VOCAL CORDS AND THE EARS 303 



drum. The ear-drum is a thin, membrane-like muscle 

 which vibrates when a sound strikes it. 



The middle ear is separated from the external ear by 

 the ear-drum and is connected with the throat by a tube 

 called the Eustachian tube, through which air can pass 

 in and out of the middle ear. Air comes out of the Eus- 

 tachian tube when the air pressure in the outer ear de- 

 creases, and air goes into the tube when the air pressure 

 outside increases. The purpose is to keep the air pres- 

 sure in the middle ear the same as in the outer ear, in 

 order that the ear-drum may keep its position and not 

 bulge in or out, or be burst. The middle ear also has 

 three small bones called the hammer, anvil, and stirrup, 

 which form a chain from the ear-drum to a membrane 

 closing an oval opening into the internal ear. These 

 bones transmit the vibrations of the ear-drum, caused by 

 sound waves, to the membrane of the inner ear. 



The inner ear is composed of several irregular connected 

 cavities. The three semicircular canals, at right angles 

 to each other, have nothing to do with hearing, but enable 

 the body to keep its balance and know its position even 

 with the eyes closed. The coiled part of the inner ear, 

 called the cochlea because it resembles a snail shell, con- 

 tains a watery fluid and also the ends of the nerves of 

 hearing. This fluid, when disturbed by sound waves, 

 affects the hair-like parts which connect with the nerves. 



209. Hearing. When an explosion or a vibrating body 

 produces a sound wave, the outer visible part of the ear 

 reflects the wave into the auditory canal. The ear-drum 

 is made to vibrate and the three bones in the middle ear 

 carry this vibratory motion to the membrane of the inner 

 ear. The liquid in the inner ear transmits the vibrations 

 to the hair-like projections of the cells containing the 



