THE VOICES OF NATURE. 137 
head. This concha makes a curve much like the 
curve a deaf man makes with his hand behind his ear 
to catch the sound. Animals often have to raise their 
ears to catch the sound well, but ours stand always 
ready. When the air-waves have passed in at the 
hole of your ear, they move all the air in the passage, 
be, 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 mem- 
brane, 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 whole of the inner chamber 
and the tube E, which runs down into the throat behind 
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. 
