the air is therefore forced out through the nose of the bellows. It is in the same way that the blood is forced out of the heart through the aorta. The only difference is that the heart contracts itself, instead of having it done, as in the case of the bellows, by hands and handles. Again, when you move the handles of the bellows apart, as represented in Fig. 19, you

FIG. 19.

enlarge the room in the bellows, and so the air rushes in to fill the vacant space. In like manner, when the heart dilates, or enlarges itself, there is more room in it, and the blood rushes in to fill it up.

6. Another comparison, to illustrate the contraction and dilatation of the heart, is this. Fasten a tube to the neck of an india- rubber bottle, and fill it up with water. Put the end of the tube in a vessel of

Illustrate the action of the heart by the comparison of the indiarubber bottle.

water. If now you press the sides of the ball together, some of the water in it is forced out into the vessel, just as blood is forced out through the aorta, when the heart contracts. If now you stop pressing the ball, and let it take its round shape again, the water rushes into it from the vessel. For the same reason, when the heart dilates or becomes larger, the blood rushes into it.

7. I will now explain to you the manner in which the heart contracts and dilates. The heart is made up of muscular fibres, which have the power of shortening themselves, as you saw in chapter second, §7 and §8. Now suppose one of these fibres, as seen at a, in Fig. 20, shortens itself so as to be like b, the space that

a

FIG. 20.

00 00

is inclosed in it becomes smaller, just as in the case of the bellows. In c and d you see the same thing represented when several fibres are together. If the fibres in c become shorter, so as to be as in d, the space they inclose is smaller. You readily see from this, that when all the fibres of the heart are shortened, the space in it is lessened, and a part of the blood is forced out.

8. You can see by the same figures how the heart

Explain by the Figures the action of the muscular fibres of the heart when it contracts and dilates.

b

dilates or enlarges itself. If the contracted or shortened fibre 6 lengthens so as to be as a, the space enclosed by it becomes larger. And so also of any number of fibres. It is supposed that the enlarged or dilated state of the heart is its natural state of rest, when the fibres are not acting, but are quiet. That is, the heart is really at work only when it contracts, When it dilates it merely ceases to act, and lets itself go back to its natural size by its own elasticity, as it is termed. It is just as the india-rubber ball goes back to its natural roundness when you stop pressing it.

FIG. 21.

9. The fibres of the heart are not arranged in the regular form in which they are represented in the above figures. They meet each other, and cross each other in various ways. But the effect of their contraction is as described. You can see, for example, by figure 21, that it will make no difference in the effect, whether a single fibre go all around, as in a, or whether two fibres lap on to each other, as

α

in b, and are fastened together. And the same can be said of any number of fibres.

10. When the heart beats, these fibres shorten themselves, and the blood is forced out into the arteries. Then, as the fibres relax, the blood comes into the heart from the veins. And so the heart by turns contracts and enlarges, just as you contract and enlarge the bellows in working them, as you blow the fire.

Is the heart in action, or is it at rest, when it dilates? How are the fibres of the heart arranged? Give the comparison made in §10 between the action of these fibres and the action of the bellows.

the arteries so

11. Let us look now at some things in which the arteries and veins differ from each other. You see veins lying just under the skin in various parts of the body, but you do not see the arteries. They all lie deeper than these veins that you see. The reason is this. It would be dangerous to have near the skin as some of the veins are. For the heart is pumping the blood directly into them with great force. And therefore if an artery is cut, it bleeds much more than a vein of the same size, and its bleeding is not as easily stopped. For this reason the Maker of our bodies has, as we may say, laid the arteries deep, so that they cannot often be cut in the accidents that happen to us.

12. You can see that special pains are taken in some cases to guard the arteries. Thus the large artery of the arm, when it comes to the joint at the elbow, does not pass over the bones, where it would be apt to get wounded. It lies deep on the inside of the elbow, under the stout tendon that you feel there. So at the knee, the artery is deep in the ham at the back of the joint, in a space between two jutting parapets of bone, as we may call them.

13. There are only a few places in the body where arteries of any size are very near the surface. In such cases it is because they could not possibly be laid in any better way. One of these is the wrist, where the physician commonly feels the pulse. Another is on the temples. In some persons who are very thin you

How do the arteries and the veins differ from each other in their situation? What is the reason of this difference? Mention some cases in which special pains are taken to guard the arteries.

can see the artery on the temples beating, and can count the pulse there without being obliged to feel it.

14. As the heart pumps the blood into the arteries with so much force, they are made much stronger than the veins are. If they were not, they would often burst, as you have seen the hose of a fire engine do. But the arteries are made so strong that this is a very uncommon accident.

15. What is called the pulse I will explain to you. When the heart contracts it gives a sudden motion or impulse to all the blood in all the firm arteries. The blood all moves at once. The motion is not like a wave, going from the heart in all directions. The blood at a distance from the heart is moved at the same time with the blood near the heart. It is this motion or impulse that you feel when you put your finger upon an artery. The impulse thus felt is called the pulse. You can feel the pulse wherever you can feel an artery. It is everywhere. In a young infant you can both feel and see the pulse in the open space on top of its head, where the bones are not joined together. This is the pulse of the arteries of the brain. When the heart beats very strongly, as it does in a high fever, this pulse in the brain is very manifest.

16. If a vein be cut, the stream from it is a steady one, because the blood flows in the veins back to the heart slowly and steadily. But if an artery be cut, the stream is not steady but spouts out by jerks or

In what places in the body are the arteries very near the surface, and why? How do the arteries and veins differ in strength, and why i Explain what the pulse is. Where can you feel the pulse?