MOTHER: The tunnel is round, like a tube, and I must tell you that these tubes are in every part of your body. Some are quite large, some are small, and some are so tiny that you could not see them if you should try. They are like a tree with its trunk dividing into large branches, and these into smaller ones, till at last they become little twigs. The largest tubes for carrying blood through the body are called arteries. The smaller ones are called veins. The arteries carry fresh, bright, clean blood to every part of your body-house. It bounds along with a hop, skip, and jump, as though it were in a hurry to get to work. The arteries have very strong walls, and, as I told you, the blood soon finds itself in the heart. HELEN: Which room did it go into first? MOTHER: When the blood is fresh and clean it goes into the top room on the left side. It keeps coming in until the room is filled full. Then the little folding doors open, and the blood is crowded into the lower left room, the doors fly back, and-- AMY: But please tell us about the doors. MOTHER: They are made so that the blood could not get back into the top room if it wished; for they never swing but one way, and some small cords hold them in place. These doors are called valves. When the lower room is filled, the walls press together, and the blood is forced into the largest blood tube in the body, the walls of which are so very smooth, that the blood passes along with a merry bound. The tube keeps growing smaller the farther we go from the heart, and branches into many smaller tubes. PERCY: And how far does the blood go? MOTHER: Perhaps it first takes a trip through the trunk of your body, down through your right leg, and on to the end of your big toe. The tubes at last become very small, and there are so many of them that they are like a network of the finest lace. A hair would seem like a big rope beside them. They are so very tiny that you can not see them. Their walls are thinner than tissue paper, and they are so close together that you can not touch your skin with the point of a needle without touching some of them. When the blood comes to these tiny tubes, it does not travel so fast as at first, and as it passes along, the muscles pick it to pieces, take the part they want as food, and load the blood down with waste which they can not use. When they are so hungry, the blood is glad to feed them and give them the oxygen, which makes them warm. AMY: Did it stay long in those little tubes?