Sir M. Foster

26. You have already learnt to recognize the blood-vessels of the rabbit, and to distinguish two kinds of blood-vessels—the arteries, which in a dead animal generally contain little or no blood, and have rather firm stout walls; and the veins, which are generally full of blood, and have thinner and flabby walls. The arteries when you cut them generally gape and remain open; the veins fall together and collapse. The larger the arteries, the stouter and firmer they are, and the greater the difference between them and the veins.

You have also studied the capillaries in the frog’s foot; you have seen that they are minute channels, with the thinnest and tenderest walls, forming a close network in which the smallest arteries end, and from which the smallest veins begin.

You have moreover been told that all over your own body, in every part, there are, though you cannot see them, networks of capillaries like those in the frog’s foot which you can see; that all the arteries of your body end in capillaries, and all the veins begin in capillaries. Let me repeat that, one or two structures excepted, there is no part of your body in which, could you put it under a microscope, you would not see a small artery branching out and losing itself in a network of capillaries, out of which, as out of so many roots, a small vein gathers itself together again.

In some places the network is very close, the capillaries lying closer together than even in the frog’s foot; in others the network is more open, and the capillaries wider apart; but everywhere, with a few exceptions which you will learn by and by, there are capillaries, arteries, and veins.

Suppose you were a little lone red corpuscle, all by yourself in the quite empty blood-vessels of a dead body, squeezed in the narrow pathway of a capillary, say of the biceps muscle of the arm, able to walk about, and anxious to explore the country in which you found yourself. There would be two ways in which you might go. Let us first imagine that you set out in the way which we will call backwards. Squeezing your way along the narrow passage of the capillary in which you had hardly room to move, you would at every few steps pass, on your right hand and on your left, the openings into other capillary channels as small as the one in which you were. Passing by these you would presently find the passage widening, you would have more room to move, and the more openings you passed, the wider and higher would grow the tunnel in which you were groping your way. The walls of the tunnel would grow thicker at every step, and their thickness and stoutness would tell you that you were already in an artery, but the inside would be delightfully smooth. As you went on you would keep passing the openings into similar tunnels, but the further you went on, the fewer they would be. Sometimes the tunnels into which these openings led would be smaller, sometimes bigger, sometimes of the same size as the one in which you were. Sometimes one would be so much bigger, that it would seem absurd to say that it opened into your tunnel. On the contrary, it would appear to you that you were passing out of a narrow side passage into a great wide thoroughfare. I dare say you would notice that every time one passage opened into another the way suddenly grew wider, and then kept about the same size until it joined the next. Travelling onwards in this way, you would after a while find yourself in a great wide tunnel, so big that you, poor little corpuscle, would seem quite lost in it. Had you anyone to ask, they would tell you it was the main artery of the arm. Toiling onwards through this, and passing a few but for the most part large openings, you would suddenly tumble into a space so vast that at first you would hardly be able to realize that it was the tunnel of an artery like those in which you had been journeying. This you would learn to be the aorta, the great artery of all; and a little further on you would be in the heart.

Suppose now you retraced your steps, suppose you returned from the aorta to the main artery of the arm, and thus back through narrower and narrower tunnels till you came again to the spot from which you started, and then tried the other end of the capillary. You would find that that led you also, in a very similar way, into wider and wider passages. Only you could not help noticing that though the inside of all the passages was as smooth as before, the walls were not nearly so thick and stout. You would learn from this that you were in the veins, and not in the arteries. You would meet too with something, the like of which you did not see in the arteries (except perhaps just close to the heart). Every now and then you would come upon what for all the world looked like one of those watch-pockets that sometimes are hung at the head of a bedstead, a watch-pocket with its opening turned the way you were going. This you would find was called a valve, and was made of thin but strong membrane or skin. Sometimes in the smaller veins you would meet with one watch-pocket by itself, sometimes with two or even three abreast, and I dare say you would notice that very frequently, directly you had passed one of these valves, you came to a spot where one vein joined another.

Well, but for these differences, your journey along the veins would be very like your journey along the arteries, and at last you would find yourself in a great vein, whose name you would learn to be the vena cava, or hollow vein (and because, though there is but one aorta, there are two great “hollow veins,” the superior vena cava or upper hollow vein), and from thence your next step would be into the heart again. So you see, starting from the capillary (you started from a capillary in the arm, but you might have started from any capillary anywhere), whether you go along the arteries or whether you go along the veins, you at last come to the heart.

Before we go on any further we must learn something about the heart.

27. Go and ask the butcher for a sheep’s pluck. There will most probably be one hanging up in his shop. Look at it before he takes it down. The hook on which it is hanging has been thrust through the windpipe. You will see that the sheep’s windpipe is, like the rabbit’s, all banded with rings of cartilage, only very much larger and coarser. Below the windpipe come the spongy lungs, and between them lies the heart, which perhaps is covered up with a skin and so not easily seen. Hanging to the heart and lungs is the great mass of the liver. When you have got the pluck home, cut away the liver, cut away the skin (pericardium, it is called) which is covering the heart, if it has not been cut away already, and lay the lungs out on a table with the heart between them. You will then have something very much like what

Image unavailable: Fig. 5.—Heart of Sheep, as seen after Removal from the Body, lying upon the Two Lungs. The Pericardium has been cut away, but no other Dissection made. R.A. Auricular appendage of right auricle; L.A. auricular appendage of left auricle; R.V. right ventricle; L.V. left ventricle; S.V.C. superior vena cava; I.V.C. inferior vena cava; P.A. pulmonary artery; Ao, aorta; ÁÓ, innominate branch from aorta dividing into subclavian and carotid arteries; L. lung; Tr. trachea. 1, solid cord often present, the remnant of a once open communication between the pulmonary artery and aorta. 2, masses of fat at the bases of the ventricle hiding from view the greater part of the auricles. 3, line of fat marking the division between the two ventricles. 4, mass of fat covering the trachea.
Fig. 5.—Heart of Sheep, as seen after Removal from the Body, lying upon the Two Lungs. The Pericardium has been cut away, but no other Dissection made.

R.A. Auricular appendage of right auricle; L.A. auricular appendage of left auricle; R.V. right ventricle; L.V. left ventricle; S.V.C. superior vena cava; I.V.C. inferior vena cava; P.A. pulmonary artery; Ao, aorta; ÁÓ, innominate branch from aorta dividing into subclavian and carotid arteries; L. lung; Tr. trachea. 1, solid cord often present, the remnant of a once open communication between the pulmonary artery and aorta. 2, masses of fat at the bases of the ventricle hiding from view the greater part of the auricles. 3, line of fat marking the division between the two ventricles. 4, mass of fat covering the trachea.

is represented in Fig. 5. If you could look through the front of your own chest, and see your own heart and lungs in place, you would see something not so very very different.

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

Image unavailable: 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; 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.
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; 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.

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 A², 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.

Image unavailable: Fig. 7.—Diagrammatic Sections of Veins with Valves. 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.
Fig. 7.—Diagrammatic Sections of Veins with Valves.

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

Image unavailable: Fig. 8.—Right Side of the Heart of a Sheep. 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.
Fig. 8.—Right Side of the Heart of a Sheep.

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.

Image unavailable: Fig. 9.—The Orifices of the Heart seen from above, the Auricles and Great Vessels being cut away. 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.
Fig. 9.—The Orifices of the Heart seen from above, the Auricles and Great Vessels being cut away.

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.

water will immediately fill the auricle and run over. If you look at the membrane carefully as it comes bulging up, you will notice that it is made up of three pieces joined together as is shown in Fig. 9 (lv. 1, lv. 2, lv. 3). These three pieces form the valve between the right auricle and ventricle, called the tricuspid, or three-peaked valve. Why it is so called you will understand if you lay open the right ventricle by cutting with a pair of scissors from the auricle into the ventricle along the side of the heart, or by cutting away the front of the ventricle as has been done in Fig. 8. You will then see that the valve is made up of three little triangular flaps, which grow together round the opening with their points hanging down into the cavity of the ventricle (Fig. 10, t.v.) They do not, however, hang quite loosely. You will notice fastened to the sides of the flaps, thin delicate threads, the other ends of which are fastened to the sides of the ventricle, and often to little fleshy projections called papillary muscles (Fig. 8, P.P.)

How do these valves act? In this way. When the ventricle is empty, and blood or water or any other fluid is poured into it from the auricle, the valves are pushed on one side against the walls of the ventricle, and thus there is a great wide opening from the auricle into the ventricle. But as the ventricle fills, the blood or water gets behind the flaps and floats them up towards the auricle. The more fluid in the ventricle the higher they float, until when the ventricle is quite full they all meet together in the middle of the opening between the auricle and ventricle and completely block it up. But why do they not turn right over into the auricle, and so open up again the wrong way? Because of those little threads (the chordÆ tendineÆ, as they are called) which fasten them to the walls of the ventricle. The flaps float back until these threads are stretched quite tight, and the threads are just long enough to let the flaps reach to the middle of the opening, but no further. The tighter the threads are stretched the closer the flaps fit together, and the more completely do they block the way from the ventricle back into the auricle.

The tricuspid valve, then, lets blood flow easily from the right auricle into the right ventricle, but prevents it flowing from the ventricle into the auricle.

31. Now look at the cavity of the ventricle. Its walls are fleshy, that is muscular, and you will notice that they are much stouter and thicker than those of the auricle. Besides the opening from the auricle there is but one other, which is at the top of the ventricle, side by side with the former. If you put a penholder or your finger through this second opening, you will find that it leads into the large vessel which you have already learnt to recognize as the pulmonary artery (Fig. 5, P.A.)

Slit up the pulmonary artery from the ventricle with a pair of scissors, as has been done in Fig. 8, P.A. You will notice at once the line where the red soft flesh of the muscular ventricle leaves off, and the yellow firmer material of which the artery is made begins. Just at that line you will see a row of three (perhaps you may have cut one of the three with your scissors) most beautiful, watch-pocket valves, made on just the same principle as those in the veins, only larger, and more exquisitely finished. These are called semilunar valves, because each pocket is of the shape of a half-moon. Lift them up carefully and see how tender and yet how strong they are. There is no need to tell you the use of these. You know it at once. They are to let the blood flow from the ventricle into the pulmonary artery, and to prevent the blood going back from the artery into the ventricle.

On the right side of the heart we have, then, two great valves, the tricuspid valve between the auricle and the ventricle, and the semilunar valve between the ventricle and the pulmonary artery. These let the blood flow easily one way, but not the other. If you doubt this, try it. Put a tube into either the superior or inferior vena cava of a fresh heart, tying the other vena cava and another tube into the pulmonary artery. If with a funnel you pour water into the tube in the vein, it will run through auricle and ventricle and out through the tube of the pulmonary artery as easily as possible; but if you try to pour water the other way down the pulmonary artery, you will find you cannot do it; the tube gets blocked directly, and only a few drops come back through the heart into the vein.

Now slit up the pulmonary artery as far as you can, and note when you cut it how stout and firm are its walls. You will find that it soon divides into two branches, one for the right lung, one for the left. Each of these, when it gets to the lung, divides into branches, and these again into others, as far as you can follow them. You know from what you have learnt already that these branches end in capillaries all over the lungs.

32. Not far from the two main branches of the pulmonary artery you will find, covered up perhaps with fat and other matters, some tubes which you will at once recognize as veins, and if you open any one of these you will find that you can put a thin rod into it, and that it leads in one direction to the lungs, and in the other into the left side of the heart. These are the pulmonary veins, and if you slit them right up you will find they open (by four openings) into a cavity on the left side of the heart, almost exactly like that cavity on the right side which we called the right auricle (Fig. 11). This cavity is, in fact, the left auricle; out of it there is an opening into the left ventricle, very like the opening from the right auricle into the right ventricle. It too is guarded by flap valves, exactly like the tricuspid valve, only there are but two flaps instead of three (Fig. 9, m.v. 1, m.v. 2). Hence this valve is called the bicuspid, or more frequently the mitral valve. Its flaps have little threads by which they are fastened to the walls of the ventricle, and in fact, except for there being two flaps instead of three, the mitral valve is exactly like the tricuspid valve, and acts exactly the same way.

If you cut with a pair of scissors from the auricle into the ventricle, you will find the left ventricle (Fig. 11) very much like the right ventricle, only its walls are very much thicker, so much thicker that the left ventricle takes up the greater part of the heart. You will see this if you now look at the outside of a fresh heart.

The auricles are so small and so covered up by fat that from the outside you can hardly see them at all. What you chiefly see are two little fleshy corners, one of each auricle (Fig. 5, R.A. L.A.), often called “the auricular appendages.” By far the greater part is taken up by the ventricles—and if you look you will see a band of fat slanting across the heart (Fig. 5, 3). This marks the line of the fleshy division, or septum as it is called, between the two ventricles. You will notice that the point or apex of the heart belongs altogether to the left ventricle.

To return to the inside of the left ventricle. Up at the top of the ventricle, close to the opening from the auricle, there is one other opening, and only one. If you put your finger into this, you will find that it leads into a tube which first of all dips under or behind the pulmonary artery and then comes up and to the front again. This tube is what you already know as the aorta. If you slit it up from the ventricle (and to do this you must cut through the pulmonary artery), you will find that on the left side, as on the right, the red fleshy wall of the ventricle suddenly changes into the yellow firm wall of the artery, and that just at this line there are three semilunar valves exactly like those in the pulmonary artery.

On the left side of the heart, then, we have also two valves, the mitral between the auricle and the ventricle, and the semilunar between the ventricle and the aorta. These let the blood pass one way and not the other. You can easily drive fluid from the pulmonary veins through auricle and ventricle into the aorta, but you cannot send it back the other way from the aorta.

These then are the reasons why the blood will only pass one way, the way I said it did. There are sets of valves opening one way and shutting the other. These valves are the tricuspid between the right auricle and right ventricle, the pulmonary semilunar valves between the right ventricle and the pulmonary artery, the mitral valve between the left auricle and the left ventricle, the aortic semilunar valves between the left ventricle and the aorta, and the valves which are scattered among the veins of the body. Of these by far the most important are the valves in the heart: they do the chief work; those in the veins do little more than help.

33. Well, then, we understand now, do we not? why the blood, if it moves at all, moves in the one way only. There still remains the question, Why does the blood move at all?

You know that during life it does keep moving. You have seen it moving in the web of a frog’s foot—and whenever any part of the body can be brought under the microscope, the same rush of red corpuscles through narrow channels may be seen. You know it moves because when you cut a blood-vessel the blood runs out. If you cut an artery across, the blood gushes out from the end which is nearest the heart; if you cut a vein across, the blood comes most from the end nearest the capillaries. If you want to stop an artery bleeding, you tie it between the cut and the heart; if you want to stop a vein bleeding, you tie it between the cut and the capillaries. You understand now why there is this difference between a cut artery and a cut vein. And you see that this is by itself a proof that the blood moves in the arteries from the heart to the capillaries, and in the veins from the capillaries to the heart.

The blood is not only always moving, but moves very fast. It flies along the great arteries at perhaps ten inches in a second. Through the little bit of capillaries along which it has to pass it creeps slowly, but manages sometimes to go all the way round from vein to vein again in about half a minute.

It is always moving at this rapid rate, and when it ceases to move, you die.

What makes it move?

Suppose you had a long thin muscle, fastened at one end to something firm, and with a weight hanging at the other end. You know that every time the muscle contracted it would pull on the weight and draw it up. But suppose, instead of hanging a weight on to the muscle, you wrapped the muscle round a bladder full of water. What would happen then each time the muscle contracted? Why, evidently it would squeeze the bladder, and if there were a hole in the bladder some of the water would be squeezed out. That is just what takes place in the heart. You have already learnt that the heart is muscular. Each cavity of the heart, each auricle, and each ventricle is, so to speak, a thin bag with a number of muscles wrapped round it. In an ordinary muscle of the body, the bundles of fibres of which the muscle is made up are placed carefully and regularly side by side. You can see this very well in a round of boiled beef, which is little more than a mass of great muscles running in different directions. You know that if you try to cut a thin slice right across the round, at one part your carving-knife will go “with the grain” of the meat, i.e. you will cut the fibres lengthways; at another part it will go “against the grain,” i.e. you will cut the fibres crossways. In both parts, the bundles of fibres will run very regularly. But in the heart the bundles are interlaced with each other in a very wonderful fashion, so that it is very difficult to make out the grain. They are so arranged in order that the muscular fibres may squeeze all parts of each bag at the same time.

Each cavity of the heart, then, auricle or ventricle, is a thin bag with a network of muscles wrapped round it, and each time the muscles contract they squeeze the bag and try to drive out whatever is in it. There are more muscles in the ventricles than in the auricles, and more in the left ventricle than in the right, for we have already seen how much thicker the ventricles are than the auricles, and the left ventricle than the right; and the thickness is all muscle.

And now comes the wonderful fact. These muscles of the auricles and ventricles are always at work contracting and relaxing, shortening and lengthening, of their own accord, as long as the heart is alive. The biceps in your arm contracts only when you make it contract. If you keep quiet, your arm keeps quiet and your biceps keeps quiet. But your heart never keeps quiet. Whether you are awake or whether you are asleep, whether you are running about or lying down quite still, whatever you are doing or not doing, as long as you are alive your heart keeps on steadily at work. Every second, or rather oftener, there comes a short sharp squeeze from the auricles, from both exactly at the same time, and just as the auricles have finished their squeeze, there comes a great hug from the ventricles, from both at the same time, but a much stronger hug from the left than from the right; and then for a brief space there is perfect quiet. But before the second has quite passed away, the auricles have begun again, and after them the ventricles once more, and thus the contracting and relaxing of the walls of the heart’s cavities, this beat of the heart as it is called, this short snap of the two auricles, this longer, steadier pull of the two ventricles, have gone on in your own body since before you were born, and will go on until the moment comes when friends gathering round your bedside will say that you are “gone.”

34. But how does this beat of the heart make the blood move? Let us see.

Remember that you have, or when you are grown up will have, bottled up in the closed blood-vessels of your body about 12 lbs. of blood. You have seen that the heart and the blood-vessels form a system of closed tubes; the walls are in some places, in the capillaries for instance, very thin, but they are sound and whole—and though the road is quite open from the capillaries through the veins, heart, and arteries to the capillaries again, there is no way out of the tubes except by making a breach somewhere in the walls.

This closed system of heart and tubes is pretty well filled by the 12 lbs. of blood.

What then must happen each time the heart contracts?

Let us begin with the right ventricle. Suppose it is full of blood. It contracts. The blood in it, squeezed on all sides, tries to go back into the right auricle, but the tricuspid flaps have been driven back and block the way. The more the blood presses on them, the tighter they become, and the more completely they shut out all possibility of getting into the auricle.

The way into the pulmonary artery is open, the blood can go there. But stay, the artery is already full of blood, and so are the capillaries and veins in the lung. Yes, but the artery will stretch ever so much. Take a piece of pulmonary artery, and having tied one end, pump or pour water into the other; you will see how much it will stretch. Into the pulmonary artery, then, goes the blood, stretching it in order to find room. As the ventricle squeezes and squeezes, until its walls meet in the middle, all the blood that was in it finds its way out into the artery. But the beat of the ventricle soon ceases, the squeeze is over and gone, and back tumbles the blood into the ventricle, or would tumble, only the first few drops that shoot backwards are caught by the watch-pocket semilunar valves. Back fly these valves with a sharp click (for the things of which we are speaking happen in a fraction of a second), and all further return is cut off. The blood has been squeezed out of the ventricle, and is safely lodged in the pulmonary artery.

But the pulmonary artery is ever so much on the stretch. It was fairly full before it received this fresh lot of blood; now it is over-full—at least that part of it which is nearest to the heart is over-full. What happens next? What happens when you stretch a piece of india-rubber and then let it go? It returns to its former size. The ventricle has stretched the piece of pulmonary artery near it, beyond the natural size, and then (when it ceased to contract) has let it go. Accordingly the piece of pulmonary artery tries to return to its former size, and since it cannot send the blood back to the ventricle, squeezes it on to the next piece of the artery nearer the capillaries, stretching that in turn.

This again in turn sends it on the next piece—and so on right to the capillaries. The over-full pulmonary artery, stretched to hold more than it fairly can, empties itself through the capillaries into the pulmonary veins until it is not more than comfortably full. But the pulmonary veins also are already full,—what are they to do? To empty the surplus into the left auricle. Oftener than every second there will come a time when they can do so.

For at the same time that the right ventricle pumped a quantity of blood into the pulmonary artery and safely lodged it there, the left ventricle pumped a like quantity into the aorta, safely lodged it there, and was left empty itself. But just at that moment the left auricle began to contract and to squeeze the blood that was in it.

Where could that blood go? It could not go back into the pulmonary veins, for they were already full, and the blood in them was being pressed behind by the over-full pulmonary arteries. But it could pass easily into the empty ventricle—and in it tumbled, the mitral flaps readily flying back and opening up a wide way. And so the auricle emptied itself into the ventricle. But now the auricle ceases to contract—its walls no longer squeeze—it is empty and wants filling, and so comes the moment when the pulmonary veins can pour into it the blood which has been driven into them by the over-full pulmonary artery.

Thus the right ventricle drives the blood into the over-full pulmonary artery, the pulmonary artery overflows into the pulmonary veins, the pulmonary veins carry the surplus to the empty left auricle, the left auricle presses it into the empty left ventricle, the left ventricle pumps it into the aorta—(the stretching of the aorta and of its branches is what we call the pulse)—the over-full aorta overflows just as did the pulmonary artery, through the capillaries of the body into the great venÆ cavÆ—through these the blood falls into the empty right auricle, the right auricle drives it into the empty right ventricle, and the full right ventricle is the point at which we began.

Thus the alternate contractions of auricles and ventricles, thanks to the valves in the heart and in the veins, pump the blood, stroke by stroke, through the wide system of tubes; and thus in every capillary all over the body we find blood pressed upon behind by over-full arteries, with a way open to it in front, thanks to the auricles, which are, once a second or oftener, empty and ready to take up a fresh supply from the veins. Thus it comes to pass that every little fragment of your body is bathed by blood, which a few moments ago was in your heart, and a few moments before that was in some other part of your frame. Thus it is that no part of your body can keep itself to itself; the blood makes all things common as it flies from spot to spot. The red corpuscle that a minute ago was in your brain, is now perhaps in your liver, and in another minute may be in a muscle of your arm or in a bone of your leg: wherever it goes it has something to bring, and something to fetch. A restless heart is for ever driving a busy blood, which wherever it goes buys and sells, making perhaps an occasional bargain as it shoots along the great arteries and great veins, but busiest of all as it lingers in the narrow pathways of the capillaries.

35. When you look down upon a great city from a high place, as upon London from St. Paul’s, you see stretching below you a network of streets, the meshes of which are filled with blocks of houses. You can watch the crowds of men and carts jostling through the streets, but the work within the houses is hidden from your view. Yet you know that, busy as seems the street, the turmoil and press which you see there are but tokens of the real business which is being carried on in the house.

So is it with any piece of the body upon which you look through the microscope. You can watch the red blood jostling through the network of capillary streets. But each mesh bounded by red lines is filled with living flesh, is a block of tiny houses, built of muscle, or of skin, or of brain, as the case may be. You cannot see much going on there, however strong your microscope; yet that is where the chief work goes on. In the city the raw material is carried through the street to the factory, and the manufactured article may be brought out again into the street, but the din of the labour is within the factory gates. In the body the blood within the capillary is a stream of raw material about to be made muscle, or bone, or brain, and of stuff which, having been muscle, or bone, or brain, is no longer of any use, and is on its way to be cast out. The actual making of muscle, or of bone, or of brain, is carried on, and the work of each is done, outside the blood, in the little plots of tissue into which no red corpuscle comes.

The capillaries are closed tubes; they keep the red corpuscles in their place. But their walls are so thin and delicate that they let the watery plasma of the blood, the colourless fluid in which the corpuscles float, soak through them into the parts inside the mesh. You probably know that many things will pass through thin skins and membranes in which no holes can be found even after the most careful search. If you put peas into a bladder and tie the neck, the peas will not get out until the bladder is untied or torn. But if you were to put a solution of sugar or of salt into the bladder, and place the bladder with its neck tied ever so tightly in a basin of pure water, you would find that very soon the water in the basin would begin to taste of sugar or salt—and that without your being able to discover any hole, however small, in the bladder. By putting various substances in the bladder, you will find that solid particles and things which will not dissolve in water keep inside the bladder, whereas sugar and salt, and many other things which dissolve in water, will make their way through the bladder into the water outside, and will keep on passing until the water in the basin is as strong of sugar or salt as the water in the bladder. This property which membranes such as a bladder have of letting certain substances pass through them is called osmosis. You will at once see how important a part it plays in your own body. It is by osmosis chiefly that the raw nourishing material in the blood gets into the little islets of flesh lying, as we have seen, in the meshwork of the capillaries. It is by osmosis chiefly that the worn-out stuff from the same islets gets back into the blood. It is by osmosis chiefly that food gets out of the stomach into the blood. It is by osmosis chiefly that the waste, worn-out matters are drained away from the blood, and so cast out of the body altogether. By osmosis the blood nourishes and purifies the flesh. By osmosis the blood is itself nourished and kept pure.

There are two chief things by which the blood, and through the blood the body, is nourished. These are food and air. The air we have always with us, we have no need to buy it or toil for it; hence we take it as we want it, a little at a time, and often. We gather up no store of it; and cannot bear the lack of it for more than a few moments.

For our food we have to labour; we store it up in our bodies from time to time, at intervals of hours, in what we call meals, and can go hours or even days without a fresh supply.

Let us first of all see how the blood, and, through the blood, the body, is nourished by air.