962 A MANUAL OF PHYSIOLOGY 



despairing of further analysis, content themselves with the con- 

 clusion that the organ of Corti vibrates as a whole. Some of 

 these theories will be again referred to in considering what is the 

 greatest problem of the physiology of hearing, viz. : 



The Perception of Pitch Analysis of Complex Sounds. 

 As the eye, or, rather, the retina plus the brain, can perceive 

 colour, so the labyrinth plus the brain can perceive pitch. The 

 colour-sensation produced by ethereal waves of definite fre- 

 quency depends on that frequency ; and upon the frequency of 

 the aerial vibrations depends also the pitch of a musical note. 

 But there is this difference between the eye and the ear : that 

 while the sensation produced by a mixture of rays of light of 

 different wave-length is always a simple sensation that is, 

 a sensation which we do not perceive to be built up of a number 

 of sensations, which, in other words, we do not analyze the 

 ear can perceive at the same time, and distinguish from each 

 other, the components of a complex sound. When a number 

 of notes of different pitch are sounded together at the same 

 distance from the ear the disturbance which reaches the mem- 

 brana tympani is the physical resultant of all the disturbances 

 produced by the individual notes, and it strikes upon the mem- 

 brane as a single wave. ' A single curve describes all that the 

 ear can possibly hear as the result of the most complicated 

 musical performance. ... In the complicated sound the varia- 

 tions of the pressure of the air are more abrupt, more sudden, 

 less smooth, and less distinctly periodic than they are in softer, 

 purer, and simpler sound. But the superposition of the different 

 effects is really a marvel of marvels ' (Kelvin). The ear or 

 brain must, therefore, possess the power of resolving the complex 

 vibrations into their constituents, else we should have a mixed or 

 blended sensation, and not a sensation in which it is possible to 

 distinguish the constituents of which it is made up. Several 

 hypotheses have been proposed to explain this physiological 

 analysis of sound, on the assumption that the analysis takes place 

 in the labyrinth. The most important, in spite of certain defects, 

 is still that of Helmholtz. 



Helmholtz attempted to explain the perception of pitch on 

 the assumption that in the internal ear there exists a series 

 of resonators, each of which is fitted to respond by sympathetic 

 vibration to a particular note, while the others are unaffected ; 

 just as when a note is sung before an open piano it is taken up 

 by the string which is attuned to the same pitch and ignored by 

 the rest. Let us suppose that a given fibre of the auditory nerve 

 ends in an organ which is only set vibrating by waves impinging 

 on it at the rate of 100 a second, and that the end-organ of another 

 fibre is only influenced by waves with a frequency of 200 a second. 



