part 12

110 Our bodies can be looked upon as machines which are kept going by fuel. As food is burned up in the body cells it gives us heat and other sorts of energy. All through our lives—from birth to death—we must be supplied with energy. Even when we are sleeping we are using up some food to keep our bodies warm. Our lungs and heart keep on working. Some animals sleep through the cold winter weather: snakes, for example. When they are asleep their rate of breathing and pulse rate and temperature go down, and less food is needed. Only animals that can store enough fat in their bodies to keep them alive through the winter can sleep through the months when food is hard to get.

111 While our bodies are at rest the large muscles are not in use. It is these large muscles which pull on our bones and let us move as we want to. Hard work with our large muscles uses up energy quickly so that we come to feel hungry and want to eat. The more we know about the parts of our bodies and their work, the more new questions come to our minds, questions such as: How is it possible for all the different parts of us to grow from one cell? How is it possible for all the millions of cells in our growing bodies to go on doing what they do without our being conscious of their workings?

112 Through about two billion (2,000,000,000) years, scientists say, the development of plants and animals on our earth has been going on. At first both were very small, much less than a pin point in size. We might say that the first plants or animals were bodies living in one-roomed houses, too small for the eye to see. All the housekeeping went on in one room. Such cells take food in and let waste out. They do not give birth to new cells but divide to become two daughter cells. These three pictures show the division of a cell into two separate cells, each of which may in turn divide into two more.

113 The earth itself is thought to be more than twice as old as life on it. Our picture of what it was like two billion (2,000,000,000) years ago is of a great ball about as hot on its surface as boiling water. In time, as the earth's surface became less hot, the thick clouds round it, from which water went on raining down, got thinner. Then more sunlight could get through to the surface of the earth. Sunlight is necessary for life on earth, as air and water are.

114 Life began, people think, in the salt sea. Near the land, where the waters were not deep, very small one-celled organisms came into being. The cells in our bodies are of many different sorts but they all have water in them. Our bodies are seventy per cent water. This water has in it about the same amount of salt, common table salt, as there was in the water of the sea in which the earliest organisms swam. That water was a little less salt than sea water is today. In billions of years the rivers have washed down more and more salt from the land into the sea. The sea has become more salt. The water in our bodies does not become more salt. Even if we eat a great amount of salt in our food the body automatically controls how much it will keep in the blood stream. And the same is true if we let very much salt out through the skin when we get overheated. (See page 91.) Then we have to eat something that will give us new supplies of salt.

115 The first living things, though they were too small to be seen, were eating, growing organisms which divided to become others like them. Under a microscope, a cell looks like a bit of clear jelly with a thin wall round it. Very small openings in the walls of the cell let food in and waste out. In every cell there is a part like a little ball. This is the nucleus, which organizes the work of the cell. Though cells were discovered two and a half centuries ago, it is only in the last hundred years that knowledge of the work of the nucleus has developed. It is this nucleus which keeps the cell working at what has to be done. It is to the cell what London is to Britain, New Delhi to India, Beijing to China. It is the seat of government.