v THE SENSE OF HEARING 245 



By means of Ewald's theory it is possible to exteud to auditory 

 sensation the principle demonstrated by Mach for tactile and 

 visual sensation, vi/. that it is not necessary in explanation of 

 the sensations of the different tones to consider the separate fibres 

 or groups of fibres of the auditory nerve as specifically differen- 

 tiated. They may all be identical inter se. The sensations of 

 different tones depend on the spatial distribution of the peripheral 

 excitation. It is the auditory image that characterises the tone, 

 and this image is always well defined and easy to recognise, even 

 when, owing to the absence of certain waves, it is interrupted or 

 imperfect. These peripheral images caused by tones need not 

 (as in Helmholtz' theory) have any absolute, but only a relative 

 value. Only when they are a combination of different regular or 

 irregular spatial periods do they assume definite significance in 

 the sensory centres. The peripheral sound-images are only the 

 local signs, which are perceived in our consciousness as tones of 

 different pitch, strength, and timbre. 



Consonance arises when the auditory images corresponding to 

 two or more simultaneous tones are superposed and interposed in 

 regular alternation. Beats or intermittent tones are produced 

 when the rhythmical impulses required for the production of 

 stationary waves are not all equal so that rhythmical lacunae 

 occur in the sound-images. 



Ewald's theory explains our faculty of arranging tones in 

 series according to their pitch. We judge the spatial differences 

 between the sound -images as differences in tone. Different 

 individuals are more or less musically gifted, according to their 

 capacity of appreciating the different spatial relations between 

 the manifold acoustic images formed simultaneously on the 

 vibrating membrane of the organ of Corti. The ability to 

 judge the relative value of two notes that approximate closely 

 in the number of their vibrations depends on the power of 

 estimating the minute spatial differences between the auditory 

 images. Obviously this must vary considerably in different 

 individuals. 



Lastly, from the phylogenetic point of view, Ewald's theory 

 makes it comprehensible how a gradual, uniform, general evolution 

 of the whole apparatus of hearing was advantageous to future 

 generations, whatever the specific acoustic circumstances of their 

 environment. 



The theory of acoustic images is also supported by the facts 

 of pathology and experimental physiology. Deafness to deep 

 tones when the apical convolution of the organ of Corti is affected 

 in man, or artificially destroyed in the dog, is the natural con- 

 sequence of the shortening of the acoustic scale at its lower end. 

 On the other hand, pathology shows that deafness to high tones 

 may be observed in cases of cochlear disease in which no segment 



