CHAPTER XXXIII. 



HEARING. 



Just as impulses traveling along the optic nerves can only 

 give rise, in the sensorium, to impressions of light, so impulses 

 communicated to the portio molUs of the seventh pair of cranial 

 nerves can only excite impressions of sound, and any stimulation 

 of that nerve gives rise to sound sensations. 



The peripheral end of the special nerve of hearing is dis- 

 tributed to an organ of very peculiar construction situated in 

 the internal ear, which, from its complexity, has been called the 

 labyrinth. The nerve ending is spread out between layers of 

 fluid, so that it must be affected by very gentle forms of stimu- 

 lation ; and, when we know its delicacy, we can hardly be sur- 

 prised that even sound vibrations suffice to stimulate this terminal 

 to transmit a nerve impulse to the brain. But the organs of 

 hearing of mammalia and man are so deeply placed in the petrous 

 part of the temporal bone that a special mechanism has to be 

 adopted to convey the sound with sufficient intensity from the 

 air to the fine nerve terminals. These beautiful contrivances 

 make up a complex piece of anatomy which will be briefly 

 referred to presently. 



SOUND. 



Before attempting to describe the complex mechanisms by 

 means of which the sound is conveyed to the nerve endings, some 

 notion must be formed of what sound is from a merely physical 

 standpoint. Without the sense of hearing one cannot form any 

 idea of sound, and here the knowledge of sound ends with many 

 people, since they only think of it as something they can hear. 

 A physicist, however, regards sound in a very different way. He 

 knows that it is caused by a kind of motion known as the vibra- 

 tions of elastic bodies, such as a tense string, a metal rod, or an 

 elastic membrane. These vibrations, being communicated to the 

 air, are conveyed by it to our nerve endings, where they set up 



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