138 THE FAIRY-LAND OF SCIENCE. 
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 
to the middle of the drumhead so that it moves to 
and fro every time this membrane quivers. The head 
of this bone fits into a hole in the next bone e, the 
anvil, and is fastened to it by muscles, so as to drag it 
along with it ; but, the muscles being elastic, it can 
draw back a little from the anvil, and so give it a blow 
each time it comes back. This anvil, e is in. its turn 
very firmly fixed to the little bone /, shaped like a 
stirrup, which you see at the end of the chain. 
This stirrup rests upon a curious body L, which 
looks in the diagram like a snail-shell with tubes 
coming out of it. This body, which is called the 
labyrinth, is made of bone, but it has two little 
windows in it, one h covered only by a membrane, 
while the other has the head of the stirrup f resting 
upon it. 
Now, with a little attention you will understand 
that when the air in the canal be shakes the drum- 
head c to and fro, this membrane must drag with it 
the hammer, the anvil, and the stirrup. Each time 
the drum goes in, the hammer will hit the anvil, and 
drive the stirrup against the little window ; every time 
it goes out it will draw the hammer, the anvil, and the 
stirrup out again, ready for another blow. Thus the 
stirrup is always playing upon this little window. 
Meanwhile, inside the bony labyrinth L there is a 
fluid like water, and along the little passages are 
very fine hairs, which wave to and fro like reeds ; and 
