Part 5

=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

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

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[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 _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.

=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

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.

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

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.=