16. How we Use the Nerves.—If you happen to touch your hand to a hot stove, what takes place? You will say that your arm pulls the hand away. Do you know why? Let us see. The nerves of feeling in the hand tell the nerve cells in the brain from which they come that the hand is being burned. The cells which feel cannot do anything for the hand, but some of their branches run over to another part of the brain, which sends nerves down to the muscles of the arm. These cells, through their nerve branches, cause the muscles to contract. The cells of feeling ask the cells which have charge of the muscles to make the muscles of the arm pull the hand away, which they do very quickly.
17. So you see the nerves are very much like telegraph or telephone wires. By means of them the brain finds out all about what is happening in the body, and sends out its orders to the various organs, which may be called its servants.
18. An Experiment.—A man once tried an experiment which seemed very cruel. He took a dove and cut open its skull and took out its large brain. What do you think the effect was? The dove did not die at once, as you would expect. It lived for some time, but it did not know anything. It did not know when it was hungry, and would not eat or drink unless the food or water was placed in its mouth. If a man gets a blow on his head, so hard as to break his skull, the large brain is often hurt so badly that its cells cannot work, and so the man is in the same condition as the poor dove. He does not know anything. He cannot think or talk, and lies as though he were asleep.
19. By these and many other facts we know that the large brain is the part with which we remember, think, and reason. It is the seat of the mind. We go to sleep because the large brain is tired and cannot work any longer. We stop thinking when we are sound asleep, but sometimes we do not sleep soundly, and then the large brain works a little and we dream.
20. What the Little Brain Does.—The little brain[B] thinks too, but it does not do the same kind of thinking as the large brain. We may use our arms and legs and many other parts when we wish to do so; and if we do not care to use them we may allow them to remain quiet. This is not the case with some other organs. It is necessary, for example, that the heart, the lungs, and many other organs of the body should keep at work all the time. If the large brain had to attend to all of these different kinds of work besides thinking about what we see, hear, and read, and other things which we do, it would have too much work to do, and would not be able to do it all well. Besides, the large brain sometimes falls asleep. So the large brain lets the little brain do the kinds of work which have to be attended to all the time, and the little brain keeps steadily at work when we are asleep as well as when we are awake.
21. What the Spinal Cord Does.—If you tickle a person's foot when he is asleep, he will pull it up just as he would if he were awake, only not quite so quickly. What do you suppose makes the muscles of the leg contract when the brain is asleep and does not know that the foot is being tickled? And here is another curious fact. When you were coming to school this morning you did not have to think about every step you took. Perhaps you were talking or looking over your lessons; but your legs walked right along all the time, and without your thinking about them. Can you tell how?
22. It would be too much trouble for the large brain to stop to think every time we step, and the little brain has work enough to do in taking care of the heart and lungs and other organs, without keeping watch of the feet when we are asleep, so as to pull them up if some mischievous person tickles them. So Nature puts a few nerve cells in the spinal cord which can do a certain easy kind of thinking. When we do things over and over a great many times, these cells, after a time, learn to do them without the help of the large brain. This is the way a piano-player becomes so expert. He does not have to think all the time where each finger is to go. After the tunes have been played a great many times, the spinal cord knows them so well that it makes the hands play them almost without any effort of the large brain.
SUMMARY.
1. The part of the body with which we think is the brain.
2. The brain is found filling the hollow place in the skull.