CHAPTER XLVI 



SOUND SENSATIONS 



281. What we hear. Certain kinds of movements, or vibra- 

 tions, of the air, when they make an impression upon our 



nerves, give 



Fig. 99. Lateral line in brook trout 



The line running along each side from the gill cover 



to the tail is made up of nerve end-organs that are 



sensitive to certain kinds of vibrations in the water 



US the feehng of '' A^," just as certain other 

 kinds of vibrations give us the feeling of "yellow." The eye 



with which we see is an 

 organ whose nerve end- 

 ings receive impres- 

 sions from vibrations in 

 the ether at the rate of 

 from 400,000,000,000 

 to 800,000,000,000 

 per second. If the vi- 

 bration is much more 

 rapid or much slower, 

 the eye nerves are not 

 affected by them. Sound is a much slower vibration of the 

 air or of solids. Between 16 or 20 vibrations in a second and 

 25,000 to 40,000 per second the human ear discovers sounds 

 of varying pitch. In the middle register, which includes most 

 of the sounds with which we are familiar, as the pitch of the 

 human voice, the ear can distinguish very slight differences of 

 pitch. It is possible for a trained ear to distinguish more than 

 1000 shades of pitch in one octave. 



282. Perception of other vibrations. Among lower animals the 

 range of vibrations that can be perceived varies considerably ; some 

 animals are quite insensitive to soimds of what we would consider 

 the common range of pitch, while some insects can perceive a much 

 higher tone than any human being can hear at all. The earthworm, 



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