CHAPTER XXXIII. 

 HEARING. 



JUST as impulses travelling along the optic nerves can only 

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

 communicated to the portio mollis 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 distrib- 

 uted 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 stimulation ; 

 and, when we know its delicacy, we can hardly be surprised that 

 even sound vibrations suffice to stimulate this terminal to trans- 

 mit 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 com- 

 plex 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 



