Why do chickens and turkeys need strong muscles
in their legs?

7. Why do swallows need strong breast muscles?

8. What makes the muscles of the blacksmith's arm
so strong?

9. What will make your muscles strong?

10. What will make them weak?

11. What does alcohol often do to the muscles?

12. Can fatty muscles work well?

13. Why does not drinking beer make one stronger?

CHAPTER III.

NERVES.

[Illustration: H]OW do the muscles know when to move?

You have all seen the telegraph wires, by which messages are sent from
one town to another, all over the country.

You are too young to understand how this is done, but you each have
something inside of you, by which you are sending messages almost every
minute while you are awake.

We will try to learn a little about its wonderful way of working.

In your head is your brain. It is the part of you which thinks.

As you would be very badly off if you could not think, the brain is your
most precious part, and you have a strong box made of bone to keep it
in.

[Illustration: _Diagram of the nervous system._]

We will call the brain the central telegraph office. Little white cords,
called nerves, connect the brain with the rest of the body.

A large cord called the spinal cord, lies safely in a bony case made by
the spine, and many nerves branch off from this.

If you put your finger on a hot stove, in an instant a message goes on
the nerve telegraph to the brain. It tells that wise thinking part that
your finger will burn, if it stays on the stove.

In another instant, the brain sends back a message to the muscles that
move that finger, saying: "Contract quickly, bend the joint, and take
that poor finger away so that it will not be burned."

You can hardly believe that there was time for all this sending of
messages; for as soon as you felt the hot stove, you pulled your finger
away. But you really could not have pulled it away, unless the brain had
sent word to the muscles to do it.

Now, you know what we mean when we say, "As quick as thought." Surely
nothing could be quicker.

You see that the brain has a great deal of work to do, for it has to
send so many orders.

There are some muscles which are moving quietly and steadily all the
time, though we take no notice of the motion.

You do not have to think about breathing, and yet the muscles wor