Several of the muscles in the forearm run into tendons in the wrist because if the muscle part were to extend along the wrist, this part of the arm would be large and clumsy instead of graceful and slender. Some of these tendons may be seen to move by bending the wrist and then working the fingers. =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. =Using the Muscles keeps the Body Well.=--All muscles must have more blood when they are used so that the heart is made to beat faster and stronger by exercise. In this way its valves and walls become able to do more work. Such a heart not only does its work better in a well person, but is able to keep pumping when the body is weakened by disease. Many persons die because the heart gets too weak to push the blood through the body.