Part 10

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

FIG. 6.--Diagram of the Heart and Vessels, with the Course of the Circulation, viewed from behind so that the proper left of the Observer corresponds with the left side of the Heart in the Diagram.

L.A. left auricle; L.V. left ventricle; Ao. aorta; A^{1}. arteries to the upper part of the body; A^{2}. arteries to the lower part of the body; H.A. hepatic artery, which supplies the liver with part of its blood; V^{2}. veins of the upper part of the body; V^{2}. 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 A^{2}. 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, A^{2}, 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 vein=, H.V., which does what the portal vein ought to have done but did not; it opens straight into the vena cava.

The other branch of the aorta of which we are speaking goes straight to the liver, and is called the =hepatic artery=, H.A.: there it breaks up in the liver into small arteries, and then into capillaries, which mingle with the capillaries of the portal vein, and form one system, out of which the hepatic veins spring. So you see it makes a great difference to a red corpuscle which is travelling along the lower part of the aorta A^{2}, whether it takes a turn into the branch going to the alimentary canal, or whether it goes straight on into, for instance, a branch going to some part of the leg. In the latter case, having got through a set of capillaries, it is soon back into the vena cava and on its road to the heart. But if it takes the turn to the alimentary canal, it finds after it has passed through the capillaries and got into the portal vein, that it has still to go through another set of capillaries in the liver before it can pass through the hepatic vein into the vena cava.

This then is the course of the circulation. Right side of the heart, pulmonary artery, capillaries of the lungs, pulmonary vein, left side of the heart, aorta, capillaries somewhere, sometimes two sets, sometimes one, vena cava, right side of the heart again. A little corpuscle cannot get from the right to the left side of the heart without going through the capillaries of the lungs. It cannot get from the left side of the heart to the right without going through some capillaries somewhere in the body, and if it should happen to take the turn to the stomach, it has to go through two sets of capillaries instead of one.

You see, you really have two circulations, and you have two hearts joined together into one. If you were very skilful you might split the heart in half and pull the two sides asunder, and then you would have one heart receiving all the veins from the body and sending its arteries (branches of the pulmonary artery) all to the lungs, and another heart receiving all the veins from the lungs and sending its arteries (branches of the aorta) all over the body. And you would have two circulations, one through the lungs, and another through the rest of the body, both joining each other. Very often two circulations are spoken of, and because the lungs are so much smaller than the rest of the body, the circulation through the lungs is called the lesser circulation, that through the rest of the body the greater circulation.

=29.= I have described the circulation as if the blood always went in one direction from the right side of the heart to the left, from arteries to veins, the way the arrows point in the diagram. And so it does. It cannot go the other way round. =Why does it go that way? Why cannot it go the other way round?=

The reasons are to be found partly in the heart, partly in the veins.

=In the veins the blood will only pass from the capillaries to the heart.= Why not from the heart to the capillaries? You remember the little watch-pocket-like valves, here and there, sometimes singly, sometimes two or three abreast. =You remember that the mouths of the watch-pockets were always turned towards the heart.= Now suppose a crowd of little corpuscles hurrying along a vein towards the heart. When they came to one of these watch-pocket valves they would simply trample it down flat, and so pass over it without hardly knowing it was there, and go on their way as if nothing had happened. But suppose they were journeying the other way, from the heart to the capillaries. When they came to the open mouth of a watch-pocket valve, some of them would be sure to run into the pocket, and then the pocket would bulge out, and the more it bulged out the more blood would run into it, until at last it would be so full of blood that it would press close against the top of the vein, as is shown in Fig. 7 (or, if there were two or three, they would all meet together), and so quite block the vein up. If you doubt this, make a watch-pocket out of a piece of silk or cotton, fasten it on to a piece of brown paper, and roll the paper up into a tube, so that the valve is nicely inside the tube. If you pour some peas down the tube with the mouth of the valve looking away from you, they will run through at once; but if you try to pour them the other way, your tube will soon be choked, and if you carefully unroll the tube you will find the watch-pocket crammed full of peas.

In the upper, the blood is supposed to be flowing in the direction of the arrow, towards the heart; in the lower, the reverse way. C, capillary side; H, heart side. ]

=The valves in the veins, then, let the blood pass easily from the capillaries to the heart, but won’t let it go the other way.= If you bare your arm you may see some of the veins in the skin, in which the blood is running up from the hand towards the shoulder. If with your finger you press one of these veins back towards the hand it will swell up, and if you look carefully you may see little knots here and there caused by the bulging out of the watch-pocket valves. If you press it the other way, towards the elbow, you will empty it easily, and if with another finger you prevent the blood getting into it from behind, that is from the hand, the vein will remain empty a very long time.

The presence of valves in the veins, then, is one reason why the blood moves in one direction, but other reasons, and these the chief ones, are to be found in the heart.

Let us now go back to the sheep’s heart.

=30.= You know from the diagram that the two great veins, the superior and inferior vena cava, open into the right auricle. If you slit up these two veins in the sheep’s heart, you will find that they end by separate openings in a small cavity, the inside of which is for the most part smooth, and the walls of which, made, as you will at once see, of muscle, are not very thick. This small cavity is the right auricle, shown in Fig. 8, R.A., where the great veins have not been slit up, but the front of the auricle has been cut away. In this auricle, beside the openings into the two great veins and another one which belongs to a vein coming from the heart itself (Fig. 8, b) there is quite a large one, leading straight downwards, into which you

R.A. cavity of right auricle; S.V.C. superior vena cava; I.V.C. inferior vena cava; (a piece of whalebone has been passed through each of these;) a, a piece of whalebone passed from the auricle to the ventricle through the auriculo-ventricular orifice; b, a piece of whalebone passed into the coronary vein.

R.V. cavity of right ventricle; tv, tv, two flaps of the tricuspid valve: the third is dimly seen behind them, the a, piece of whalebone, passing between the three. Between the two flaps, and attached to them by chordæ tendineæ, is seen a papillary muscle, PP, cut away from its attachment to that portion of the wall of the ventricle which has been removed. Above, the ventricle terminates somewhat like a funnel in the pulmonary artery, P.A. One of the pockets of the semilunar valve, sv, is seen in its entirety, another partially.

1, the wall of the ventricle cut across; 2, the position of the auriculo-ventricular ring; 3, the wall of the auricle; 4, masses of fat lodged between the auricle and pulmonary artery. ]

can put your three fingers. This is the opening into the right ventricle; and you will have no difficulty in putting your fingers from the auricle into the ventricle and bringing them out again.

P.A. pulmonary artery, with its semilunar valves; Ao. aorta, do.

R.A.V. right auriculo-ventricular orifice with the three flaps (lv. 1, 2, 3) of tricuspid valve.

L.A.V. left auriculo-ventricular orifice, with m.v. 1 and 2, flaps of mitral valve; b, piece of whalebone passed into coronary vein. On the left part of L.A.V. the section of the auricle is carried through the auricular appendage; hence the toothed appearance due to the portions in relief cut across. ]

But hold the heart in one hand with the auricle upwards, and try to pour some water into the ventricle. The first few spoonfuls will go in all right, and then you will see some thin white skin or membrane come floating up into the opening and quite block up the entrance from the auricle into the ventricle; the

R.A.V. right auriculo-ventricular orifice surrounded by the three flaps, t.v. 1, t.v. 2, t.v. 3, of the tricuspid valve; these are stretched by weights attached to the chordæ tendineæ.

L.A.V. left auriculo-ventricular orifice surrounded in same way by the two flaps, m.v. 1, m.v. 2, of mitral valve; P.A. the orifice of pulmonary artery, the semilunar valves having met and closed together; Ao. the orifice of the aorta with its semilunar valves. The shaded portion, leading from R.A.V. to P.A., represents the funnel seen in Fig. 8. ]