p r 



Ampulla 



Section of ampulla in motion 



HOW OUR BALANCING ORGAN WORKS 



In the rapid movement of a plane, every turn, bank, climb, or dive involves the 

 centrifugal effects upon the semicircular canals. As a result, the pilot is frequently 

 confused and unable to judge his position or direction except by means of special 

 instruments. In steady movement the hairs and the fluid in the canals move together 

 and there is no sensation. In a quick start or turn or stop, the fluid in the canals 

 holds back or runs ahead and so bends the sensitive hairs 



sea strike against a floating buoy, setting it in motion. In other air-breathing 

 vertebrates the hearing organ is very much like our ow^n (see illustration, 

 p. 289). The stretched membrane, or drum, is the receiving area for sound 

 vibrations in many diflerent types of animals. In some insects and spiders, 

 however, the sound waves are received by fine, stretched hairs connected 

 with nerve fibers or by fine hairs standing out on the antennae. 



Animals differ very much as to the range of sound vibrations they can 

 perceive. Some animals are quite insensitive to sounds that nearly all hu- 

 man beings can hear, while some insects can perceive a much higher pitch 

 than any human being. The human ear discovers sounds of various pitch 

 when the vibrations of the air are at least 16 to 20 per second but not more 

 than 25,000 to 40,000 per second. In the middle register, which includes the 

 range of the human voice and most familiar sounds, we can distinguish 

 very slight differences in pitch. A trained ear can distinguish more than 

 1000 shades of pitch in one octave. 



Chemical Sensitiveness Protozoa are attracted by the presence of vari- 

 ous kinds of bacteria, but they are repelled by various chemical substances. 



287 



