You remember that I told you in Part First how the sap circulates in a plant or a tree. It goes up in one set of pipes, and goes down in another set. Just so it is with the blood in your body; it is always in motion. There are two different sets of pipes for it to go back and forth, as there are in the plant for the sap; these two sets of pipes are called arteries and veins.
The heart.
The blood in your body is kept in motion by a pump that works all the time, night and day. This pump is in your chest. It is the heart. Put your ear to the chest of some one, and you can hear its working as it pumps out the blood. You can hear it in your own chest sometimes when it works very hard. When you have been running very fast you can hear it.
The heart pumps the blood out at every beat into a large artery. From this great main pipe other pipes or arteries branch out every where, and from these branches other branches go out; dividing in this way, like the branches of a tree, the arteries at last are very small.
The capillaries.
At the ends of the arteries there are exceedingly small vessels. They are called capillaries, from the Latin word capilla, which means a hair. They are really smaller than the finest hairs, for you can not see them. When you cut your finger you divide a great many of these vessels, and the blood oozes out from them. When any one blushes, these capillaries in the skin of the face are very full of blood, and this causes the redness. It is the blood in these little vessels that makes the lips red. These capillaries are every where, so that wherever you prick with a pin the blood will ooze out.
The blood goes out from the heart by one set of pipes, and comes back to the heart by another set. It goes out from the heart by the arteries, as I have just told you; it comes back to the heart by the veins.
How arteries are guarded more than veins, and why.
The veins lie, some of them, very deep, and some just under the skin. You see some of them under the skin in your arm and hand. But you can not see the arteries; they nearly all lie deep. Think of the reason of this. If an artery of any size is wounded, it is not easy to stop its bleeding, for the heart is pumping blood right through it; but it is easy to stop the bleeding of a wounded vein, because the blood is going in it quietly back to the heart. Now it is because it is so dangerous to wound arteries that God has placed them so deep that they can not easily be wounded.
The maker of our bodies has guarded the arteries in another way. He has made them much stronger than the veins. If they were not made very strong they would now and then burst. You sometimes see the hose of a fire-engine burst when they are working the engine very hard; but, though your heart pumps away sometimes so fast and hard, as when you have been running, not one of all the arteries gives way; but they would often burst if they were not made stronger than the veins are.