Corpuscles.
Amy: What do they look like?
Mother: You can not see them at all unless you should look through a mi´cro-scope. The red cor´pus-cles are shaped like a little biscuit with a dimple in the middle. The white ones keep changing their shape in a very wonderful way. First they are round, then square, then three-cornered, and they take on ever so many other shapes. There are several millions of these little red and white fellows in a single drop of blood.
Elmer: But you said it went through a dark tunnel to get to the heart. Please tell us about that.
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.