XXXV.] Tilt: SEKSES. 101 



networks, the purpose of which appears to he to obtain the nerves of 

 necessary extent of sensitive surface ; and the optic nerve ^^J^^^ ' 

 branches out into a network through the retina (which smelJ, aud 

 organ derives its name from this fact), for the purpose of sensitive 

 providing a surface sensitive to light, on wliich the image '^^'''^'Ces. 

 of the object looked at is to be formed. But the nerves of 

 hearing terminate in a totally different way from any of 

 these.^ No image has to be formed in the ear analogous to Distribu- 

 the retinal image in the eye, and consequently no sentient ]/e°rve°s of^*^ 

 surface is needed like the retina, nor is an extended ^f?^""g 

 sentient surface needed for any other purpose. Conse- 

 quently the nerves of hearing are not spread out at their 

 terminations, but are concentrated togetlier in " Corti's 

 organ." There are about three thousand of these nerve- 

 fibres ; and we have good reason to believe that each one 

 of these is sensitive, not to all sounds, but only to sounds 

 of a pitch altogether or nearly identical with that to which 

 it is itself strung. So tliat different sounds, when heard at 

 the same time, are heard by different nerves, and make 

 each its own impression on the consciousness : unlike 

 colours, or smells, or tastes, which, when mixed, are felt by 

 the same nerves, and make a combined impression on the 

 consciousness. 



In order to explain how sounds act each on its own Laws of 

 appropriate nerve, we must refer to the general laws of suikmous 

 acoustic vibrations. Every stretched string has a period 

 of vibration belonging to itself, which is not affected by vibration 

 the manner in which it is caused to vibrate, but is constant constant 

 so long as its length and its tightness are unchanged ; in s;„„e 

 other words, the number of vibrations which a string makes ^^t'l'ig- 

 in a second is constant for the same string. The pitch of a 

 sound depends on the number in a second of the sonorous 

 vibrations, which, when transmitted thiough the air to the 

 ear, excite the sensation of sound ; so that the same string, 

 by its vibrations, always produces sound of the same pitch ^otu con- 

 — a single vibration of the string, of course, producing a ^f ""**"'' 

 single sonorous vibration, or sound-A^'ave. The lav/s of string. 



1 l'"<;r the fullowiiig details, see the review of TyndaUV teetures oji 

 Soiuid, in lliB Ediiiliurgh Ueview of January lSfi8v ^ 



