There are other physical signs produced by the action of the heart besides this beating, and these are certain sounds that the heart produces when it is in action - two sounds, called the first and second sounds of the heart.

The first sound is long and dull, and the second, which succeeds it immediately, is a short sharp sound; they are represented by the sounds produced in the pronunciation of the words rub, dub, the second one being said very sharply. These sounds can be heard by placing the ear against the chest-walls of a person.

The first sound probably has several causes; the most efficient cause is the action of contraction in the muscular walls of the ventricle; the second sound is certainly produced by the sharp closing of the semilunar valves, after the blood is forced into the arteries. After the blood has been forced into these arteries they recoil on it, and force it so sharply against those semilunar valves, that they come together with a click.

Now, it is by means of the alterations in these sounds of the heart that physicians are often able to tell whether there is heart disease, and to distinguish between various heart diseases.

I meant to point out to you that it is not necessary for the left ventricle of the heart to be strong enough, as was long thought, to force the blood, not only through the arteries, and through the capillaries of the tissues, but also back again through the veins to the heart You will see at once that this is not necessary, for you know that liquids always find their own level If you had two vessels containing a liquid and communicating with one another, no matter what their size, the liquid would stand at the same height in both, and so it is with the blood-vessels in the body.

When the heart has driven the blood through the arteries, and through the capillaries, there is a column of blood in the arteries and capillaries supporting the blood in the veins up to the heart; and the fact that the veins are three times the size of the arteries does not make any difference; the height of the column of blood is the same. What the ventricle has to do is to force the blood through the arteries, and through the capillaries, and then the mere properties of a fluid will do the rest.

You will see at once that the ventricles have a great deal more to do than the auricles; the two ventricles have to drive the blood through considerable distances, and through an immense quantity of extremely fine tubes, with a great amount of resistance; the two auricles have nothing to do but to take the blood as it comes in by the veins, and let it run on through the valves, which offer no resistance, and just at the last to give a little squeeze; this is why the walls of the two auricles are very thin, and the walls of the two ventricles very thick, strong, and muscular. You will see at once, that as the left ventricle has to force the blood through the aorta, and all over the body, through myriads of capillary vessels in all parts of the body, and at such distances, its walls must be very thick indeed, and they are much thicker than the walls of the right ventricle, which has not to force the blood nearly so far. There is a remarkable exception to all this. The blood that goes from the great aorta or great artery to certain organs in the abdomen, viz., to the stomach, to the intestines, to the pancreas or sweetbread, and to the spleen, goes from the great artery of the body by means of small arteries, just as the blood that goes to the hands or feet does, and flows in capillaries in the walls of those organs. These capillaries run together, and form veins just in the same way as the capillaries in the hands or feet do, but these veins do not go into the great vena cava inferior, into which all the rest of the blood from the lower part of the body goes; they run together, and form one large vein, which goes by the name of the portal vein; this does not go straight into the vena cava inferior, but does a very remarkable thing; it goes into the liver, and in the liver it divides up into branches, like an artery. It is the only large vein in the body which does any such thing; it goes into the liver together with the proper artery of the liver, which also divides up into branches, and these two sets of branches run together and end in one and the same set of capillaries.

These capillaries run together and form a large vein, which leaves the liver, and runs into the vena cava inferior. That circulation of the blood from the stomach, intestines, pancreas, and spleen, through the liver, goes by the name of the portal circulation. One of the most remarkable things in the bodies of animals is this large vein, which divides like an artery.

Remember that an artery is a vessel in which blood is going from the heart; a vein is a vessel in which blood is going towards the heart.

What is the shortest course that a particle of blood can take in the human body? A good many of you might probably say it is to go through the lesser circulation from the right ventricle through the lungs, and to the left auricle, but no, it is not so by any means; the shortest course is to go from the left ventricle of the heart, then through one of those little arteries that go into the heart itself, into the capillary vessels in the walls of the heart, through these capillary vessels into the veins of the walls of the heart, and through the veins into the little vein, which empties from the walls of the heart into the right auricle; that is a part of the greater circulation, but is entirely confined to the walls of the heart.