252 



OUR BODIES AND HOW WE LIVE 



of smell. In the sense of hearing, impressions are made upon 

 the nerves by wavelike vibrations in the surrounding air. 



All sounds are caused by the vibration of something in 

 the atmosphere. The object struck sends out these vibra- 

 tions to the surrounding 

 air, which carries forward 

 a series of waves in all 

 directions. 1 



345. The Organ of Hear- 

 ing. The air waves are 

 received, and the impres- 

 sion made by them is sent 

 to the brain, by a special 

 apparatus, called the organ 

 of hearing. It is lodged in 

 the temporal bone, one of 

 the thick, inner bones which 

 form the base of the skull. 

 The ear, the organ of 

 hearing, is far more com- 



FIG. 155. The Pinna, or Auricle. 



plicated than any of the organs of sense yet described. It 

 is second only in importance to the eye, the organ of sight. 



The ear is divided into three parts, the outer, the middle, 

 and the inner ear. 



346. The Outer Ear. The outer ear consists of a plate 

 of gristle, shaped somewhat like a shell, known as the pinna, 



1 We shall understand these air waves better if we throw a stone into a 

 pool of water and watch the result. We see a series of tiny circular ripples 

 gradually spread themselves over the surface of the water from the spot 

 where the stone fell. This exactly represents the waves of sound caused 

 by the vibration of bodies in the air. These air waves travel with wonder- 

 ful rapidity. The usual velocity of sound is about eleven hundred feet 

 a second. 



