372 ADVANCED PHYSIOLOGY 



When a drum head is loosely drawn, the drum will sound 

 very well if struck hard enough with the sticks, but if 

 the drummer merely touches the head or hits it very gently, 

 no sound of consequence comes from it. If, on the other hand, 

 the drum head is drawn tight by shoving down the straps on 

 the cords at the sides of the drum, the merest movement of 

 the sticks on the head produces a very distinct noise. 



In the middle ear the two tiny muscles, mentioned on 

 page 365, are so placed that they can tighten the membranes 

 bordering on the middle ear, so that the least movement of 

 sound waves in the air can be detected. The tensor tympani 

 draws the ear drum tight, and the stapedius pulls on the stapes 

 and adjusts the membrane over the foramen ovale; the 

 ear is thus put into condition for perceiving very faint sounds. 



Quality of Sound. As has already been pointed out, the 

 quality of one's voice is determined not entirely by the nature 

 of the vocal cords, but by the size and shape of the cavities of 

 the trachea, the pharynx, the nose and some smaller cavities 

 in the bones of the upper part of the nose. The air in these 

 cavities is set in vibration by sound waves, and the people 

 who are listening to one's voice get the effect of all the inci- 

 dental influences of the cavities upon the sound. How 

 marked the influence of these centers is may be proved by 

 merely closing the nasal passages grasping the nose between 

 the fingers while talking. 



DEAFNESS 



Defective hearing is due to a variety of circumstances. In 

 elderly people it is generally caused by the stiffening of the 

 ear drum or other delicate membranes so that they are not 

 so sensitive to slight sound waves as formerly, or it may be 

 due to the fact that the bones of the middle ear have become 

 more or less rigid and do not move readily. 



In younger people, and in those who become temporarily 

 deaf, the trouble usually is that the Eustachian tubes have 



