This artery, the great aorta, gives off branches. The first branches that it gives off are two small branches a little above the semilunar valves, and these go into the substance of the heart. They are called the coronary arteries, and the blood which goes through these two little branches into the walls of the heart nourishes the walls themselves. Then the aorta turns round, forming an arch, and gives off branches to the head and arms. You will recognise the name of the branches to the head, viz. the carotid arteries. Then it goes down through the chest close to the vertebral column, giving off branches as it goes to the chest-walls, etc., and passes through a hole in the diaphragm into the abdomen, giving off branches to the different organs in the abdomen; lastly, it subdivides into two branches, one for each lower limb. Its branches go on giving off smaller branches the farther they go, so that from that great artery branches gQ to all parts of the body. This is the artery which starts from the left lower cavity of the heart.

These branches become finer and finer, and when they become very fine they have a considerable quantity of muscular tissue in their walls, and this is involuntary muscular tissue; they end in still finer tubes of what we call structureless membrane, which, because of their exceeding fineness, are named capillary vessels, from a word meaning a hair, though they are much finer than a hair.

The blood then is pumped into the great aorta, and, by contrivances already explained, forms a kind of wave which goes on and extends the different pieces of artery on its way, each piece of artery recoiling upon it and pushing it farther, and it goes on right into the very small arteries. We can actually detect that wave in some places, as in the wrist, where we are able to feel the pulse, which is caused by the wave we have just been describing.

When the blood passes through the smallest arteries, and is squeezed by their muscular walls into the capillary vessels, this wave is lost, and through these it travels at a perfectly uniform rate. Now there are several reasons for that One reason is, that the farther you go from the heart the greater the sectional area of all the arteries becomes; the smaller the arteries get the more there are, and they are immensely out of proportion to the size of the artery at its beginning, so that if you take all the small arteries at a certain distance from the heart their area is very much larger than the arch of the aorta as it leaves the heart, so that the blood that is started through this large artery very fast must go very slowly when it comes to the small ones, and consequently when it reaches the capillary vessels it goes quite slowly and uniformly, and you can tell how it is that though this arteiy is filled with blood by jerks of three ounces of blood at a time, when it reaches the capillary vessels it should run uniformly and slowly; because, suppose you had a large cistern with a small tap in the bottom of it, and you filled this cistern with water by throwing in buckets of water at the rate of one every half minute, and turned the tap on, you would be throwing in water just in the same kind of way as the blood is pumped by the heart, but the water would go on running at the tap in a continuous stream, just as the blood flows through the capillaries, and not in jerks.

These capillaries in the different tissues of the body run together and form little vessels which we call veins. These little vessels have very thin walls, not strong, with little muscular tissue in them; they run on together and form larger veins; these join one another, and so on. Now whereas in the arteries there were no valves at all after those valves just at the beginning of the great aorta, there are throughout the veins little flaps of membrane sticking out from their walls, and sticking out in such a way that blood can pass between them from the capillaries, but whenever any blood is forced back again towards the capillaries these flaps are so constructed that they close up the passage; so that the blood can only go one way in the veins, namely from the capillaries in the tissues of the body towards the heart.

These veins then go on running together, joining one another, continually forming larger veins; these larger veins end in two large veins called vence cavce, which means hollow veins. One comes from all the lower part of the body, and is called the vena cava inferior, and the other comes from the upper part of the body, the head, the arms, etc., and is called the vena cava superior; these two large veins enter into the right auricle of the heart, having no valves at their entrance into it.

The blood that has been pumped from the left ventricle goes on therefore through the veins, passing the valves, until it gets by one or other of the two great veins into the right auricle of the heart; it goes on through the valves which we call the tricuspid valves, simply because there is no reason why it should not, they offer no resistance to it, and the ventricle is at the time empty, so the blood flows on into the right ventricle.

Before I go on I must mention that there is another vessel that enters into this right auricle, and that is the vein that brings back the blood from the substance of thp heart. The two coronary arteries which go into the walls of the heart end in capillaries which form veins, ultimately joining in the coronary vein, which enters the right auricle by itself, and is protected by a valve.

So that where the large veins open into the auricle tRere are no valves, and the blood flows on through the auricle into the ventricle and gradually fills it; when it has got nearly full, the walls of the auricle, which are' muscular walls though very thin, contract upon the remainder of the blood in the auricle and press it equally in all directions, against the blood in the veins to a certain extent, because there are no valves at the entrance of the veins, and on past the tricuspid valves into the ventricle, filling the latter and floating up the valves.