ARTERIAL AND VENOUS BLOOD.

It is at the extremity of the capillary arteries, as we have said, that the incomprehensible prodigy of the nourishment of our organs is accomplished. This done, the next thing is for the blood to return to its starting-point; and here recommence those infinitesimally minute wonders of which we have already spoken. Close upon the capillary _arteries_ follow the capillary _veins_, equally fine and imperceptible as the others. These take possession of the blood everywhere at once, without allowing it a moment's respite, and it is thenceforth on its road of return, travelling back again to the heart.

Where do the veins begin? where do the arteries end? No one can say precisely, since the last ramifications of each elude the eye of man, however much it may be aided by the admirable instruments which his genius has invented. Nevertheless, although no one has ever ascertained the fact by sight, there is one thing I can tell you--namely, that our minute veins are a continuation of our minute arteries, and that it is the same ca.n.a.l which as it lengthens out turns from an artery into a vein, without any interruption; the substances destined for the nourishment of the organs pa.s.sing through its walls, as moisture pa.s.ses through our skin when we perspire.

But if n.o.body has seen this, say you, how can they know it for a fact?

Let me explain. In man, and in the animals which come nearest to man in structure, it has never been seen; but it has been seen elsewhere.

This requires a little explanation, and you will not regret my giving it hereafter. It has its interest, I a.s.sure you.

When you put your hand on your throat, how does it feel to you?

_Warm_, does it not? And when you take hold of a kitten or a bird, how do they feel? _warm_ in the same way. Now, then, can you tell me whence comes this warmth? But to save time I will answer the question myself. It comes from their and your _blood_, which is itself warm, and we shall soon see why. You have no idea of all the curious facts wrapt up in that little phrase, "You are warm-blooded;" your blood is warm.

But it has not got warm of itself; bear that well in mind.

Now if you touch a frog, a lizard, or a fish, how do they feel to you?

Cold, of course, you answer. But I ask why? A question you will answer in the same way as the other. Because their blood is cold, they are "cold-blooded."

Precisely; and while you are about it you may add that, if their blood be cold, it is because it has not been warmed as yours is. Do not be impatient, we shall make all this clear at the proper time and place.

Now in the cold-blooded animals, such as serpents, frogs, tortoises, lizards, fishes, and others, the blood circulates as it does in us, and what is more, it does so, thanks to a machinery very similar to our own. But, as you may imagine, a machine which produces warmth must be constructed in a more perfect manner than a machine which produces no warmth; and to speak truth, without flattering you, there is a little difference between you and a frog, and it seems natural enough that the body of a frog should be more clumsy in structure than yours.

It is the old story of the poor man being not so well lodged as the rich; but putting aside rich and poor, who are all human beings alike, let us take one of those lovely dolls who walk, and move their arms and head, and say papa! and mamma! and compare it with a cheap bazaar doll which you can get for a penny. Both are made, in the main, in one way. Each has two arms, two legs, a mouth, a nose, eyes, &c.; but what a difference in the details of the two! and what infinitely more pains have been bestowed on one than on the other!

Well, cold-blooded animals are, so to speak, _penny doll_ animals, by comparison with ourselves. Like us they have arteries and veins, but there is not near so much workmanship in them; and that marvellous delicacy of the capillary extremities, which in man and in the warm-blooded animals drives the close observer to despair, does not exist to trouble us in these others. It is true that with the naked eye we are still unable to see everything, even in them; but with the help of the microscope the whole is laid open to us--the extremities of the arteries and the extremities of the veins; and it was here that what I was telling you of, just now, was observed and discovered,--namely, that the end of the artery changes into a vein, without any interruption in the tube. It was these very observations upon fishes and frogs, which eventually gained the day in favor of Harvey's ideas on the circulation of the blood, at which the learned men of his own age had laughed so much. He was dead by that time it is true, as has happened but too often in such cases, but do not let us pity him too much! He who has had the rare good-fortune to lay hold of a new truth, and launch it into the world, is sufficiently recompensed in advance. If he also craves after the flattering voice of man's approbation, and the toylike pleasure of personal triumph, he is after all but a child, unworthy of the great part G.o.d has given him the privilege of playing.

A child, did I say? Then how rude you must have thought me, dear child!

And as a punishment, you are perhaps going to remind me that I have once more fallen into my old bad habit of wandering away from my subject. Never mind, I am going to return to it at once.

How can one distinguish--you will ask me--an artery from a vein, so as to be able to determine which is a vein and which an artery?

In many ways, I reply. First of all, an artery, as I told you lately, is composed of three coats, of which the princ.i.p.al, _i.e._. the inner one, is tough and elastic, whereby the artery is enabled to force the blood forward in its turn, but which is also the reason of arterial cuts being so dangerous; for in such cases the wounded tube remains wide open; being held so by the stiffer inner coat; and thus the blood is allowed to run out indefinitely. Now this inner coat is wanting in the veins, whose walls sink in together when a cut is made in them, so that it is much easier to stop the flow of the blood in them.

Furthermore, the veins are furnished inside at intervals with little doors, similar to those we noticed at the entrance of the _auricles_ and _ventricles_ of the heart. You remember those important _valvelets_, on which depends so much of the mechanism; which permit the blood to pa.s.s in one direction, but will not allow it to return back in the other?--well, the little doors of the veins, which are also called _valvelets_, do exactly the same work. They open in the direction of the heart, to allow the blood to pa.s.s on, but it finds them fast closed if it wants to go back; so that as soon as it has forced one pa.s.sage there is no longer any hope of its return, and thus by degrees it gets nearer and nearer to the heart without any possibility of escape. There is nothing similar to this in the arteries, which the blood traverses in a single bound from the impetus it receives from the heart.

Finally--and this is most important--the blood which is found in the veins is no longer the same as that which fills the heart.

No longer the same? you exclaim--have we then two sorts of blood in our bodies? Most certainly, my dear child; but you would not have suspected it; for when you accidentally p.r.i.c.k or cut yourself, or when your nose bleeds, it is always the same sort of blood that comes out--that fine red liquid which everybody knows so well by sight. This is because the blood flows at once from the small arteries and small veins, and what you see is a mixture of the two. The same mixture issues from all wounds, whether small or great, and on this account people are unanimous in declaring that blood is red; a statement which is not true of either arterial or venous blood, separately. The last is black, as you might convince yourself if you had courage enough, and should happen to be in the room with any one who was going to be bled,--a rare event, happily, in these enlightened days.

In such a case it is always a vein which is opened, the reason of which you will understand, after what I said of the danger of cutting the arteries. You would there, fore see a reddish black jet of liquid spout from under the lancet; much blacker than red, however--that is _venous_ blood. When, on the other band, an artery has been accidentally cut, what comes out is quite different. It is a rosy, frothy fluid, almost like milk and carmine dissolved in it, which has been whipped up with a stick; this is called _arterial_ blood.

Nothing is more simple, as you perceive, than to distinguish an artery from a vein; you have only to ascertain what is inside of it. When the blood goes out to our organs to nourish them, it is _arterial_; when it is returning back after having nourished them, it has become _venous_.

But what--you will ask--is it going to do now at the heart, towards which it is on its road? It is going to seek there a fresh impetus which shall send it once more into the lungs, where it will again become _arterial_, _i. e._ and once more capable of affording nourishment to the organs. Therein lies the whole secret, and the why and the wherefore of the CIRCULATION.

This is easily said, dear child; but suppose that you do not comprehend it? Well, you need not be ashamed. There is no possibility of comprehending it until one has learnt what RESPIRATION is--so here we are stopped short.

To-morrow, then, when we will begin with the study of this third part of the History of Nutrition; and if the first two have amused you, I feel pretty sure you will not find this last one dull.

LETTER XVIII.

ATMOSPHERIC PRESSURE.

When we have been laboring very hard, my dear child, and want to rest for a minute, we say, _Let us take breath_; because breathing is an action which takes place of itself, requiring neither effort nor attention on our part.

But, if it takes place of itself, it does not explain itself; consequently, when I say to you, _Now, let us take breath_, this is not a signal for my having a rest, for I have undertaken to explain Respiration to you.

If you were a German, I would remind you of what so often happens when you put a fork into a dish of sour-krout. You want to lay hold of a little bit merely, but the strips of cabbage-leaf are twisted one within the other, and hang together in spite of you, so that withoutintending it you get hold of a whole plateful at once.

Now this Respiration affair is something like the sour-krout story--begging your pardon for the comparison. I should have liked to give you only a small plateful--a child's plateful--of it; but I feel the explanations coming, hanging one upon the other; and, whether I will or no, I must treat you like a grown-up person, and we must give up for once the nice little doll's dinners with which we began.

In my opinion, you will lose nothing by the change if you will but pay attention; for about that soft little breath of yours, which is always coming and going over your pretty lips, there are many more things to be learnt than you have heard of yet. As I said just now, you will find you have got hold of a plateful all at once. A good appet.i.te to you!

To prevent confusion we will divide the subject into two parts. I shall explain to you first, _How we breathe?_--a very curious question, as you will see. And afterwards we will examine, _Why we breathe?_--which is still more interesting.

First, I must tell you that air is heavy, and very heavy too; a thousand times more so than you may suppose. The air we breathe, through which we move backwards and forwards, that air is _some_thing, remember, although we do not see it; and when there is a wind, that is to say, when the air is in motion, like a stream of water running down a hill, we are forced to acknowledge its being something, for we see it throw down the largest trees and carry along the biggest ships. But without going so far out of the way for examples, try--you who run so well--to run for two minutes against a strong wind: and then you shall tell me whether the air is something or nothing. But if it be something it must have weight, for all substances have; paper as well as lead; with this sole difference, that the weight of lead is greater in proportion to its size than that of paper. Now a sheet of paper is very light, is it not? and you would be puzzled perhaps to say what it weighs. But many sheets of paper placed one upon the other, end by forming a thick book which has its undeniable weight; and if some one were to heap upon your head a pile of large books, like those you see on your papa's shelves, the end might be that you would be crushed to death.

In the same way, a small amount of air is by no means heavy; but you can conceive that a great quant.i.ty of it gathered together may end by weighing a great deal. Now get well into your head the fact, that we, here, on the surface of the earth, are at the bottom of an immense ma.s.s of air, extending to somewhere about forty or fifty miles above our heads. Let us say forty to make more sure, for learned men have not yet been able to calculate the precise height to a nicety; and for my own part, I think we have done wonders to get so near the mark even as this. But can you picture to yourself the distance which forty miles high really is? I will help you to form some idea.

One mile contains 5,280 feet, and your papa is six feet high. One mile high would therefore be 880 times as high as your papa, But this is a mere nothing--only one mile's height. In forty miles there would be no less than 211,200 feet; and setting papas aside, of whom it would take 35,200, one on the top of the other, to go so far into the sky, let us think of the height of the tallest buildings you know; church and cathedral towers for instance. Now the towers of many parish churches are 150 feet high; the towers of York Minister not 300. At that rate it would take 1,408 ordinary parish church-towers, or upwards of 704 York Minster towers, piled one above the other, to reach to the end of the forty miles of air above our heads. I leave you to judge what would be the weight of a ma.s.s of paper piled up as high as that.

You may safely grant then, that this ma.s.s or pile, or if you like it better, this _column_ of air (for that is the proper expression), must be of considerable weight; as is still further made certain by the fact of its having been weighed, so that I can even name the weight to you if you wish to hear it. Bear in mind too, that the weight of a column of air will be in proportion to its _superficial extent_--to its breadth and width, that is; for, as you may suppose, a column as large in extent as one of the towers of York Minster will weigh a good deal more than one the size of a single brick.

But wait; here is a book on the table which will serve me for a measure, and as you will probably find the same on your mamma's table, you can follow my measurement. It is a French Grammar. The back is seven inches long and four and a quarter wide. That is, there are four and a quarter rows, each seven inches long. In other words, the back contains nearly--and let us call it quite, for convenience' sake--thirty inches side by side. Thirty _square inches_ as it is called. Measure your mamma's copy and you will see. Now, can you guess the weight of the column of air forty miles high which this volume supports? Upwards of four cwt.; 450 lbs., that is to say. If you want to be very exact, here is the rule. Air presses on all bodies at the rate of fifteen pounds to every square inch; so now you can make the calculation for yourself.

But I suspect you had no idea you were so strong; for I see you tossing up the book, heavily laden as it is, like a feather.

Comfort yourself. There is no magic in the matter. If a very strong man were to push you on one side, could you resist him? Certainly not. But if another man of equal strength were to push you at the same time on the other side, what would happen? Well, you would remain quietly in your place, without troubling yourself more about one than the other, the two forces mutually destroying each other. And this is the case here. While the air above your book is weighing down upon it with a force of 450 lbs., the air below it presses against it underneath with an equal weight, and this destroys the effect of the other. From 450 lbs. take 450 lbs., and nothing remains. Your grammar has nothing to carry after all, and you may toss it about as you please, without deserving much credit for the effort.

"What are you telling me?" you inquire. "If I put a stone on the top of my head, I can feel its weight easily enough; but if I put my hand on the top of the stone I no longer feel anything. How can the air below the stone press against it? And talking of columns--how pleasant it would be, for instance, if the people who go up the Monument were to have the weight of it on their heads when they get to the top!"

Well said, little one. And your objection reminds me of an argument which distracted my head as a lad, when I first heard the pressure of air explained by a good fellow who did not trouble himself to be quite as exact as you and I are in our discussions. I was told that the surface of the body, or the skin of a large man, measured sixteen feet square, which is equal to the surface of a table four feet long and four broad. Now, you know that in four feet there are forty-eight inches, and on the surface of the table are forty-eight rows, with forty-eight inches in each, or 2,304 square inches; so that a man's surface is 2,304 square inches, and the weight his body supports is 34,560 lbs., or upwards of fifteen tons--always at the rate of fifteen pounds to every square inch, you understand. Now, I was constantly asking myself how it happened that in entering a house one never seemed to get rid of this almost fabulous weight, since the roof of the house must naturally interpose itself between the air-column of forty miles high and the man who would then only have some few feet of air above his head. The roof would support the rest, that was clear. From whence, then, came the 34,560 lbs. which seemed to weigh as heavily as before; since, whether on the threshold of the door, while still under shelter of the roof, or two steps outside in the open air, under the tremendous column forty miles high, one never felt a bit lighter, not even to the extent of the weight of a single sheet of paper? This was a difficulty from which I could never extricate myself.

I found out the answer to the riddle afterwards, and a very simple one it is.

Air does not, in point of fact, _weigh down_ like a solid fifty pounds' weight, which has no impulse but to descend, and has nothing to do with anything above it. It _presses against_ rather, like a spring, which, having been compressed, tries to resume its natural position with a force equal to that which holds it back. Ask some one to show you the spring of a watch, and you will understand this better.

Each atom of air is a spring of matchless elasticity, which nothing can break, which never wears out, which one can always compress, if one employs force sufficient, and which is always ready to expand indefinitely, in proportion as the compressing power is withdrawn.

Now, consider the column of air outside the door, where there is a pile of such springs forty miles high. The lower ones have to bear up all their comrades, which press upon them with their united weight, and these make desperate efforts to repulse the tremendous pressure, and to spread out in their turn. They endeavor to escape in every direction--to the right, to the left, above, below; but caught between the earth, which will not give way, and the compact ma.s.s of all the columns of air which surrounds the earth in every direction, and of which the lower part is equally compressed everywhere, they struggle unceasingly, but in vain; indefatigable, but powerless. You live in the midst of those little wrestlers, and naturally bear the punishment of the injury done to them. They press against you as against every thing else--before, behind, on all sides--with a force equal to thatwith which they are themselves compressed, or I would say, equal to the weight by which they are so horribly squeezed and contracted: so that, in fact, you bear this weight not only on your head and shoulders, as you might at first suppose, but also all along your body and limbs, under your arms, under your chin, in the hollow of your nostrils, everywhere.

Now we will suppose you to enter the house; and what do you find there?

Outer air, which on its part has got in by the door, the window, and every little crevice in the wall. The column outside the roof no longer presses upon it, but what is the gain of that?

It was compressed when it got in, and the little springs will struggle as a matter of course, quite as much on this side of the door as on the other. The protecting roof has so little power that were it not itself protected by the air outside, the pressure of which keeps it in its place, the air within would shiver it into a thousand fragments in its efforts to get loose.

You laugh; but wait till I explain myself further. I will take the case of a miniature house to make the matter pleasanter to you; one fifteen feet long, fifteen feet wide, and with a flat roof, the most economical plan as regards s.p.a.ce. Fifteen feet are five yards, and as the multiplication table tells us that five times five make twenty-five, our roof will in this case be twenty-five square yards (_i. e._ 225 square feet) in superficial extent, or _area_; it is not much, and you will find few as small.

Would you like to calculate the force with which the millions and thousand millions of little spring imps imprisoned under that poor unfortunate roof would press against it? We settled before that the quant.i.ty of them brought to bear upon a square inch had the power to push at the rate of fifteen pounds. Were they to push against a square yard (a surface 1296 times greater than the square inch) it would therefore be 19,440 lbs. This being so for one square yard, calculate for twenty-five square yards, and you will have the amount of pressure against our roof--viz. 486,000 lbs--merely that! And now tell me what cottage roof in the world was ever built so as to be able to stand against such a weight?

Perhaps though, you can scarcely appreciate the amount of heaviness, 486,000 lbs. Well, 486,000 lbs. is nearly 217 tons; and one of those railway trucks that you see laden with coals at the stations can carry, perhaps, from eight to ten tons, without breaking down. Say ten tons as an outside estimate, and then think of piling the contents of twenty-one such trucks on your roof, and yet you would still be short of the weight of air which is bearing down upon it. I need scarcely say now that were you to take away the air from within the roof, theair without would smash both it and the whole cottage flat, as a giant at a fair strikes an egg flat with one blow of his fist. To show you how in another way: take a moderate sized column or pillar, such as you see sometimes in a n.o.bleman's grounds, of about the weight of the twenty-one tons, and set it up like a chimney on the roof of our cottage, then walk away to a little distance and watch what will happen!