HERMANN VON HELMHOLTZ 



This organ, as already said, consists of a complicated 

 series of sacs and tubes filled with fluid. In certain 

 situations the walls of the sac contain highly-differen- 

 tiated epithelial structures, which are intimately 

 related to the terminal filaments of the auditory 

 nerve. 1 The problem is to explain how the pres- 

 sures transmitted by the foot of the stapes affect 

 these terminal structures in such a way as to excite 

 sensations corresponding to the pitch, intensity and 

 quality of tones. 



Two small sacs, the utricle and saccule, are the first 

 structures that receive the impulses of the base of the 

 stapes. The utricle communicates with the semi- 

 circular canals, and the saccule with the long spiral 

 duct of the cochlea. The oval window, into which 

 the base of the stapes fits, is covered by a membrane. 

 Suppose the base of the stapes to be pushed in by the 

 pressure of a wave of sound, then, since the delicate sacs 

 and canals are all inclosed in cavities in the bone, having 

 rigid walls, it is clear that, as the fluid is practically 

 incompressible by the force applied, no movements 

 could be communicated to any of the delicate nerve- 

 endings. But one part of the osseous wall, next the 

 tympanum or middle ear, has a round opening, also 

 covered by a membrane. Thus, when the base of 

 the stapes is pushed inwards, the membrane cover- 



1 In some parts of this chapter I make free use of an article on 

 Hearing in Schafer's advanced Text-Book of Physiology, vol. ii., written by 

 my pupil, Dr Albert A. Gray, and myself. 



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