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.

Now you must fancy from that that there must be a great many of them, and that they must be very small; they are about the 3200th part of an inch across; 3200 would lie along an inch. That, however, does not give you much idea of their size. I will give you a better idea, not so much of their size as of the number of them that are in the blood. Suppose I take a cubic inch of blood, how many do you think there would be in it? there would be seventy thousand millions - that seems very astonishing, but still it does not give you the least idea how many there are. I will give you an idea in this way. Suppose that you were to begin counting, and go on day and night without any intermission, and that you were to count one hundred a minute, it would take you, making allowance for leap years, just 1331 years to count the red corpuscles in one cubic inch of blood; so that if William the Conqueror had begun counting when he came over to England, and had gone on day and night at that rate, he would not now have got through two-thirds of the number contained in a cubic inch of blood. Besides these there is another lot of corpuscles called white corpuscles, which are larger than the red corpuscles, and are irregular in shape; they have a peculiarity in that they have one or more smaller bodies inside them, and I may tell you that it is generally believed that the red ones are developed from these small bodies that are inside the white ones. They are not nearly so numerous as the red corpuscles, but in the proportion of something like three or four to 1000 red ones. Where they come from we shall see hereafter. The blood when it is run out of the blood-vessels does not remain liquid, but a thick clot forms in it; this is called coagulation of the blood; the importance of it to us is that when cut or abraded surfaces bleed, clots form, and plug up the cut ends of the capillary vessels, so helping to stop the bleeding.

There is another set of vessels in the body besides the blood-vessels, and these vessels do not form a closed circulatory system of tubes like the blood-vessels. They begin almost everywhere in the tissues of the body as thin fine vessels, and they run together into solid-looking bodies that are called glands. These vessels contain a fluid called lymph, because it is like water, and the vessels themselves, because they contain this fluid, are called lymphatic vessels, and the glands lymphatic glands.

These glands are more numerous in some parts of the body than in others; there are a great many about the intestines, and a good many in the neck and under the arms. The lymphatic vessels empty into cavities in the glands, and then other rather larger vessels start away from the other side of these bodies, and run on until they come to some more of them, and so these glands are joined together by the lymphatic vessels running from one to the other. They are like veins in that they have valves in them, which only allow the fluid to go one way, viz. from the smaller vessels towards the larger ones. The lymphatics from the lower part of the body all run together into a receptacle placed against the bodies of the lumbar vertebr‘. This receptacle goes by the name of the receptacle of the chyle; it is connected with a tube about the size of a goose quill running up close to the left side of the bodies of the dorsal vertebr‘ into the neck, and emptying itself into the junction of the vein from the left arm and the great vein from the left side of the head (called the jugular vein), and where it empties itself into the veins there is a valve, which is so arranged that no blood can go out of the veins into this tube, which is called the thoracic duct, but the fluid that is in that duct can go into the blood that is in the veins - that is the course of the lymphatic vessels of the lower part of the body, and of the left side of the chest and head and left arm. But the lymphatics of the right arm and right side of the head and neck, and the upper part of the right side of the chest, do not go into the thoracic duct at all, but empty themselves into the veins in a corresponding place on the other side; so that ultimately all the lymph that comes from all the tissues of the body gets into the great veins which run into the upper vena cava, and so into the right auricle of the heart.