PERCY: About the size of the fist of the person in whom it is found. AMY: Then the baby’s heart is about as big as his dear little hand. MOTHER: Can you describe its shape? HELEN: I think it is something like that of a pear or a strawberry, with the small end down. [Illustration: _The heart._] MOTHER: Here is a picture that will help us in learning its shape. I think I have not yet told you that the trunk of the body is divided into two large rooms. There is a partition running crosswise, called the di´a-phragm (di´a-fram). This gives us a large upper room, where we find the engine and bath room. The kitchen, eating room, store room, and waste rooms are in the lower part of the trunk, below the di´a-phragm. But we want to talk about the heart now. We have found about how large it is and what it is shaped like; let us next take a peep inside and learn, if we can, how it does its work. ELMER: Didn’t you tell us once that the heart was made of muscles? MOTHER: Yes; the outside walls are made of little strong muscles, and the inside is hollow. It is divided into four rooms. Each has its own name, but we will not try to learn them now. There is a wall reaching from top to bottom, and as it has no door, nothing can pass through from one side to the other. Then there are cross walls, or partitions, with folding doors in them, so there is an up-stairs and down-stairs room on each side. There are big pipes, or tubes, leading in or out from each room. They are called veins, or ar´ter-ies. The veins carry the blood _to_ the heart, while the arteries carry it _away_. HELEN: But, mother, what makes the heart beat? MOTHER: I thought that would be about the first thing you would wish to know, and I will explain the best I can. When the muscles which make up the heart draw together, the rooms inside become small, and the blood in them is squeezed out. When the muscles slacken, the rooms become larger, and the blood rushes in and fills them again. So the blood keeps coming in and going out of the heart all the time, and it causes it to make the movement which we call beating. AMY: How fast does it beat? MOTHER: In very little children it beats from one hundred and twenty to one hundred and forty times a minute. In grown people it beats sixty or seventy times, and when the body-house has grown old and feeble, it beats slower still. Percy, you may run up and down stairs and then tell us if you see any difference in your heart-beats. PERCY: I believe they are twice as many as they were when I was sitting still. MOTHER: Hardly as many as that, but the heart beats much more quickly. Can you think of anything else that makes the action of the heart faster? HELEN: When I was frightened this morning I could hear my heart go thump, thump, and I am sure it seemed to be in a hurry.