HERMANN VON HELMHOLTZ 



tions. The minute size of the structures does not 

 present any difficulty ; because, however minute the 

 vibrators might be, if they had different periods, they 

 must act in obedience to the same principles of 

 resonance as larger bodies do outside the ear. In 

 1863, Helmholtz was of opinion that the different 

 degrees of tension in the arches of Corti indicated 

 capacity for vibrating at different periods. Soon 

 after, it was shown by Hasse that these rods do not 

 exist in birds, animals presumably capable of appre- 

 ciating tones ; and Hensen pointed out that the 

 membrana basilaris, on which the rods rest, consisted 

 of transverse fibres, which varied in length approxi- 

 mately from -g^jth of an inch, at the base of the cochlea, 

 to - 6 Vh of an inch at its apex. This led Helmholtz 

 to suggest that it is probably the breadth of the 

 membrana basilaris in the cochlea which determines 

 the tuning. He pointed out that the membrane 

 was in a state of tension transversely, while it has 

 only little tension in the longitudinal direction, and 

 that such a membrane had very different properties 

 from that of a membrane which had the same tension 

 in all directions. The membrane, in fact, behaves 

 like a system of stretched strings, bound together by 

 a semifluid substance. Each string or fibre would 

 act independently of the others, and would be set 

 into vibration by an impulse to the fluid in the scala 

 vestibuli, corresponding to its period. Consequently, 

 if a part of the membrane were called into action, one 

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