85 o SPECIAL SENSES. 



pitch, which have been discussed rather fully under the head of the physics of sound. 

 As far as the terminal filaments of the auditory nerve are concerned, it is evident that 

 the intensity of sound is appreciated in proportion to the power of the impression made 

 upon these nerves, and this point does not demand elaborate discussion. With regard to 

 quality of sound, we have seen that this is due to the form of sonorous vibrations, and 

 that most musical tones are compound, their quality depending largely upon the relative 

 power of the harmonics, partial tones, etc. We have also seen that consonating bodies 

 repeat by influence, not only the actual pitch of tones, but their quality. If there be in 

 the cochlea an anatomical arrangement of rods or fibres by which the sonorous vibra- 

 tions, conveyed to the ear by the atmosphere, are repeated, there is reason to believe 

 that the quality, as well as the pitch, is reproduced. Narrowing down the question, 

 then, to its most interesting and important point, viz., the appreciation of differences in 

 the pitch of musical tones, we inquire whether there be in the cochlea any arrangement 

 by which the pitch can be repeated. This inquiry can only be answered by a study of 

 the anatomical arrangement of the structures connected with the terminal filaments of 

 the nerves, and by the application of physical laws. 



The arrangement of the rods which enter into the structure of the organ of Corti has 

 afforded a theoretical explanation of the final mechanism of the appreciation of pitch. 

 Until we come to the internal ear, the action of different portions of the auditory appa- 

 ratus is simply to conduct and repeat sonorous vibrations ; and the sole function of these 

 accessory parts, aside from the protection of the organs, is to convey the vibrations to 

 the terminal nervous filaments. Whatever be the functions of the membrana tympani in 

 repeating sounds by influence, it is certain that this membrane possesses no true auditory 

 nerves, and that the auditory nerves only are capable of receiving impressions of sound. 

 Thus, hearing, and even the appreciation of pitch, is not necessarily lost after destruction 

 of the membrana tympani; and, if sonorous vibrations reach the auditory nerves, they 

 will be appreciated and appreciated correctly. With this point clearly understood, we 

 are prepared to study the probable functions of the organ of Corti. 



When we consider the organ of Corti, with its eight thousand or more rods of differ- 

 ent lengths arranged with a certain degree of regularity, a number more than sufficient 

 to represent all the tones of the musical scale, we are not surprised that eminent physi- 

 ologists regard them as capable of repeating all the shades of tone heard in music. 

 Helmholtz formularizes this idea in the theory that tones conveyed to the cochlea throw 

 into vibration those elements of the organ of Corti which are tuned, so to speak, in unison 

 with them. According to this hypothesis, the rods of Corti constitute a harp of several 

 thousand strings, played upon, as it were, by the sonorous vibrations. 



It would be difficult to imagine any thing more satisfactory and simple than such an 

 hypothesis as we have just quoted. Attention and education enable persons endowed 

 with what is called a musical ear to discriminate between different tones with great 

 accuracy. Experiments have shown that the situation of the actual appreciation of 

 tones may be restricted to the cochlea ; and, in the cochlea, the only anatomical arrange- 

 ment, as far as we know, which points toward an appreciation of the pitch of different 

 tones is that of the rods x>f Corti. Still, it must be remembered that the cochlea is so 

 situated as to be removed from the possibility of experimental investigation to prove the 

 theory; and we must carefully study the anatomical arrangement of the parts and the 

 possible application of physical laws to the supposed vibration of the rods. 



Viewing the question from its anatomical aspect, it is by no means certain that the 

 rods of Corti are so attached and stretched that they are capable of separate and indi- 

 vidual vibrations. It has not been demonstrated that certain of these rods vibrate under 

 the influence of certain tones or that they are tuned in accord with certain tones. Hensen, 

 who has written elaborately upon the very question under consideration, denies the accu- 

 racy of the theory of Helmholtz, basing his opinion upon the anatomical arrangement of 

 the rods of Corti, and he assumes that it is a physical impossibility for the different rods 



