MOTHER: Yes; moving quickly, fright, anger, or joy makes this busy pump work more quickly. Sadness and grief cause it to work slowly. It beats faster when we are standing than when we sit still, and the motion is slower when we lie down than when we are sitting. ELMER: Why did the doctor put his finger on my wrist when I was sick, mother? MOTHER: He wanted to know how your heart was working, so he felt your pulse. Sometimes when people are ill it beats very, very fast, and sometimes it moves more slowly than it should. AMY: What is the pulse? MOTHER: If I use any words that you do not understand you must ask what they mean. The pulse is the beating or throbbing of the arteries caused by the blood flowing through them from the heart. Have you noticed how the water sometimes goes in jerks as it is pumped through the hose pipe in the garden? It is that way with the heart. Each beat sends the blood through the arteries in jerks, and when we place our fingers on them, we can tell how fast the heart is beating. That is called the pulse. AMY: Sometimes I think that I can hear my heart beating. MOTHER: Each time it beats it makes two sounds, and they can be heard if the ear is placed over the heart. The doctor can tell by these sounds whether the heart is working all right. PERCY: But I should think it would get tired out if it keeps at work all the time. MOTHER: So it would if it had no rest. Every part of the body must rest. Between the heart-beats there is just a little rest, and, though the time is very short, yet if it were all put together it would amount to six or eight hours a day. HELEN: If the heart beats sixty or seventy times a minute, I wonder how many times it beats in a day. MOTHER: You may do a little figuring to find out. Seventy beats a minute, sixty minutes an hour, and twenty-four hours a day. ELMER: I have it. It would be more than one hundred thousand. ┌─────────┐ │ 70 │ │ 60 │ │ ———- │ │ 4,200 │ │ 24 │ │ ———- │ │ 16800 │ │ 8400 │ │ ———- │ │ 100,800 │ └─────────┘ MOTHER: And this means hard work, too; for if all it does in twenty-four hours were done at once, it would be equal to lifting one hundred and twenty tons of stone one foot from the ground. PERCY: Whew! I should think this was a powerful little force-pump, sure. MOTHER: But what would you think of a man who made his heart beat six thousand times more in twenty-four hours, which means that it must lift seven tons more than it should? Amy: But I thought the heart kept working of itself. Then how _could_ any one make it do more?