Chapter XIII: Introduction: § I (3)

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.

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.
]

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.

_P.V._ pulmonary veins opening into the left auricle by four
openings, as shown by the styles or pieces of whalebone placed in
them: _a_, a style passed from auricle into ventricle through the
auriculo-ventricular orifice; _b_, a style passed into the coronary
vein, which, though it has no connection with the left auricle, is,
from its position, necessarily cut across in thus laying open the
auricle.

_M.V._ the two flaps of the mitral valve (drawn somewhat
diagrammatically): _pp_, papillary muscles, belonging as before to
the part of the ventricle cut away; _c_, a style passed from
ventricle in _Ao._ aorta; _Ao_^{2}. branch of aorta (see Fig. 5,
_Áó_); _P.A._ pulmonary artery; _S.V.C._ superior vena cava.

1, wall of ventricle cut across; 2, wall of auricle cut away around
auriculo-ventricular orifice; 3, other portions of auricular wall
cut across; 4, mass of fat around base of ventricle (see Fig. 5,
2).
]

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.

HOW THE BLOOD IS CHANGED BY AIR: BREATHING. § VI.

=36.= I have already said, perhaps more than once, that our muscles burn, burn in a wet way without giving light. And when I say our muscles, I might say our whole body, some parts burning more fiercely than others.

You have learnt from your Chemistry Primer (Art. 2, p. 2) what happens when a candle is placed in a closed jar of pure air. The oxygen gets less, carbonic acid comes in its place, and after a while the candle goes out for want of oxygen to carry on that oxidation which is the essence of burning. You also know that exactly the same thing would happen if you were (only you need not do it) to put a bird or a mouse in the jar instead of a candle. The oxygen would go, carbonic acid would come, and the little flame of life in the mouse would flicker and go out, and after a while its body would be cold.

But suppose you were to put a fish or a snail in a jar of pure fresh water, and cork the jar tight. There seems at first sight to be no air in the jar. But there is. If you were to take that fresh water, and put it under an air-pump, you could pump bubbles of air out of it; and if you were to examine these bubbles you would find them to contain oxygen and nitrogen, with very little carbonic acid. =The water contains dissolved air.= After the fish or the snail had been some time in the jar, you would see its flame of life flicker and die out, just like that of the bird in air; and if you then pumped the air out of the water you would find that the oxygen was nearly gone and that carbonic acid had come in its place.

You see, then, that air can be breathed, as we call it, even when it is dissolved in water.

Now to return to our muscle. When you were watching the circulation in the frog’s foot, you could tell the artery from the vein, because in the artery the blood was flowing _to_ the capillaries, and in the vein _from_ them. Both artery and vein were rather red, and of about the same tint of colour. But if you could see in your own body a large artery going to your biceps muscle, and a large vein coming away from it, you would be struck at once with the difference of colour between them. The artery would look bright scarlet, the vein a dark purple; and if you were to prick both, the blood would gush from the artery in a bright scarlet jet, and bubble from the vein in a dark purple stream. And wherever you found an artery and a vein (with a great exception of which I shall have to speak directly), the blood in the artery would be bright scarlet, and that in the vein dark purple. Hence we call the bright scarlet blood which is found in the arteries =arterial blood=, and the dark purple blood which is found in the veins =venous blood=.

What is the difference between the two? If you were to pump away at some arterial blood, as you did at the water in which you put your fish, you would be able to obtain from it some air, or, more correctly, some gas; a great deal more gas, in fact, than you did from the water. A pint of blood would yield you half a pint of gas. This gas you would find on examination not to be air, _i.e._ not made up of a great deal of nitrogen and the rest oxygen. (Chemical Primer, Art. 9.) There would be very little nitrogen, but a good deal of oxygen, and still more carbonic acid.

If you were to pump away at some venous blood you would get about as much gas, but it would be very different in composition. The little nitrogen would remain about the same, but the oxygen would be about half gone, while the carbonic acid would be much increased.

This, then, is one great difference (for there are others) between venous and arterial blood, that while =both contain, dissolved in them, oxygen, nitrogen, and carbonic acid, venous blood contains less oxygen and more carbonic acid than arterial blood=.

=37. In passing through the capillaries on its way to the vein, the blood in the artery has lost oxygen and gained carbonic acid.= Where has the oxygen gone to? Whence comes the carbonic acid? To and from the islets of flesh between the capillaries, to the bloodless muscular fibre or bit of nerve or skin which the blood-holding capillaries wrap round. The oxygen has passed from the blood within the capillaries to the flesh outside; from the flesh outside the carbonic acid has passed to the blood within the capillaries. And this goes on all over the body. Everywhere the flesh is breathing blood, is breathing gas dissolved in the blood, just as a fish breathes water, _i.e._ breathes the air dissolved in the water.

Goes on everywhere with one great exception. There is one great artery, with its branches, in which blood is not bright, scarlet, arterial, but dark, purple, venous. There are certain great veins in which the blood is not dark, purple, venous, but bright, scarlet, arterial. You know which they are. The pulmonary artery and the pulmonary veins. The blood in the pulmonary veins contains more oxygen and less carbonic acid than the blood in the pulmonary artery. =It has lost carbonic acid and gained oxygen, as it passed through the capillaries of the lungs.=

=38.= What are the lungs? As you saw them in the rabbit, or as you may see them in the sheep, they are shrunk and collapsed. We shall presently learn why. But if you blow into them through the windpipe, which divides into branches, one for either lung, you can blow them out ever so much bigger. They are in reality bladders which can be filled with air, but which, left to themselves, at once empty themselves again.

They are bladders of a peculiar construction. Imagine a thick short bush or tree crowded with leaves; imagine the trunk and the branches, small and great, down to the veriest twigs, all hollow; imagine further that the leaves themselves were little hollow bladders, stuck on to the smallest hollow twigs, and made of some delicate, but strong and exceedingly elastic, substance. If you blew down the trunk you might stretch and swell out all the hollow leaves; when you left off blowing they would all fall together, and shrink up again.

Around such a framework of hollow branches called =bronchial-tubes=, and hollow elastic bladders called =air-cells=, is wrapped the intricate network of pulmonary arteries, veins, and capillaries, in such a way that each air-cell, each little bladder, is covered by the finest and most close-set network of capillaries, very much as a child’s india-rubber ball is covered round with a network of string. Very thin are the walls of the air-cell, so thin that the blood in the capillary is separated from the air in the air-cell by the thinnest possible sheet of finest membrane. As the dark purple blood rushes through the crowded network, its carbonic acid escapes through this thin membrane, from the blood into the air, and oxygen slips from the air into the blood.

Thus the dark purple venous blood coming along the pulmonary artery, as it glides in the pulmonary capillaries along the outside of the inflated air-cells, by loss of carbonic acid and gain of oxygen is changed into the bright scarlet blood of the pulmonary veins.

This then is the mystery of our constant need of air. =The flesh of the body of whatever kind, everywhere all over the body, breathes blood, making pure arterial blood venous and impure, all over the body except in the lungs, where the blood itself breathes air, and changes from impure and venous to pure and arterial.=

=39.= Through the capillaries of the muscle a stream of blood is ever flowing so long as life lasts and the heart has power to beat; every instant a fresh supply of bright, pure, arterial blood comes to take the place of that which has become dark, venous, and impure. Without this constant renewal of its blood the muscle would be choked, and its vital flame would flicker and die out.

In the lungs, the air filling the air-cells would if left to itself soon lose all its oxygen and become loaded with carbonic acid; and the blood in the capillaries of the lungs would no longer be changed from venous to arterial, but would travel on to the pulmonary vein as dark and impure as in the pulmonary artery. =Just as the blood in the muscle must be constantly renewed, so must the air in the lungs be continually changed.=

How is this renewal of the air in the lungs brought about?

In the dead rabbit you saw the lungs, shrunk, collapsed, emptied of much of their air, and lying almost hidden at the back of the chest (Fig. 1, _G.G._) The cavity of the chest seemed to be a great empty space, hardly half filled by the lungs and heart. But this is quite an unnatural condition of the lungs. Take another rabbit, and before you touch the chest at all, open the abdomen and remove all its contents--stomach, liver, intestines, &c. You will then get a capital view of the =diaphragm=, which as you already know forms a complete partition between the chest and the belly. You will notice that it is arched up towards the chest, so that the under surface at which you are looking is quite hollow. If you hold the rabbit up by its hind legs with its head hanging down, and pour some water into the abdomen, quite a little pool will gather in the shallow cup of the diaphragm.

In the rabbit the diaphragm is very transparent; you can see right through it into the chest, and you will have no difficulty in recognizing the pink lungs shining through it. =You will notice that they cover almost all the diaphragm--in fact they fill up the whole of the cavity of the chest that is not occupied by the heart.=

If you seize the diaphragm carefully in the middle with a pair of forceps, and pull it down towards the abdomen, you will find that you cannot create a space between the lungs and the diaphragm, but that the lungs follow the diaphragm, and are quite as close to it when it is pulled down as when it is drawn up.

=In other words, when the diaphragm is arched up as you find it on opening the abdomen, the lungs quite fill the chest; and when the diaphragm is drawn down and the cavity of the chest made bigger, the lungs swell out so that they still fill up the chest.=

=40.= How do they swell out? By drawing air in through the windpipe. If you listen, you will perhaps hear the air rush in as you pull the diaphragm down--and if you tie the windpipe, or quite close up the nose and mouth, you will find it much harder to pull down the diaphragm, because no fresh air can get into the lungs.

Now prick a hole through the diaphragm into the cavity of the chest, without wounding the lungs. You will hear a sudden rush of air, and the lungs will shrink up almost out of sight. They are no longer close against the diaphragm as they were before; and if you open the chest you will find that they have shrunk to the back of the thorax as you saw them in the first rabbit. The rush of air is partly a rush of air out of the lungs, and partly a rush of air into the chest between the chest walls and the outside of the lungs.

But before you lay open the chest, pull the diaphragm up and down as you did before you made the hole in the diaphragm. You will find that you have no effect whatever on the lungs. They remain perfectly quiet, and do not swell up at all. By working the diaphragm up and down, you only drive air through the hole you have made, =in and out of the cavity of the chest=, not =in and out of the lungs= as you did before.

We see then that the =chest is an air-tight chamber=, and that the lungs, when the chest walls are whole, =are always on the stretch=, are on the stretch even when the diaphragm is arched up as high as it can go.

Why is it that the lungs are thus always on the stretch? Because the chest is air-tight, so that no air can get in between the outside of the lungs and the inside of the chest wall. You know from your Physics Primer (Art. 29, p. 34) that the atmosphere is always pressing on everything. It is pressing on all parts of the rabbit; it presses on the inside of the windpipe and on the inside of the lungs. It presses on the outside of the abdomen, and so presses on the under surface of the diaphragm, and drives it up into the chest as far as it will go. But it will not go very far, because its edges are fastened to the firm walls of the chest. The air also presses on the outside of the chest, but cannot squeeze that much, because its walls are stout.

If the walls of the chest were soft and flabby, the atmosphere would squeeze them right up, and so through them press on the outside of the lungs; since they are firm it cannot. The chest walls keep the pressure of the atmosphere off the outside of the lungs.

The lungs then are pressed by the atmosphere on their insides and not on their outsides; and it is this inside pressure which keeps them on the stretch or expanded. When you blow into a bladder, you put it on the stretch and expand it because the pressure of your breath inside the bladder is greater than the pressure of the atmosphere outside the bladder. If, instead of making the pressure inside _greater_ than that outside, you were to make the pressure outside _less_ than that inside, as by putting the bladder under an air-pump, you would get just the same effect; you would expand the bladder. That is just what the chest walls do; they keep the pressure outside the lungs less than that inside the lungs, and that is why the lungs, as long as the chest walls are sound, are always expanded and on the stretch.