Semicircular 

 canals 



Cochlea 



B,F 



BALANCING ORGAN IN MAN 



In vertebrates the balancing organ consists essentially of three hollow rings lined 

 with sensitive nerve endings. These three rings correspond to the three planes of 

 the space in which we move about. We do not ordinarily "feel" the balance, but in 

 skating, dancing, flying, tightrope walking, in all physical activities that involve rapid 

 changes in the body's position, the co-ordination of movements depends largely 

 on these canals 



tains some floating grains of sand (see illustration, p. 288). An experimenter 

 removed the sand from several crayfish and replaced it v^^ith iron particles. 

 This did not affect the movements of the crayfish; but w^hen he brought 

 a magnet near one of these animals, it behaved as if the side tow^ard 

 the magnet were down. This experiment shows that the statocyst works 

 through the displacement of the solid particles in the course of the animal's 

 movements. It also shows the automatic character of some of the animal's 

 adjustments. 



Hearing When we hear the low roar of the airplane engine becoming 

 steadily louder, it does not occur to us that we are touched by anything. We 

 think of the sound as coming from a great distance, as we think of the air- 

 plane itself going a great distance. Yet we may reasonably think of our 

 hearing organs as highly specialized touch receptors. For according to the 

 studies of physicists, sensations of sound correspond to vibrations in the air 

 — actual air movements striking upon our eardrums much as waves of the 



286 



