376 



THE SENSORIAL rUNCTIONS. 



§ 2. Physioloiry of Hearing in Man. 



That part of the organ of hearing, which, above all 

 others, is essential to the performance of this func- 

 tion, is the acoustic nerve, of which the fibres are 

 expanded, and spread over the surface of a fine 

 membrane, placed in a situation adapted to receive 

 the full impression of the sonorous undulations 

 which are conveyed to them. This membrane, then, 

 with its nervous filaments, may be regarded as the 

 immediate organ of the sense ; all the other parts 

 constituting merely an accessory apparatus, de- 

 signed to collect and to condense the vibrations of 

 the surrounding medium, and to direct their con- 

 centrated action on the auditory membrane. 



I have endeavoured, in Fig. 390, to exhibit, in 

 one view, the principal parts of this complicated 



organ, as they exist in man, in their relative situa- 

 tions, and of their natural size ; thereby affording 

 a scale by which the real dimensions of those 



