232 ANIMAL LIFE 



the case of man the sense of smell is not nearly so well 

 developed as among many of the other vertebrates. This 

 inferiority is largely due to degeneration through lessened 

 need; for in Indians and primitive races the sense of 

 smell is keener and better developed than in civilized 

 races. Where man has to make his living by hunting, and 

 has to avoid his enemies of jungle and plain, his special 

 senses are better developed than where the necessity of 

 protection and advantage by means of such keenness of 

 scent and hearing is done away with by the arts of civi- 

 lization. 



122. The sense of hearing. Hearing is the perception 

 of certain vibrations of bodies. These vibrations give rise 

 to waves sound waves as they are called which proceed 

 from the vibrating body in all directions, and which, com- 

 ing to an animal, stimulate the special auditory or hearing 

 organs, that transmit this stimulation along the auditory 

 nerve to the brain, where it is translated as sound. These 

 sound waves come to animals usually through the air, or, 

 in the case of aquatic animals, through water, or through 

 both air and water. 



The organs of hearing are of very complex structure 

 in the case of man and the higher vertebrates. Our ears, 

 which are adapted for perceiving or being stimulated by 

 vibrations ranging from 16 to 40,000 a second that is, for 

 hearing all those sounds produced by vibrations of a rapid- 

 ity not less than 16 to a second nor greater than 40,000 to 

 a second are of such complexity of structure that many 

 pages would be required for their description. But among 

 the lower or less highly organized animals the ears, or au- 

 ditory organs, are much simpler. 



In most animals the auditory organs show the common 

 characteristic of being wholly composed of, or having as 

 an essential part, a small sac filled with liquid in which 

 one or more tiny spherical hard bodies called otoliths are 

 held. This auditory sac is formed of or lined internally by 



