8. Most of the nerve branches pass out through a large opening at the back of the skull, in one large bundle called the spinal cord. 9. The spinal cord runs down through a canal in the backbone, and all along gives off branches to the various parts of the body. 10. It gives us pain to prick or hurt the flesh in any way, because when we do so we injure some of the little nerve branches of the brain cells. 11. When we suffer, we really feel a pain in the brain. We know this because if a nerve is cut in two, we may hurt the part to which it goes without giving any pain. 12. We have different kinds of nerves of feeling. 13. There are other nerves besides those of feeling. These are nerves of work. 14. The nerves of work have charge of the heart, the lungs, the muscles, the liver, the stomach, and every part of the body which can work or act. 15. The brain and nerves control the body and make all the different parts work together in harmony, just as a general controls an army. 16. The brain uses the nerves very much as a man uses the telephone or telegraph wires. 17. With the large brain we remember, think, and reason. 18. The little brain does the simple kind of thinking, by means of which the heart, lungs, and other vital organs are kept at work even when we are asleep. 19. The spinal cord does a still more simple kind of work. It enables us to walk and to do other familiar acts without using the large brain to think every moment just what we are doing. CHAPTER XXIII. HOW TO KEEP THE BRAIN AND NERVES HEALTHY. ~1. Uses of the Brain.~--What do you think a boy or girl would be good for without any brain or nerves? Such a boy or girl could not see, hear, feel, talk, run about, or play, and would not know any more than a cabbage or a potato knows. If the brain or nerves are sick, they cannot work well, and so are not worth as much as when they are healthy. ~2. The Brain Sympathizes with Other Organs.~--Did you ever have a headache? Did you feel happy and good-natured when your head ached hard, and could you study and play as well as when you are well? It is very important that we should keep our brain and nerves healthy, and to do this we must take good care of the stomach and all other organs, because the brain sympathizes with them when they are sick. ~3. We must have Pure Air.~--How do you feel when the school-room is too warm and close? Do you not feel dull and sleepy and so stupid that you can hardly study? This is because the brain needs good, pure blood to enable it to work well. So we must always be careful to have plenty of pure air to breathe.