366 COORDINATION AND SENSATION 



cause deafness (Fig. 155). This may come about with- 

 out pain and so gradually that one does not think of seek- 

 ing medical aid. Such masses are easily removed by the 

 physician, the hearing being then restored. Both for 

 painful disturbances of the ear and for the gradual loss 

 of hearing, the physician should be consulted. 



The Hearing of School Children. School children not 

 infrequently have defective hearing and for this reason 

 are slow to learn. The hearing is easily tested with a 

 watch, the normal ear being able to hear the watch tick 

 at a distance of at least two feet. Pupils with defective 

 hearing should, of course, have medical attention, and in 

 the classroom should be seated where they can hear to 

 the best advantage. 



Summary. Sound waves constitute the external stimuli 

 for the sensation of hearing. They consist of progressive 

 vibratory movements of the air that originate in vibrating 

 bodies. Through the larynx and the ear, sound waves are 

 utilized by the body in different ways, but chiefly as a 

 means of communication. The larynx produces sound 

 waves which are reenforced and modified by the air pas- 

 sages. The ear supplies suitable conditions for the action 

 of sound waves upon nerve cells. Both the ear and the 

 larynx are constructed with special reference to the nature 

 and properties of sound waves, and they illustrate the body's 

 ability to adjust itself to, and to make use of, its physical 

 environment. 



Exercises. I . For what different purposes are sound waves em- 

 ployed in the body ? 



2. How do sound waves originate ? How are they transmitted ? 

 How do they differ from the waves on water ? 



3. How are sound waves able to act as nerve stimuli ? 



4. Describe two methods of reenforcing sound waves. Which 

 method is employed in the body ? 



