How the Muscles do their Work.—A tiny nerve thread runs from the spinal cord or brain to every muscle thread. Messages sent through the nerve threads to the muscles make them act. A muscle can act in only two ways ([Fig. 84]). It can become shorter or longer. When it gets shorter, we say it contracts. When it stretches out, it is said to relax.
A muscle cannot contract more than one fourth of its length. To pull the forearm up, the brain sends a message to the muscle fixed by one end at the shoulder and by the other end to a bone at the elbow. The muscle at once becomes shorter and thicker, as may be felt by placing the fingers on it. Although it shortens only two inches it is fastened to the bone so near the elbow that it draws the hand up two feet.
PRACTICAL QUESTIONS
1. Of what use are the bones?
2. What animals have bony skeletons?
3. What can you say of the form of bones?
4. How many bones in the body?
5. Name six bones.
6. What part of the arm has two bones side by side?
7. How many ribs have you?
8. Explain how a broken bone should be cared for.
9. Point out and name two kinds of joints.
10. What are ligaments?
11. Of what is a muscle made?
12. How many muscles in the body?
13. How many tendons can you feel in your wrist?
CHAPTER XIX
THE MUSCLES AND HEALTH
Making the Muscles Strong.—No persons use all of the five hundred muscles in the body every day. In slow walking only about twenty muscles are used, while in running more than four times that number are called into action. Muscles which are not used get lazy and weak.
Every time a muscle is made to act the blood vessels enlarge and bring to it more blood to supply food. The more food the muscle has the stronger it grows. The right arm is used more than the left in most persons. This makes it so much stronger that some boys can lift twenty-five pounds more with the right arm than they can with the left.