HO^V yINlMALS KNOIV THINGS 107 



discovered their presence in the building. The discovery 

 was undoubtedly made by the sense of smell. These 

 moths have very elaborately developed antenna;, finely 

 branched or feathered, affording opportunity for the ex- 

 istence of very many smelling-pits." 



Hearing is the perception of certain vibrations of bod- 

 ies. These vibrations give rise to waves — sound-waves 

 as they are called — which proceed from the vibrating 

 body in all directions, and which, coming to an animal, 

 stimulate the special auditory or hearing organs, which 

 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 

 (fig- 73)' which are adapted 

 for perceiving or being stimu- 

 lated by vibrations ranging from 

 16 to 40,000 a second — that 

 is, for hearing all those sounds 

 produced by vibrations of a 

 rapidity not less than 16 to a 

 second nor greater than 40,000 



to a second— are of such com- p,^ yj.-Diagram of human 

 plexity of structure that many ear; ^7, external opening; /' 

 '^^ • J r bones of the ear; /, labyrintli 



pages would be required tor r, cochlea or "snail shell " 



their description. But among "; «;ditory nerve. (After 

 '^ Headly.) 



the lower or less highly organ- 

 ized animals the ears, or auditory organs, are much 

 simpler. 



In most animals the auditory organs show the com- 

 mon characteristic of being wholly composed of, or hav- 

 ing, as an essential part, a small sac filled with liquid in 



