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.

When you make a hole into the chest, and let the air in between the outside of the lungs and the chest wall, the pressure of the atmosphere gets at the outside of the lungs; there is then the same atmospheric pressure outside as inside the lungs; there is nothing to keep them on the stretch, and so they shrink up to their natural size, just as does the bladder when you leave off blowing into it, or when you take it out of the air-pump.

When before you made the hole in the diaphragm you pulled the diaphragm down, you still further lessened the pressure on the outside of the lungs; hence the pressure inside the lungs caused them to swell up and follow the diaphragm. But this put the lungs still more on the stretch, so that when you let go the diaphragm and ceased to pull on it, the lungs went back again to their former size, emptying themselves of part of their air and pulling the diaphragm up with them. When there is a hole in the chest wall, pulling the diaphragm down does not make any difference to the pressure outside the lungs. They are then always pressed upon by the same atmospheric pressure inside and outside, and so remain perfectly quiet.

When in an air-tight chest the diaphragm is pulled down, the pressure of the atmosphere drives air into the lungs through the windpipe and swells them up. When the diaphragm is let go, the stretched lungs return to their former size, emptying themselves of the extra quantity of air which they had received.

Suppose now the diaphragm were pulled down and let go again regularly every few seconds: what would happen? Why, every time the diaphragm went down a certain quantity of air would enter into the lungs, and every time it was let go that quantity of air would come out of the lungs again.

V.C.I. the vena cava inferior; O. the oesophagus; Ao. the aorta; the broad white tendinous middle (B) is easily distinguished from the radiating muscular fibres (A) which pass down to the ribs and into the pillars (C D) in front of the vertebrÆ.