234 PHYSIOLOGY CHAP. 



He further calculated that there must be a second combination 

 tone, which must be higher than the two primary tones, to which 

 he gave the name of summational tone, because it results from 

 the sum of their vibrations. He assumed that the summational 

 tone must be far more difficult to distinguish than the differential 

 tone, because it is weaker, and that it can only be detected when 

 the primary tones are very loud, and the ear brought close to 

 the organ-pipe used as the sounding instrument. It must be 

 said, however, that many physicists and physiologists with a 

 keen and well-trained ear are unable to perceive the summational 

 tone of Helmholtz. Eighi is inclined to regard the summational 

 tone as a purely theoretical mathematical deduction. 



XL We must now return to the main problem in the 

 physiology of audition namely, the mechanism by which the 

 more or less compound tones give rise in the organ of Corti to 

 neural excitations, which in consciousness assume the form of 

 auditory perceptions, with all their various quantitative and 

 qualitative characters. To appreciate the difficulty of solving 

 this psycho-physical problem, it is necessary to remember that 

 our auditory apparatus is capable not only of perceiving funda- 

 mental tones and sounds, but also of analysing and distinguishing 

 the separate elementary components of the most complex 

 vibrations, which result from the algebraic sum of a great 

 number of coincident tones and sounds. 



Space forbids us to enter into the various and imperfect 

 theories put forward in explanation of the phenomena of hearing ; 

 we can, only investigate those which, however incomplete and 

 inadequate, have found wide acceptance, because they agree best 

 with the general principles of psycho-physics. 



From 1877 to quite recent times the illustrious name of 

 Helmholtz fathered an ingenious theory of audition, which was 

 based on the law of resonance and the extension of Johannes 

 Muller's law of specific energy to the separate fibres of the 

 auditory nerve. 



According to Helmholtz it is impossible that excitation of 

 one and the same fibre of the auditory nerve can give rise in 

 consciousness to sensations of tones of different pitch. Seeing 

 that, so far as we know, the stimulation of a nerve invariably 

 produces the same effect, whatever the stimulus that arouses it, 

 and that the quality of the effect remains the same, whatever 

 the rhythm of the stimulation, it follows according to Helmholtz 

 that one and the same fibre of the auditory nerve cannot 

 respond to tones of different pitch, but must always evoke the 

 sensation of a tone of uniform pitch. So that to explain the 

 faculty by which the ear perceives different notes of the musical 

 scale it is necessary to extend the law of specific energy to the 

 component fibres of the cochlear nerve, and to assume that the 



