4. Many of the muscles are joined to the bones by strong white cords called tendons.
5. Muscular fibres can contract so as to lessen their length. It is in this way that the muscles perform their work.
6. All bodily motions are due to the action of the muscles.
7. Most of the muscles act only when we wish them to do so. Some muscles, however, act when it is necessary for them to do so, whether we will that they should act or not, and when we are asleep as well as when we are awake.
CHAPTER XXI.
HOW TO KEEP THE MUSCLES HEALTHY.
1. How to Make the Muscles Strong.—With which hand can you lift the more? with the right hand or with the left? Why do you think you can lift more with the right hand than with the left? A blacksmith swings a heavy hammer with his right arm, and that arm becomes very large and strong. If we wish our muscles to grow large and strong, so that our bodies will be healthy and vigorous, we must take plenty of exercise.
2. Effects of Idleness.—If a boy should carry one hand in his pocket all the time, and use only the other hand and arm, the idle arm would become small and weak, while the other would grow large and strong. Any part of the body which is not used will after a time become weak. Little boys and girls who do not take plenty of exercise are likely to be pale and puny. It is important that we should take the proper amount of exercise every day, just as we take our food and drink every day.