1

In this engraving, o and q show the stumps of the two great veins, which bring back the blood that has been distributed to all parts of the body by means of the arteries. They are called the vena cava. There are, however, one or two other large veins which bring back blood. These you see at p. The right auricle is at n; b is the right ventricle; k represents the pulmonary artery, through which the blood is sent to be changed in the lungs-of

which 77 are the right and left branches; m m show the great veins-pulmonary veins-that bring back the blood from the lungs into the left auricle; a, the left ventricle; cef, the great aorta, through which blood is sent out to all parts of the body; and g h i, the branches. of this artery which carry blood to the head, neck and arms. The little arrows point always in the direction in which the blood runs. The letter s points out the coronary arteries, which carry blood to the heart itself.

But I must explain to you, more fully, the motion of the heart. The blood which returns from the lungs, through m m, and that which returns from all the rest of the body, through o p q, enters both the right and left auricles at the same instant, and also in the same instant flows through these auricles into the two ventricles.

Thus both sides of the heart fill in the same instant. Now let us suppose them filled. What is next to be done? The heart contracts-shrinks-and compresses the blood with as much force as a strong man could compress it with his hand. But suppose you held in your hand a fleshy sack of blood that

contained two or three ounces, with little hollow branches, that parted into ten thousand more into which the blood could flow, but could not get out at their sides or extremities. Suppose them now all full, and the sack full, too. If you press the sack hard with your hand, what will happen? Why, the blood, you will tell me, will go out of it into the branches. It will; but it will be as likely to go into one as another, provided it is equally large.

But there is another difficulty. As soon as I cease to press the sack, and the blood has an opportunity to do so, it will run back into it again. So you may, perhaps, at first view, suppose the heart would do. As soon as it should cease to contract, and begin to relax, so that its cavities or chambers would hold just as much as they did before, the blood would run back into it. Why should it not? No motion like that in our bodies would ever, in this way, be produced.

I have told you what one might naturally think, who knew nothing about the circulation. But let us see for a moment what the facts

are.

When the two auricles, one on each side of the heart, are full of blood, they contract at the same time, and press the blood into the two ventricles. If you ask why this blood is not just as likely to go back into the veins again, when the auricles contract, as to go into the ventricles, I will give you two reasons. First, the veins are already full, and the mass of blood in them is flowing onward and pressing towards the auricles; and to force the blood back into them would be somewhat like pushing it up hill. But secondly, there are little clappers, or valves, as they are called, in the sides of the veins, which, like so many small swinging doors, hang down against the sides of the veins, so long as the blood in them. is running towards the auricles. But as soon as the auricles contract, and the blood attempts. to get back by the way it came, the valves spread out and form a kind of floor or partition, which obstructs it.

Now the ventricles both contract; and as was the case with the two auricles, they both contract in the same instant. This contraction pushes their blood into the arteries, as I have before told you. The right ventricle pushes

its blood into the pulmonary artery, whence it goes into the lungs; and the left ventricle pushes its blood into the great aorta, through which it goes to every part of the body.

Why does not the blood, when the ventricle contracts, go back into the auricle? Because there are valves between them, which immediately spread out, like so many flaps or clappers, and form a sort of partition or floor, as the valves do in the veins, and prevent it. They do not, it is true, prevent every drop of it from returning. A very small quantity gets back, but none worth mentioning.

We have seen how the blood gets out of the auricles into the ventricles, and why it goes into the ventricles, rather than back into the veins. I will now say a little more about the structure of the veins.

The valves, of which I have spoken, are found in the larger veins all over the body; and now comes the reason why the blood can run up hill. The pressure in the veins is all the while diminishing, as you may easily see, on the side towards the heart, even though it is the up hill side; and as the arteries, at their extremities, are all the while pouring their