802 HEARING, SMELL, AND TASTE. 



limits of hearing, each vibration or series of vibrations must produce its 

 effect on the auditory nerves, according to the measure of its intensity and 

 period. Out of those effects, out of the sensory impulses to which the several 

 vibrations thus give rise, are generated our sensations of the noise or of the 

 sound. 



The vibrations of a musical sound (and since noises are so imperfectly 

 understood, we may, with benefit, chiefly confine ourselves to musical 

 sounds), as they pass through the air (or other medium) are not discrete ; 

 the vibrations corresponding to the fundamental tone and overtones do 

 not travel as so many separate waves ; they all together form one com- 

 plex disturbance of the medium ; and it is as one composite wave that 

 the sound falls on the membrana tympaui, and passing through the audi- 

 tory apparatus, breaks on the terminations of the auditory nerve. And 

 when two or more musical sounds are heard at the same time, the same 

 fusion of the waves occurs. Since we can distinguish several tones reach- 

 ing our ear at the same time, it is clear that we must possess in our 

 minds or in our ears some means of analyzing these composite waves of 

 sound which fall on our acoustic organs, and of sorting out their con- 

 stituent vibrations. 



716. There is at hand a simple and easy physical method of analyzing 

 composite sounds. If a person standing before an open piano sings out any 

 note, it will be observed that a number of the strings of the piano will be 

 thrown into vibration, and on examination it will be found that those strings 

 which are thus set going correspond in pitch to the fundamental tone and to 

 the several overtones of the note sung. The note sung reaches the strings 

 as a complex wave, but these strings are able to analyze the wave into its 

 constituent vibrations, each string taking up those vibrations and those vibra- 

 tions only which belong to the tone given forth by itself when struck. If we 

 suppose that each terminal fibril of the auditory nerve is connected with an 

 organ so far like a piano-string that it will readily vibrate in response to a 

 series of vibrating impulses of a given period and to none other, and that 

 we possess a number of such terminal organs sufficient for the analysis of all 

 the sounds which we can analyze, and that each terminal organ so affected 

 by particular vibrations gflves rise to a sensory impulse and thus to a sensa- 

 tion of a distinct character if we suppose these organs to exist, our appre- 

 ciation of sounds is in a large measure explained. Tn the organ of Corti 

 we find structures, the arrangement of which irresistibly suggests to us that 

 these are the organs we are seeking. We have only to suppose that of the 

 long series of rods of Corti, varying regularly as these do from the bottom 

 to the top of the spiral, in length and in the span of their arch, each pair 

 will vibrate in response to a particular tone, and the whole matter seems 

 explained. But the more the subject is inquired into, the more complex and 

 difficult it appears ; and we are obliged to conclude that the part played by 

 the rods of Corti is only a subordinate part of the function of the whole 

 organ of Corti. 



In the first place, it is difficult to see how the rods of Corti, even if they 

 are thrown into vibration, can originate sensory impulses, for the fibrils of 

 the auditory nerve terminate in the inner and outer hair-cells, and it is in 

 these cells, and not along the course of these fibrils as they pass under and 

 between the rods of Corti, that the sensory impulses must begin. In the 

 second place, the variation in length of the fibres along the series is insuffi- 

 cient for the work assigned to them. Moreover, they appear not to be clastic. 

 Lastly, they are wholly absent in birds, who very clearly can appreciate 

 musical sounds. This last fact proves indubitably that the rods in question 

 are not absolutely essential for the recognition of tones. In the face of these 



