, are 

 able to hear much higher notes than we can detect. 



A judgment, by the sense of hearing alone, of the direction in 

 which a sound comes, is always most imperfect. 



