part 22

210 In the beginning—as long ago, as far back in the past, as we can talk about—the only sorts of changes which took place, scientists say, were physical changes. They were the sorts of changes which the science of physics is about. Here is a wheel turning round. Its motion is a physical change. Here is water boiling. And here is water turning into ice. Here are clouds forming and rain falling. These are physical changes.

211 Much later, other sorts of changes, more complex than these, began to take place. Here is a lighted candle. As it burns the heat melts the candle and the liquid goes up into the flame where it is turned into gas. So far all is physical. But in the flame chemical changes take place. Carbon from the candle and oxygen from the air come together to make carbon dioxide. At the same time water is given off, as you may see if you hold a glass cover over the flame. The cover quickly becomes clouded with little drops of water. This water was not in the wax and the candle flame gives it off however dry the air is. The water and carbon dioxide come into being through the chemical exchanges, between the lighted candle and the air, which make the flame. At the same time there are physical changes taking place. The top of the candle is melting and the liquid fuel is moving up into the flame and being turned into gas, air is coming into the flame, carbon dioxide and water are moving up from it and heat and light are being given out.

212 When you step on a scale, the scale goes down and the springs inside it are pulled out, and the pointer moves to a mark. These changes are physical. When you look at the pointer to see what your weight is, rays of light come from it to your eye and go through the lens in your eye to a point on the retina.

213 In a camera, rays of light from whatever you are photographing make an upside down picture on the film. So it is inside your eye. The pictures made on the retina are upside down. Everything going on so far has been physical. But in your retina more complex sorts of changes now take place. Some of these are chemical. Complex materials in the retina go through chemical changes not very unlike those that make photographs on films, and these changes cause other much more complex sorts of changes in the living cells of the retina.

214 These nerves, which make us able to see, are an outgrowth from the brain to the retina. From the eye impulses travel to the brain through the living cells of the nerves. There are as many as a million separate telephone lines in these nerves alone. A train of changes goes up these nerves to the back part of the brain. The cells there let us see the things that our eyes are looking at. No one, at present, has any idea of how they do this. The distance from the eye to the back of the brain is not great, but some distances from some parts of the body to the brain are long. The rate at which impulses travel through the nerves is about the same for all of them.

215 The distance from one's toe to one's brain and back may be as much as twelve feet. When you are feeling with your toe for a step to put your weight on, the impulses have to go up your nerves from your toe to your brain and back down again to the muscles which move your foot. When you are walking, almost all the muscles in your body are at work keeping you from falling down. Your muscles and nerves work together to keep you on your feet. These changes in the nerve and muscle cells are dependent on what is going on in other cells in the body in many different ways.