As soon as that happens the walls of the right ventricle contract, which is just what happened on the left side, thus pressing the blood against the valves, which are at the beginning of the large artery that leaves the right ventricle (which is, as I have already explained, the artery that goes to the lungs, called the pulmonary artery), and opening them, so the blood is forced out of the right ventricle into this pulmonary artery, and then takes place exactly what has been before described; the artery is bulged out when the blood is forced into it; it recoils and forces it against the valves, and shuts them, and so forces the blood on through the branches of the pulmonary artery, which divides into two, one to the right, and one to the left lung; in the lungs they subdivide into smaller arteries, and ultimately end in capillary vessels in the same way as the arteries do in the tissues of the rest of the body; those capillary vessels run together and form veins; these veins run on, and form four large veins, two for each lung, which, leaving the lungs, go into the left auricle of the heart; this left auricle of the heart is filled, and the blood flows from the left auricle through the bicuspid or mitral valve into the left ventricle, just in the same way as the blood flowed from the right auricle through the tricuspid valve into the right ventricle.

And now you see why it is called the circulation of the blood. You can start from one cavity of the heart, and follow the blood in its course right round, until you get into that cavity of the heart again, although the two sides of the heart do not communicate with one another in any way except through this system of tubes. You can see already, from what has been described, that there are really two circulations; there is the circulation of blood from the left ventricle, through the aorta and its branches to the different tissues of the body, through the capillaries of those tissues into the veins, through the veins back again to the heart, viz. to the right auricle; that is called the greater circulation, or systemic circulation, because it is that of the general system. And there is the circulation from the right ventricle through the pulmonary artery and its branches, through the capillaries in the lungs into the veins of the lungs, which are called the pulmonary veins, and back to the heart again, viz. to the left auricle, and this is called the lesser or pulmonary circulation. The blood in the greater circulation starts from the left, and flows back to the right side of the heart, and that in the lesser, or pulmonary circulation, starts from the right, and flows back through the lungs to the left side of the heart.

There is much more blood in the greater than in the lesser circulation, and, on the other hand, the blood travels much faster in the lesser-about five times as fast.

Another thing that I wish to impress upon you is this; the blood leaving the heart by the great aorta travels at about the rate of a foot in a second, but when it gets into the capillaries, it travels at the rate of only an inch in a minute, so that it travels slower and slower the farther it goes; and the rate becomes more and more uniform.

You will, perhaps, have thought by this description that the contraction of the auricles and ventricles of the two sides of the heart follow one another like one, two, three, four, but this is not so. When the left ventricle is contracting, and driving the blood into the great aorta, at that same time the right ventricle is contracting and driving the blood into the lungs, so that the two are contracting at one and the same time. When the two ventricles have done contracting, and are resting or dilating, they are doing it at the same time. When the blood is running from those two great veins through the right auricle, and gradually filling the right ventricle, the blood is at the same time running from the lungs through the four pulmonary veins into the left auricle and from that into the left ventricle, so that the two sides of the heart are doing their work at the same time, the two auricles are filling at the same time, and contracting at the same time; the two ventricles are filling at the same time, and contracting at the same time.

The period of action, and the period of rest, are about equal, so that although the heart beats, say seventy times a minute, it has just that same amount of rest -half that minute it is contracting, and the other half it is at rest, and that is when the heart gets its rest.

When the ventricles of the heart contract they move the heart; this contraction causes a movement of the whole heart; the base of the heart is more or less fixed in its position by means of those great vessels that leave it; the apex of the heart, on the other hand, is free; when the walls of the ventricle contract, the apex of the heart is tilted, and it butts against the chest-walls between the fifth and sixth ribs, thus causing what is known as the beating of the heart.

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.