CHAPTER XXX. 



THE SPECIAL SENSES (Cont'd). 

 Hearing 1 . 



Like light, sound travels in waves, but not as transverse waves 

 of the ether that fills space, but as longitudinal waves of con- 

 densation and rarefaction of the atmosphere itself. The magni- 

 tude of these waves is much greater and their rate of trans- 

 mission much slower than the waves of light; therefore we see 

 the flash of a gun long before we hear its sound. The several 

 qualities of sound, such as pitch, loudness and quality or timbre, 

 depend respectively on the frequency, the magnitude and the 

 contour of the waves. Sound waves are not appreciated by the 

 ordinary nerve receptors but only by those of the cochlear 

 division of the eighth nerve. These are connected, in the cochlea 

 of the internal ear, with a highly specialized receptor capable 

 of converting the sound waves into nerve impulses. The cochlea 

 consists of a bony tube wound two and one-half times as a spiral 

 around a central column, up the center of which runs the 

 end of the cochlear nerve. A longitudinal section of the 

 cochlea (Fig. 56), therefore shows us this spiral tube in sec- 

 tion at several places, and it is noticed that there projects 

 into it from the central column a ledge of bone having a C-shaped 

 free margin. From the lower lip of the C a membrane, called 

 the basilar membrane, stretches across the tube which it thus 

 divides into two canals, of which the upper is again divided into 

 two by another membrane running from the upper surface of the 

 bony ledge. 



The basilar membrane is a very important part of the mechan- 

 ism for reacting to sound waves. Resting on it is a peculiar struc- 

 ture called the Organ of Corti (Fig. 57), which in transverse sec- 

 tions of the cochlear canal is seen to be composed of two rows of 

 long epithelial cells set up on end like the rafters of a roof, with 



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