172 THE SENSES OF ANIMALS 



tubes join at the tip of the spiral so that they are really one, 

 and the fluid inside can pass from one to the other. The 

 other ends of this tube come to the surface of the bone on 

 the inner side of the middle ear cavity, and both are closed 

 by membranes stretched across the little windows through 

 which they look. The membrane that closes the upper tube 

 is attached to the stirrup-bone whose movements cause a 

 wave to run up the upper tube and down the lower one. 

 When this happens the membrane at the other window 

 bulges out or in as the one driven by the stirrup moves in or 

 out. The pressure in the outer tubes, the upper and lower 

 ones, is transferred through their membranous walls to the 

 fluid in the middle tube, which is completely cut off from 

 that in the outer ones. And it is in the middle tube that the 

 nerve endings lie. 



The floor of the middle tube contains many thousand 

 fibres stretching from side to side, and it increases in width 



UPPER TUBE 



OVERHANGING SHELF 

 ABOVE HAIR CELL 



HAIR CELL 



MIDDLE TUBE 



NERVE 



BONY SHELF 



STRETCHED 

 FIBRES 



LOWER TUBE 



Fig. 1 1 . a cross-section of one turn of the spiral cochlea, showing the upper, 

 lower and middle tubes and the bony shelf and the "hair" cells. 



