418 



LIFE ON THE EARTH 



rope from end to end, but the particles of the rope will 

 simply move up and down. It is in a similar way to this 

 that the sound waves travel, but the particles which trans- 

 mit the sound only move back and forth through small 

 distances. (Figure 134.) An echo is simply a reflection of 

 sound waves from some obstruction they meet. 



The ear, which is the sound transmitter of the body, con- 

 sists of the outer ear, which is so arranged as to catch the 



sound waves and converge 

 them upon the ear drum. 

 The ear drum is a thin 

 membrane stretched tightly 

 across a bony opening and 

 vibrates when the air waves, 

 strike it, as a drum does 

 when struck by the drum- 

 stick. On its inner side 

 the drum is attached to the 

 inner ear by a chain of 

 three bones. The sensitive 

 cells of the inner ear trans- 

 mit the impressions made by the sound vibrations through 

 the auditory nerve to the brain, where they are interpreted 

 into the sensation of sound. 



The drum head of the ear is easily broken, and therefore 

 no hard instrument should ever be thrust into the ear. 

 There is an old saying that one should never pick his ear 

 with any kind of hard instrument having a smaller point 

 than one's elbow. Immediate and skillful attention should 

 be given to any inflammation of the ear. If neglected it 

 may lead to deafness or even to an exceedingly dangerous 

 abscess in the bone back of the ear. 



CROSS SECTION OF THE HUMAN EAR 



