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Physiology. — "On the pressure of sound in Cord's organ". By 

 Prof. H. Zwaardemaker. 



(Communicated in the meeting of May 27, 1905). 



According to the hypothesis of HelmhoI;TZ-Hensen the vibrations 

 of sound, penetrating into the inner ear by way of the stapes, 

 evoke a resonance in the transversely stretched fibres of the meni- 

 brana basilaris. Strong vibrations are imparted to different fibres 

 according to the pitch ; these vibrations being communicated to the 

 sensory epithelia of Corti's organ and then becoming tlie stimulus 

 for definite nerve-fibres. We recognise the tone by the nerve-fibres 

 which are affected in this way. 



That such short fibres as the transverse fibres of the membrana 

 basilaris can resound to the relatively deep tones of the human 

 scale is explained by Helmholtz 1»^ by the resistance in the fluid 

 and in the soft cell-masses (Clausius' cells); 2"^^, by their being loaded 

 with Corti's arches on which again a w^hole system of cells rests. 



At first it was imagined that the fibre vibrates in its entire lengtli 

 as a freely stretched string. Later attention has been drawn to the 

 fact that the pars arcuata (the part over which the Corti's arches 

 vault themselves) remains largely at rest while the pars pectinata 

 (the remaining part of the string, not covered by the arches) makes 

 the greatest excursions. But then the difference in length of the 

 fibres is no longer sufficient to explain the difference in the pitch 

 for which they are tuned, so that also a difference in tension and in 

 load must be assumed.') Examining the proportions of microscopical 

 preparations and bearing in mind that the arches are more or less 

 rigid formations, one is soon convinced that the pars arcuata cannot 

 possibly resound to the deep tones of the audible scale. It is not 

 the bottom cells on their upper face that are an impediment (o this, 

 but the large vein on their lower surface. Moreover the transverse 

 fibrous structure, which is so dislinct in the pars pectinata is entirely 

 absent in the pars arcuata. The property of resounding may on 

 sufficient grounds be attributed to the stretched and loaded fibres of 

 the pars pectinata only. 



I have tried, as far as this is possible, to reproduce in a model 

 the conditions prevailing in Corti's organ. A horizontal steel string, 

 7, millimetre thick and somewiiat longer than a metre, represents 

 a transversely stretched fibre of the membrana basilaris. On this 

 rests, at one of the ends, a wooden imitation of Corti's arches. The 



1) A. A. Gray, Journal of anat. and physio). 1900, Vol. 34. p. 324. 



