Idle habits make weak muscles. So idleness is an enemy to the muscles. There is another enemy to the muscles about which I must tell you. WHAT ALCOHOL WILL DO TO THE MUSCLES. Muscles are lean meat. Fat meat could not work your joints for you as the muscles do. Alcohol often changes a part of the muscles to fat, and so takes away a part of their strength. In this way, people often grow very fleshy from drinking beer, because it contains alcohol, as you will soon learn. But they can not work any better on account of having this fat. They are not really any stronger for it. REVIEW QUESTIONS. 1. How are the joints moved? 2. Where are the muscles in your arms, which help you to move your elbows? 3. Show why joints must have muscles. 4. What do we call the muscles of the lower animals? 5. What fasten the muscles to the bones? 6. 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."