142 THE FAIRY-LAND OF SCIENCE. 
ready. When the air-waves have passed in at the 
hole of your ear, they move all the air in the passage, 
b c, which is called the auditory, or hearing, canal. 
This canal is lined with little hairs to keep out insects 
and dust, and the wax which collects in it serves the 
same purpose. But if too much wax collects, it pre- 
vents the air from playing well upon the drum, and 
therefore makes you deaf. Across the end of this 
canal, at c, a membrane or skin called the tympanum 
is stretched, like the parchment over the head of a 
drum, and it is this membrane which moves to and 
fro as the air-waves strike on it. A violent box on 
the ear will sometimes break this delicate membrane, 
or injure it, and therefore it is very wrong to hit a 
person violently on the ear. 
On the other side of this membrane, inside the ear, 
there is air, which fills the w r hole of the inner chamber 
and the tube E, which runs down into the throat be- 
hind the nose, and is called the Eustachian tube after 
the man who discovered it. This tube is closed at the 
end by a valve which opens and shuts. If you breathe 
out strongly, and then shut your mouth and swallow, 
you will hear a little " click " in your ear. This is 
because in swallowing you draw the air out of the 
Eustachian tube and so draw in the membrane c, which 
clicks as it goes back again. But unless you do this 
the tube and the whole chamber cavity behind the 
membrane remains full of air. 
Now, as this membrane is driven to and fro by the 
sound-waves, it naturally shakes the air in the cavity 
behind it, and it also sets moving three most curious 
little bones. The first of these bones d is fastened 
