232 BIOLOGY AND HUMAN LIFE 



vibration is much more rapid or much less rapid, the nerves of 

 the eye are not affected by them. Some insects, as ants, are 

 sensitive to other vibrations that make no impression at all 

 upon our retina. 



190. Hearing. When the vibrations are more than from i6 to 

 20 per second and less than from 25,000 to 40,000 per second, 

 the human ear discovers sounds of various pitch. In the middle 

 register, which includes most of the sounds with which we are 

 familiar, as the range of the human voice, the ear can distin- 

 guish very slight differences in pitch. A trained ear can dis- 

 tinguish more than 1000 shades of pitch in one octave. 



In the air-breathing vertebrates the hearing organ is very 

 much like our own, which is pictured in Fig. 115. A vibration 

 striking the eardrum is transmitted through the chain of bones 

 in the middle ear to the liquid filling the labyrinth. From this 

 liquid it is transmitted to the delicate lining of the cochlea 

 (snail shell), in which the nerve endings are located. Here some 

 of the nerve endings are stimulated by vibrations of one pitch, 

 others by those of a different pitch. The nerve fibers are con- 

 nected with special cells in the brain. 



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

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

 human beings can hear, while some insects can perceive a much higher 

 pitch than any human being can discover. The stretched membrane, or 

 drum, is the receiving area for sound vibrations in many different types 

 of animals. In some insects and spiders, however, the sound waves are 

 received by fine stretched hairs connected with nerve fibers and by fine 

 hairs standing out on the antennae, or feelers. 



191. The senses and adjustment. Most of the organs through 

 which we receive stimuli from the outer world depend upon 

 having something come in direct contact with the body. Reac- 

 tion to such stimuli is ordinarily immediate — of a reflex char- 

 acter. If an animal is to profit from its ability to react to such 

 stimuli, it must react promptly ; and if the stimulus comes from 

 possible food, then reaction must take place before the food has 

 time to get away. 



