If now you handle the heart—and if you want to learn physiology you must handle things—you will have no great difficulty in finding the great yellowish tubes marked Ao and Áó in the figure. Your butcher perhaps may not have cut them across exactly where mine has done, but that will not prevent your recognizing them. You will notice what thick stout walls they have, and how they gape where they are cut. Ao is the aorta, and Áó is a great branch of the aorta, going to the head and neck of one side, perhaps the branch along which we imagined just now that you, a poor little red blood-corpuscle, were travelling. If you were to put a wire through Áó you would be able to bring it out through Ao, or vice versâ. But what is P.A. which looks so much like the aorta, though you will find that it has no connection with it? You cannot pass a wire from the aorta into it. It also is an artery, the pulmonary[2] artery. We shall have more to say about it directly.
Now try and find what are marked in the figure as S.V.C. and I.V.C. You will perhaps have a little difficulty in this; and when you have found them you will understand why. They are the great veins of the body. S.V.C. is the superior vena cava, to form which all the veins from the head and neck and arms join, the vein in which you were journeying a little while ago. I.V.C. is the inferior vena cava, made out of all the veins from the trunk and the legs. Being veins, they have thin flabby walls; and their sides fall flat together, so that they seem nothing more than little folds of skin, and it becomes very hard to find the passage inside them. But when you have found the opening into them, you will see that you can stretch them out into quite wide tubes, and that their walls, though very much thinner than those of the aorta, so thin indeed that they are almost transparent, are still after a fashion strong. If you put a penholder or thin rod through either you will find that they both seem to lead right into the middle of the heart. With a little care you can pass a rod up I.V.C. and bring the end of it out at the top of S.V.C. Of course you will understand that both of these veins have been cut off short.
[28.] Before we go on any further with the sheep’s heart, let me tell you something about it, by help of the diagram in [Fig. 6], which is meant to represent the whole circulation. You must remember that this figure is a diagram, and not a picture; it does not represent the way the blood-vessels are really arranged in your own body. If you had no arms and no legs, and if you only had a few capillaries at the top of your head and at the bottom of your body, it might be more like than it is.
In the centre of the figure is the heart. This you will see is completely divided by an upright partition into two halves, a right half and a left half. Each half is further marked off, but not completely divided, into
L.A. left auricle; L.V. left ventricle; Ao. aorta; A1. arteries to the upper part of the body; A2. arteries to the lower part of the body; H.A. hepatic artery, which supplies the liver with part of its blood; V2. veins of the upper part of the body; V2. veins of the lower part of the body; V.P. vena portæ; H.V. hepatic vein; V.C.I. inferior vena cava; V.C.S. superior vena cava; R.A. right auricle; R.V. right ventricle; P.A. pulmonary artery; Lg. lung; P.V. pulmonary vein; Lct. lacteals; Ly. lymphatics; Th.D. thoracic duct; Al. alimentary canal; Lr. liver. The arrows indicate the course of the blood, lymph, and chyle. The vessels which contain arterial blood have dark contours, while those which carry venous blood have light contours.
two chambers, an upper chamber and a lower chamber; so that altogether we have four chambers,—two upper chambers, one on each side, marked R.A. and L.A., these are called the right and left auricles; and two lower chambers, one on each side, marked R.V. and L.V., these are called the right and left ventricles. The right auricle, R.A., opens in the direction of the arrow into the right ventricle, R.V., the opening being guarded, as we shall see, by a valve. The left auricle, L.A., opens into the left ventricle, L.V., the opening being likewise guarded by a valve; but you have to go quite a roundabout way to get from either the right auricle or ventricle to the left auricle or ventricle. Let us see how we can get round the figure. Suppose we begin with the two tubes marked V.C.S. and V.C.I., the walls of which are drawn with thin lines. These both open into the right auricle. They are the vena cava superior and inferior, which you have just made out in the sheep’s heart. From the right auricle you pass easily into the right ventricle; thence, following the arrow, the way is straight into the tube marked P.A. This is the pulmonary artery, the outside of which you saw in the sheep’s heart ([Fig. 5], P.A.) Travelling along this pulmonary artery, you come to the lungs, and after passing through branches not represented in the figure, picking your way through arteries which continually get smaller and smaller, you find yourself at last in the capillaries of the lungs. Squeezing your way through these, you come out into veins, and gradually advancing through larger and larger veins, you, still following the arrow, find yourself in one of four large veins (only one of them is represented in the diagram) which land you in the left auricle. From the left auricle it is but a jump into the left ventricle. From the left ventricle the way is open, as indicated by the arrow, into the tube marked Ao. This is intended to represent the aorta, which you have already seen in the sheep’s heart ([Fig. 5], Ao). It is here drawn for simplicity’s sake as dividing into two branches, but you have already been told, and must bear in mind, that it does not in reality divide in this way, but gives off a good many branches of various sizes. However, taking the figure as it stands, suppose we travel along A2. Following the arrow, and shooting through arteries which continually get smaller and smaller, we come at last to capillaries somewhere, in the skin or in some muscle, or in a bone, or in the brain, or almost anywhere, in fact, in the upper part of the body. Out of the capillaries we pass into veins, which, joining together and so forming larger and larger trunks, bring us at last to the point from which we started, the superior vena cava, V.C.S. If we had taken the other road, A2, we should have passed through capillaries somewhere in the lower part of the body instead of the upper, and come back by the vena cava inferior, V.C.I., instead of the vena cava superior. Starting from the right auricle, whichever way we took we should always come back to the right auricle again, and in our journey should always pass through the following things in the following order: right auricle, right ventricle, pulmonary artery, arteries, capillaries, and veins of the lungs, pulmonary vein, left auricle, left ventricle, aorta, arteries, capillaries, and veins somewhere in the body, and either superior or inferior vena cava. That is the course of the circulation. But there is something still to be added. Among the many large branches, not drawn in the diagram, given off by the aorta to the lower part of the body, there are two branches which are drawn and which deserve special notice.
One is a large branch carrying blood to the tube A.L., which is meant in the diagram to stand for the stomach, intestines, and some other organs. This branch, like all other branches of the aorta, divides into small arteries, and these into capillaries, which again are gathered up into veins, forming at last a large vein marked in the diagram V.P. and called the vena portæ or portal vein. Now the remarkable thing is that this vein does not, like all the other veins, go straight to join the vena cava, but makes for the liver, where it divides into smaller and smaller veins, until at last it breaks completely up in the liver into a set of capillaries again. These capillaries gather once more into veins, forming at last the large trunk, called the hepatic[3] vein, H.V., which does what the portal vein ought to have done but did not; it opens straight into the vena cava.