292 DYNAMICAL THEOKY OF SOUND 



The student of dynamics cannot fail to admire the beauty 

 of a theory which lends itself readily to the explanation of so 

 many complicated relations; but it is with the physiologist 

 and the anatomist that in the last resort it lies to decide 

 whether a mechanism of the kind postulated is really to be 

 found in the internal ear. In the original form of the theory 

 (1862) the resonators were identified with the structures known 

 as "Corti's rods," which are found arranged, some 3000 in 

 number, along the basilar membrane in the spiral cavity 

 of the cochlea. A disturbing discovery by Basse that these 

 structures do not occur in the ears of birds, to whom we can 

 hardly deny the perception of pitch, led to a modified form 

 of the theory. In the third edition of the Tonempfindungen 

 (1870) Helmholtz propounded the view that the resonators 

 consist of the various parts of the basilar membrane itself. 

 This membrane varies in breadth from one end to another, 

 like a very acute-angled triangle, and the tension appears to 

 be very much less in the direction of length than in that 

 of breadth. On this view the different parts could be set 

 into sympathetic vibration, much as in the case of a series 

 of strings of variable length placed side by side, except that 

 the independence of adjacent parts would be approximate 

 instead of absolute. For a full description of the complicated 

 structure of the internal ear, and for further speculations as 

 to the functions performed by its various parts, we must refer 

 to books on physiology. 



95. Combination-Tones. 



In one important respect the theory as so far developed 

 is inadequate. The explanation of consonant intervals outlined 

 in 93 assumes that one at least, and generally both, of the 

 notes concerned is complex, and contains one or more overtones 

 in addition to the fundamental. It was in fact through the 

 interference of two tones, one at least of which is an overtone, 

 that departure from the exact relation of pitch was stated to 

 make itself manifest. When both tones are pure this means 

 of definition is wanting, and on the theory of audition sketched 

 in the preceding section there appears to be no reason why 



