XV 



HEARING 



Hearing, like sight, is one of the distant-receptor senses and 

 is concerned with the perception of vibrations, though of a 

 very different kind. Unhke the electromagnetic waves of 

 light, which do not need any physical medium for their 

 transmission, the vibrations that produce sounds can travel 

 only in things that can be made to vibrate with them, solid, 

 liquid or gas. The vibrations can vary in frequency from very 

 low to very high but it is only certain of them, in the range 

 from about i6 a second to 30,000 a second at the utmojt, 

 that produce what we call sounds. They are generally, but 

 not necessarily, brought to our ears by vibrations in the air. 

 Vibrations of a higher frequency than our upper limit are 

 not only inaudible to us but imperceptible unless their inten- 

 sity is very much greater than normally occurs in nature. 

 On the other hand we can perceive vibrations of a frequency 

 below our lower limit, not as sounds, but by generalized 

 feeling. The vibrations, if strong enough, make our bodies 

 vibrate too and we become aware of them through our 

 "proprioceptive" senses which are discussed in a later 

 chapter. 



The ear is a device for converting the sound waves that 

 reach us into messages that can be sent to the brain, and 

 for distinguishing between sounds of different wave-length. 

 The sound waves do not directly stimulate the nerve endings, 

 but are first converted into touch stimuli which produce the 

 physico-chemical changes needed to start the message along 

 the nerves. As may be imagined, a complicated mechanism is 

 required to carry out these processes. As in dealing with 

 sight we shall first consider the ear in ourselves and those 

 vertebrates in which it most nearly resembles ours before 

 turning to the more different forms in other animals. 



The ear in mammals consists of much more than the 



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