The impure blood flows from the arteries into the veins through tiny canals. The veins are tubes something like the arteries, and they usually lie beside the arteries. They and the arteries are like the two tracks of a great railway line, one going from the main station, the heart, the other coming back. But the blood in the veins is not like that in the arteries. Instead of being bright red, and healthy-looking, it is dark colored. It flows more slowly, and it is full of impurities which it is carrying away from the body. The veins carry this blood to the right side of the heart, and the heart sends it to the lungs. There are many blood vessels in the lungs, and they are divided into branches running in every direction through the lung tissue. When we draw in a deep breath, we fill the lungs with fresh air. This is at once taken up by the impure blood in the branching blood vessels. The impurities are breathed out with the air that leaves our lungs, and the blood once more becomes bright red and full of new life. In this way the blood is purified. Then it is sent back to the heart, all ready to start out again through the body. The whole journey is made every two minutes. The third box, which we call the abdominal cavity, is separated from the chest by a broad, thin muscle, the diaphragm. The abdomen has a hard floor of bone, but the walls are soft, being made up only of the muscles and the lower ribs. In the abdomen are the stomach and intestines, the liver, kidneys, and other organs of which we shall learn later. Food is carried from the mouth to the stomach by the muscular tube which passes through the chest just back of the breastbone. This tube is called the esophagus. With the mouth, the stomach, and the bowels, it forms what is called the food canal. As soon as the food enters the mouth, it begins to turn from solid into liquid form, so that the blood can take it up. We chew the food, so that it may become mixed with the saliva in the mouth; then it is swallowed and goes into the stomach. This is a kind of sack which holds about a quart. Just as saliva is secreted in the mouth and acts upon the food, so in the stomach there is a fluid called the gastric juice, which aids digestion. The gastric juice mixes with the food, dissolves it, and makes it soft, so that it can pass through the lower opening of the stomach into the intestines. Here there are other juices which dissolve still more of the food, until at last it is all liquid and looks like milk. Now it is taken up by the blood through tiny canals that reach down into the intestines and absorb it. But it is not really a part of the blood yet. It must be changed still further, so the blood carries it to the liver. Here it is made a part of the blood, and is able to nourish all parts of the body.