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.

Semember 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.

The course that a particle of blood must take in the human body so as to pass through the greatest length of capillaries is to go through the great aorta and into the capillary vessels of one of those special organs mentioned in the abdomen, e.g. the stomach, through the capillaries and into the veins of that organ, and so into the portal vein; through the branches of the portal vein in the liver into the capillaries of the liver, thence into the vein which leaves the liver to join the vena cava inferior, and so to the right auricle of the heart.

A few words about this fluid called the blood.

The blood is a fluid which consists of water containing certain substances in it. Some of these substances are suspended in it, as particles of chalk or of vermilion may be suspended in water, and some are dissolved in it just as salt or sugar can be dissolved in water.

In a hundred parts of blood there are twenty-one parts of solid matters to seventy-nine of water. Now the solid matters suspended in the blood are of very great importance, and the first important matters are the little bodies which we call corpuscles (meaning little bodies). These corpuscles are of two kinds, those that are more numerous are the red ones; they are little disc-shaped bodies of what may be called a semi-solid substance, of the consistency of jelly; they are just like little gun pellets, rounded at the edges, and compressed in the centre. These red corpuscles are the things which give the blood its colour. The liquid part of the blood is not red at all, but is of a pale straw colour, almost colourless. There are so many red corpuscles in it that it is just like a little water with a lot of powdered vermilion in it; the water itself is not red, but the vermilion is red, and so it is with the blood, the corpuscles are red, the liquid part is colourless.