390 PHYSIOLOGY AND HEALTH. 



the impressions made by sonorous bodies, and to convey tliem 

 to the brain. The sonorous body causes vibrations in the air. 

 These vibrations strike upon the membrane of the tympanum. 

 The membrane acts upon the series of bones, and through 

 them upon the internal parts and the branches of the auditory 

 nerve, and then along this nerve the sonorous impression is 

 conveyed to the brain, where sensation is caused, the sound is 

 perceived, and the noise is heard. 



919. It is necessary, for hearing sound, that there should 

 be a sonorous body to create it, air to convey it, the healthy 

 ear to receive, and the brain to perceive it. 



920. There can be no sound where there is no air. If a bell 

 be rung in an exhausted receiver of an air pump, no noise is 

 made. The sound is more or less loud according to the state 

 of the air. It is conveyed more distinctly and farther in the 

 direction of the wind than in the opposite course, or in any 

 direction when the air is still. Thus we hear the sounds of 

 bells, &c., when they are at the windward better than when 

 they are at the leeward from us. 



921. // is necessary that the parts of the ear should be 

 sound for perfect hearing. When the outer canal of the ear 

 is filled, or the membrane of the tympanum is covered, with 

 wax, hearing is impaired. 



922. The eustachian tube is sometimes closed. Inflam- 

 mation of the throat, from colds, may extend to the lining of 

 this tube, and prevent the free passage of air. When this 

 happens from this or other causes, we feel an uneasy fulness 

 and pressure within the ear, and noises have an unnatural and 

 unpleasant sound. 



When this tube is closed from slight and temporary causes, 

 it can be opened by gaping, or by pressing the air into it from 

 the lungs. 



924. Hearing is impaired, and deafness, in various degrees, 

 arises, from very many causes, and from diseases in other 

 organs as well as those within the ear. Worms in the diges- 

 tive organs, scarlet fever, measles, small-pox, and influenza 

 f netimes produce this effect. 



