As you see, then, everything comes round again; and the bright idea which our professors hit upon in order to satisfy the caprice of the banker is exactly carried out in your own body, only a thousand times more perfectly than could have been done by them all, even with all their science added to all his money.

I mentioned that the shrewdest of the party boasted about making an artificial heart. But, let me tell you, there is one thing I would have defied him to imitate, by any expedient he could devise, and that is the inimitable construction of the arteries and veins, and the incomprehensible delicacy of their innumerable ramifications.

Let us talk a little about these marvellous tubes, and begin with the arteries, which have the most important part to play.

Did you ever see a doctor try the pulse of his patient? Take hold of your own wrist and search a little above the thumb. You will soon find the place and feel something beating against your finger. There is an artery which passes there, and the little beating you feel is the rebound of the pulsations, of your heart. Every time that the left ventricle, by contracting itself, chases the blood into the arteries, these, of which the tissue is very elastic, become distended all at once, and then contract again, repeating the process whenever a fresh gush of blood arrives, so that their movement is exactly regulated by the movement of the heart. It is true the two movements are in a contrary direction; that is to say, the artery becomes distended, while the heart contracts, and contracts when the heart enlarges itself; but that makes no difference to the doctor. What he wants to know is, with what force and rapidity the heart of the patient beats, and I will explain why. It is an interesting point in the history of circulation.

When you were very little—very little indeed, my dear child—your heart beat from 130 to 140 times in a minute. Afterwards the beats sank to 100 per minute; then to fewer still. At present I cannot tell you the precise number: perhaps, about ninety. When you are a grown-up young lady, it will beat about eighty times in the minute; when you are a mother, about seventy-three times; when a grandmother (if such a blessing be granted you), only from fifty to sixty times, perhaps even fewer. People tell of an old man of eighty-four whose heart beat only twenty-nine times in the sixty seconds.

Observe that in all my calculations I have taken special care to prefix the word about to the numbers mentioned. And this because, in point of fact, the heart is a capricious creature, which has no exact rules to go by. It changes its pace on every occasion—fear, joy, every emotion which agitates the soul, quickens or retards its movements; and derangements of health may be detected by its pulsations, which are infinitely varied in character. In fever, for instance, which is nothing but a race of the blood at full speed, the hearts of grown-up people beat as quickly as those of little children; sometimes, indeed, more quickly still. In certain maladies it goes with great sudden leaps, like a galloping horse; in others it trots in little jerks; while in some cases it moves slowly and wearily, and its throbs are so weak that one can scarcely feel them.

These pulsations, then, afford important revelations to the doctor. The heart is for him a gossiping confidant, who lets out the secrets of illnesses, however closely they may fancy themselves hidden in the remote depths of the body. When the doctor lays his finger on the patient's pulse, it is precisely the same thing to him as if he had laid it on his heart, only with this difference, that the one is much less difficult to do, and much sooner done than the other.

The artery of the wrist is in fact a small heart, not only because it follows all the movements of the large one, but because it carries forward the work which the other begins, and assists also in propelling the blood to the furthest extremities of the limbs, driving it on in its turn at each of its own contractions. Imagine a fire-engine, whose pipes should take up and drive forwards along their whole length the water which is thrown upon the fire, and you will have some idea of the marvellous machine which is at work in our behalf within us. Nor are you to suppose that the wrist-artery is a specially privileged one, because it has been chosen to hold intercourse with physicians. All the others are equally serviceable; and if they cannot all be used for "feeling the pulse," it is because they are generally more deeply buried in the flesh, where it is not easy to reach them.

Observe your mother when she is packing a trunk, and you will see that whatever she is most afraid maybe spoiled, she is most careful to put in the middle, so that it may be least exposed to accidents. And this is what a kind Providence has done with the arteries, which have the utmost cause to dread accidents; whilst the veins, which are much better able to bear rough usage, are allowed to wander about freely just under the skin. But when the bones happen to take up a great deal of room, and come near the skin themselves, as is the case in the wrist, the artery is forced, whether he likes it or not, to venture to the surface, and then we are able to put our fingers upon him.

And there are others in the same sort of situation; the artery of the foot for instance. But only just think how far from agreeable it would be to have to take off your shoe and present your foot to the doctor!

The artery which passes to the temple, just by the ear, is another affair. That would answer the purpose very well in fact, and I even advise you to make use of it when you want to feel your own pulse. It is more easily found than the other even, and its pulsations are still more easily perceptible. Nevertheless, when all is said and done, it is better for the doctor to take his patient by the hand than by the head. Merely as a matter of good manners.

I will now make you acquainted with the principal arteries, and the manner in which they distribute the blood through the body.

The whole of the blood driven out by the left ventricle at each of its contractions, passes into one large canal called the aorta. The aorta as it goes away at first ascends; then bends back in a curve; and from this curve, which is called the arch of the aorta (from its shape) diverge right and left, certain branch-pipes which carry the blood into the two arms and on each side of the head; and which are, in fact, the beginning, or upper end, of those whose pulsations we feel with our fingers in the two wrists and at the temples.

The supply to the upper part of the body being secured, the aorta begins to descend. But now imagine of what importance it must be, that this head-artery—the foster-father of the whole body—should be sheltered from every accident. The aorta once divided, death is inevitable; you might as well have your head cut off at once; and thus it has been fixed in the best—that is to say, the safest—place. Of course you know what is meant by the backbone or spine, called also the vertebral column, in consequence of its being made like a sort of column composed of a series of small bones fastened together, which are named vertebrae. Touch it and feel how solid it is, and how few dangers there can be for anything placed behind it. Well, that is the rampart which has been given to the Aorta. As this descends, it slips behind the heart and takes up its place in front of the vertebral column which it follows all the way down the back, just to the top of the loins. There it is, so to speak, almost unassailable; in fact hardly any cases are known of the Aorta being wounded; to get at it, it would be necessary to bestow one of those blows which used to be given in the time of the Crusades, which cut the body in two. There was an end of the Aorta, as of every thing else then; it was unfortunately not worth talking about any longer!

The next time you see a fish on the table, ask to be shown the large central bone. It is the fish's vertebral column, and it will give you an idea of your own, for it is constructed on the same plan. You will perceive a blackish thread running all along it—that is the aorta.

As it descends, the aorta sends off on its passage a great number of arteries which carry the blood into all parts of the body. Arrived at the loins it forms a fork; dividing into two great branches, which continue their descent, one on each side the body, down to the very extremities of the two feet.

As you perceive, dear child, this is not very difficult to remember. A large fork, whose two points are at the tips of the feet, the handle of which curves at the top like the crook of a crozier; from this curve come four branches, which pass into the two arms and to the two sides of the head—and this is the whole story. But of course, it would be another affair were I to enter into the detail of all the ramifications. Here it is that all engineers, past, present, and future, are baffled, defeated and outdone! Choose any place you please upon your body, and run the finest needle you can find into it what will issue from the puncture?

"Thanks for the proposal," you say; "I have no occasion to try the experiment, to discover that blood will come out."

You say that very readily, young lady; but have you ever asked yourself, what is implied by your being so sure before hand that you can bring blood from any part of your body if you choose to prick it, though never so slightly? It implies that there is not on your whole frame a spot the size of a needle's point, which has not its own little canal filled with blood; for if there were such a one, there at any rate the needle would pass in without tearing the canal, and causing the blood to flow out. And now count the number of places from the top to the bottom of your dear little self, on which one could put the point of a needle, and even when you have counted them all, do not fancy you have arrived at the number of the tiny tubes of blood. Compared to these, your needle is a coarse stake, and tears not one but a thousand of these little tubes in its passage.

That seems to you rather a strong expression, does it not? But let me make good my boldness. A needle's point is very fine, I admit; but a person who could not see it without spectacles must have very poor sight. Whereas the last subdivisions of the blood-tubes are so attenuated, that the best eyes in the world, your own included, cannot distinguish them. You are astonished at this, and yet it is nothing compared to what follows.

No doubt you have heard of the microscope,—that wonderful instrument by which you may see objects a thousand, a hundred thousand, a million times, if necessary, larger than they really are. With the microscope, therefore, as a matter of course, we can see a good many of those tiny canals which elude our unaided sight. But, alas! we discover at the same time that these are by no means the last subdivisions. The canals invisible to our naked eyes subdivide themselves again into others, and these into others again, and so it goes on, till at last—the man at the microscope can see no more, but the subdivisions still continue.

You were ready to exclaim, at my talking of thousands of canals being torn by a needle in passing through; but had I even said millions, it may be doubted whether I should have spoken the whole truth.

Besides, when you consider the office of the blood, you can easily understand that if there were a single atom of the body left unvisited by him, that atom could never be nourished. Do I say nourished? I have made here a supposition altogether inadmissible; it could have no existence at all, since it is the blood only which produces it.

These imperceptible canals of blood have been called capillaries, from the Latin word, capillus, which means a hair; because the old learned men, who had no suspicion of the wonders hereafter to be revealed by the microscope, could think of no better way of expressing their delicacy, than by comparing them to hairs. Very likely they thought even this a great compliment, but your delicate fair hairs, fine as they are, are absolute cables—and coarse cables too, believe me, compared to the capillary vessels which extend to every portion of your body.

Observe further, that each of these arterial capillaries is necessarily composed (being the continuation of the large ones) of three coats enclosed one within the other, which can be perfectly distinguished in arteries of a tolerable size; add to this that within these coats there is blood, and in the blood some thirty substances we know of, not to speak of those we do not know; and then you will begin to form some notion of the marvels collected together in each poor little morsel of your body, however minute a one you may picture to yourself.



LETTER XV.

THE NOURISHMENT OF THE ORGANS.

When I said formerly that our dear and wonderful steward the blood, was everywhere at once, you little suspected the prodigies involved in that everywhere. But you will have a glimpse of them now, when I tell you it is at the extremities of the capillary arteries that he carries on his distribution of goods, and accomplishes a mysterious act of nutrition; a wonder much greater even than that of which we have just spoken. Here, indeed, the question is no longer mechanical divisions, whose delicacy, surprising as it may be, is yet within our powers of comprehension. What is more surprising still, what moreover we cannot comprehend at all, is the delicate sensitiveness of tact—I would almost say of instinct—with which each one of the million millions of tiny atoms of which our body is composed, draws out of the blood—the common food of all—exactly that aliment which is necessary to it, leaving the rest to his neighbor, and this without ever making a mistake.

You have never thought about this; for children go on living at their ease, as if it was the simplest thing in the world to do; never suspecting even that their life is a continued miracle, and never, of course, therefore, feeling bound to be grateful to the Author of that miracle. And alas! how many hundreds of people live and die children in that respect.

But what would happen, I should like to know, if the eye took to seizing upon the food of the nail, if the hairs stopped on the way what was intended for the muscles, if the tongue absorbed what ought to go to the teeth, and the teeth what ought to go to the tongue! Yet what prevents their doing so? Can you tell me? They all drink alike out of the same cup. The same blood goes to furnish them all. The substances that it brings to the eye are the same as those which it brings to the nail; and nevertheless the eye takes from it that which makes an eye, and the nail that which makes a nail.

How is this done, do you think? that is the question.

When the doctors reply to this, that each organ has its peculiar sensibility, which makes it recognize and imbibe from the blood one particular substance and no other, they are strangely mistaken if they flatter themselves that they have really answered anything. They have done nothing but reproduce the question in other words, for it is precisely that sensibility which requires explanation, and to tell us that it exists, does not explain much, you must own. If you were to ask why you had got a headache, and some one were to reply that it was because your head ached, you would not be much the wiser I fancy.

Each of our organs, then, may be considered as a distinct being, having its separate life, and its particular likings. These organs behave towards the blood like men who recognize some friend in a crowd, and proceed to seize him by the arm; and when I told you just now that they never made a mistake, I spoke of their regular course of action in ordinary circumstances. Like men, they also make mistakes sometimes, in certain cases; and take one substance for another, or do not recognize the one they are in need of; an unanswerable proof that at other times they exercise a sort of discernment, and do not act by a sort of fatality, as one might be tempted to believe. Look at the bones, for instance. They are composed of gelatine (which cooks serve up under the name of meat-jelly, but which would be more properly called bone-jelly), and of phosphate of lime, a kind of stone of which we have spoken before, if I remember rightly, and from which they get all their solidity. Originally, the substance of the bone is entirely gelatinous, and the phosphate of lime deposits itself therein by degrees, as time goes on, and always in greater abundance as we advance in age.

Properly the bones borrow only gelatine and phosphate of lime from the blood. But when they come to be broken, their texture or tissue inflames in the fractured place; and then it changes its tastes, if I may so express myself; and, lo and behold, extracts from the blood that which forms certain little fleshy shoots, which unite together from the two sides of the fracture, and so mend the broken bone. Here is one exception to the rule.

Again, in certain diseases, the bones suddenly quarrel with the phosphate of lime; they will not hear of it any longer, they will not accept a fresh supply; and as the old wears out by degrees, by reason of the continual destruction of which I spoke the other day, the bones become more and more enfeebled, and soon can no longer support the body. A second exception this.

Finally, when old age comes on, the bones end by being so much encumbered with phosphate of lime, that they have no room to admit the fresh supply which keeps coming to them in the blood. What becomes of it then? It goes to seek its fortune elsewhere; and there are charitable souls, who forgetting their instinctive antipathies, consent to give it hospitality, though much to the prejudice of the poor old man himself, who is no longer served so well as formerly, by the incautious servants who have allowed themselves to be thus fatally beguiled; but no one consults him. It is the arteries especially, and sometimes the muscles, which take this great liberty, and it is not unusual among old people to meet with these fairly ossified—that is to say, changed into bone, thanks to the phosphate of lime with which they have consented to burden themselves. This is a third exception, and I will spare you any others.

What may we infer from all this, my dear child? Well, two things. First, that we know nothing at all about the whole affair; a fact which at once places us on a footing with the most learned philosophers in the world. Secondly, that our body is a perpetual miracle; a miracle which eats and drinks and walks, and which we must not look down upon for so doing: for God dwells therein. I should have to come back to this at every turn, if I wanted to fathom everything I have to tell you about. Each tip of hair which you grow, is an incomprehensible prodigy which would puzzle us for ever, if we did not call to our aid those eternal laws which have made us what we are, and to which it is very just our spirits should submit, since we could not exist for one second were they to cease from making themselves obeyed in our bodies.

Reflect on this, my dear little pupil. Young as you may be, you can already understand from it, that there is above you something which demands your respect. The good God, to whom your mother makes you pray every night, on your knees, with folded hands, is not so far off as you might perhaps suppose. He is not a being of the fancy, secluded in the depths of that unknown space which men call Heaven, in order to give it a name. If His all-powerful hand reaches thus into the innermost recesses of your body, His voice speaks also in your heart, and to what it says you must listen.



LETTER XVI.

THE ORGANS.

Contrary to my custom, my dear child, I made use, in the last chapter, of a new word, without giving an explanation of it.

I spoke to you of our organs, and we have not yet ascertained what an organ is.

You probably knew what I meant, because it is a word which is used in conversation and pretty well understood by everybody. But I am bent upon giving you a more exact idea of it, for the trouble will be well bestowed. If I did not do this at once it was because there is a good deal to tell about, and that would have carried me too far away from my subject.

Organ, comes from the Greek word organon, and means instrument. It was used particularly to signify instruments of music, so much so that our word "organ" comes from it. Our bodily organs then, are instruments, or tools if you like it better, which have been given to us, wherewith to perform all the acts of life; and as there is not one part of the body which is not of use to us for some purpose or other, our body is, in point of fact, from head to foot a compound of organs. Thus the hand is the tool which we make use of to lay hold of anything—so an organ; the eye is the instrument of sight—so an organ; the heart is the machine which causes the blood to circulate— so an organ; the liver fabricates the bile—it is an organ therefore; the bones are the framework which support the weight of the body—so organs; the muscles are the power which sets the bones in movement— organs also, therefore; the skin is the armor which protects them—so an organ: in fact everything within us is an organ. If there was any corner of our body which was not an organ, it would be useless to us, and we should not, therefore, have received it, because God makes nothing without a use.

Here lies the secret of that great miracle which is called life. I do not know whether you will be able to understand me thoroughly, but open your ears, as if some one was going to explain addition to you; this is not more difficult.

Life is in reality the total of an addition sum. Each one of our organs is a distinct being which has its particular nature and special office; its separate life consequently; and our individual life is the sum total of all these lesser lives, independent one of the other, but which nevertheless blend together by a mysterious combination, into one common life, which is everywhere and nowhere at the same time. It follows from this, that the more organs a being has, the greater is the sum total; the more, consequently, is life developed in him. Remember this when we begin to study life in the lower animals. In proportion as you find the number of organs diminish, you will find life diminishing in power, until we arrive at beings who have, as it were, only one organ apparent, and whose life is so insignificant, that we have some difficulty in giving an account of it, and are saying the utmost that can be said in calling it life at all.

But this comparison of life to the total of an addition sum, is too dry; and, although it has its appropriate side, yet it might give you a false idea of life; which is what always happens when one tries to solve inscrutable questions and hidden mysteries by a matter-of-fact illustration.

Let us try for something more to the purpose.

I told you that the Greek word organon was applied especially to instruments of music. Well, let us consider our organs as so many musical instruments. You have, probably, sometimes been at a concert. Each of the instruments in the orchestra performs its own part, does it not? The little flute pipes through all its holes; the double-bass pours thunder from its chords: the violin sighs with his; the cymbals clash; the Chinese bells dance to their own tinkling; all go at it in their own fashion, each independently of the other. And yet, when the orchestra is in good tune together, and well played, you hear but one sound; and to you the result of all these various noises, each of which would have no meaning alone, is music composed by some great artist whom you do not see. It is no longer a flute, a double-bass, or a violin which you hoar; it is a symphony of Beethoven's, an oratorio of Haydn's, or Mozart's overture to Don Juan.

Life is just like this. All the instruments are playing together, and there is but one music; music written by God.

But wait! when I say life is just like this, let us come to an understanding. Life is something like it, that is all, for as to telling you what life is, I shall not attempt it. I know nothing about it, do you see, though that is a painful confession to have to make to a pupil; but in this case it does not distress me, and you are welcome to hunt the world through for a master, who in this matter does know anything. I could make a hundred other comparisons, but theywould all fail in some point or other. Shall I tell you where this one fails? In an orchestra there is always a musician by the side of the instrument. Now with us we see the instrument well enough, but we cannot see the musician.

You are inclined to ask me, perhaps, why I am wasting so much paper to-day in talking to you about organs, instead of going on tranquilly with our little history of the circulation. But I told you just now that the secret of life lies in the organs, and before entering upon the history of life, I ought to have begun with them. It is there all the books begin which treat of the subject we are studying together, and if you had one in your hands at this moment, it would teach you that all creatures whatsoever are divided into those which have organs and those which have none—that is, into organic and inorganic beings [Footnote: A lump of iron is the same throughout. Each of its parts has the same properties and the same uses. It has no organs, it is an inorganic being. A rose tree has flowers, which are differently made from its leaves, and serve a different use: a root which sucks up the precious food of the earth; a bark which is of a different nature from the wood, and serves a different purpose. It has organs; it is an organic being: all animals and vegetables are organic beings.] (in stands here for not, as incomplete means not complete).

This is, in fact, the starting point for the study of nature, and there are many other things besides which I ought to have told you before I began. But we went straight ahead, without looking at what we were leaving behind, satisfied with turning aside from time to time to pay our debts.

And while I am making my confession, I ought to tell you all. You would probably only have listened to me with half an ear, if I had begun at the beginning. There is a proverb which says—"The appetite comes with eating." I do not advise you to follow this proverb too closely at dinner, for it might mislead you sadly. But it is always true when applied to learning; it is what one knows already that gives one a taste for learning more. If I have been making you bite at the organs to-day, which is rather a tough morsel, it was because I fancied that your appetite had begun to come. Was I wrong?

Let us now return to the blood which nourishes the organs.



LETTER XVII.

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 canal which as it lengthens out turns from an artery into a vein, without any interruption; the substances destined for the nourishment of the organs passing through its walls, as moisture passes through our skin when we perspire.

But if nobody 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 assure 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 God 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 principal, 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 pass 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 pass on, but it finds them fast closed if it wants to go back; so that as soon as it has forced one passage 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 prick 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 appetite 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 something, 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 quantity 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 mass 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 mass of paper piled up as high as that. You may safely grant then, that this mass 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 mass 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 space. 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 quantity 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 nobleman'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!

There, little Miss Laugher! have you at last learned to value the weight of the air, or atmospheric pressure as it is more properly called; since it is the force with which the atmosphere presses against rather than weighs upon everything on the surface of the globe? It is no joke, as you perceive, and it affords plenty of subject forreflection. I have still to prove to you that I have not been making fun of you with my calculations, and that the weight of air upon a square inch is really what I have said—viz., fifteen pounds.

Now, there is a very simple way by which we might get to know your strength, and tell its amount in figures, if one chose; namely, by putting a weight on your arms—a heap of books, if you please—and keep adding and adding to it, until those poor little arms were unable to bear any more. Then weighing what they had borne, whether we should find it to be ten or thirty pounds—I cannot guess how much it might be at this distance—one might safely say, without fear of mistake, "The strength of this young lady is equal to ten, twenty, or thirty pounds"—in other words, "she represents a weight of ten, twenty, or thirty pounds" and by a similar plan people have ascertained the strength of the air—that is, the weight which it represents. They have weighed what it is capable of carrying.

I told you lately that the whole surface of the earth was covered by an immense army of little imps—otherwise called little air-springs, which, compressed by the giant mass of their comrades above, all of whom they have to carry on their backs, are always trying to protect themselves, by pushing back everything which comes across them. Imagine the bottom of a well. Our imps are permanently installed there as a matter of course, and face to face with the water they push against it, each one doing his best, on all points at once. As the pressure is equal everywhere therefore, and always the same, there are no signs of it to be seen.

Now insert in the water the end of a tube closed below by a cork which exactly fits the interior, but which can be moved up and down in the tube by means of a bar of iron or wood which runs through it. This is called a piston, I may as well tell you as we go on.

When the piston rises in the tube, it drives before it, as it goes, the air which was already there; and which cannot slip away down the sides because the piston fits so closely to them all the way along. The result of this is, that just underneath the piston there is a place in the water to which the air cannot reach, and at that place the water has no pressure upon it at all.

Now see what happens. Pressed upon heavily by the air in every other part and place, like a mouse hunted by a cat, who finds at last a hole through which to escape, the poor water darts at this and ascends the tube close after the piston.

So far so good; but if the tube is very long, and the piston rises rather high;—at thirty-three or thirty-four feet above the level of the water it has to continue its ascent alone. The water parts company, stopping quietly behind, half-way up the tube.

"What is the meaning of this?" you will ask.

It means that the force which presses on the well-water all round the tube, and thus drives it up, has done all it can, and that our little air-imps refuse to supply any more. The water which rises in the tube has a weight of its own of course, and with this weight it presses, as it is fair it should, on the water below. In proportion as the piston rises, the column of water which follows it gets bigger and bigger, and naturally its weight increases at the same time. At last there comes a moment when this weight becomes such that its pressure on the water below is equal to that with which the air-imps are pressing on the water in the well. Thenceforth they may push as they please; no more water will go up. They are in the same position now that they were before, when their comrades (afterwards driven out by the piston) were pressing upon the same point, which had only a moment's freedom; and this water column of thirty-three or thirty-four feet holds them in check, to exactly the same extent as the gay fellows whose place it has taken.

Nothing is easier now than to calculate, even to a few grains almost, the force of the pressure of air. One can get at the weight of water, thank goodness! and it has been ascertained that our water-column will weigh fifteen pounds if the tube is a square inch in size. You will comprehend after this that it might be any size you may please to imagine, without there being the slightest alteration in the height of the column. The larger it is, the heavier will be the column of water on the one hand; but on the other, the greater will be the number of air-imps turned out; so it comes to the same thing in the end.

If you should feel any doubt about the correctness of this reasoning, you have only to try the experiment over again, in a well, filled with mercury for instance. Ask to be shown some pure mercury, which is also called Quicksilver, because one wants to express melted silver, apt to be constantly on the move; it is often to be met with in houses. Mercury weighs thirteen and a half times more than water: according to our calculations, therefore, it would take thirteen and a half times less of it than of water to bring our little air-imps to reason. And this is just what you will find happens; you will see the column of mercury stop short exactly at the moment when it has attained the orthodox weight of fifteen pounds; that is to say, at a height of twenty-eight inches.

On the other hand, take some ether. You know that delicate spirit, which smells so strong, which makes your hand feel cold if it is put upon it, and which we give to sick people to inhale. Ether weighs one-quarter less than water. In a well of ether you would therefore see something quite different, and your column would rise without being asked, to something like forty-three feet, exactly up to the point of weighing—like the others—fifteen pounds to every square inch. Air will not be replaced with less.

That, then, is the measure of its strength, or our scales are deceitful.



LETTER XIX.

THE ACTION OF THE LUNGS.

I hope I have told you enough, my dear child, to enable you fully to estimate the force with which air presses upon everything on the surface of the earth, and consequently upon our own bodies among the rest.

If you understand this, nothing is easier than to understand how air comes and goes in our lungs.

When the cook wants to light her fire with two or three hot coals, what does she do?

She takes the bellows and blows it, does she not?

But if she has no bellows at hand, what does she do? You answer at once, she blows it herself with all the strength of her lungs.

By which it would seem—does it not?—that we are a sort of living bellows, being able, in case of necessity, to act as a substitute for the wood and leather ones of common use. And if we really possess the power of doing the work of a bellows, may not this be because we have within us some little machine of the nature of a bellows?

Exactly; and this fact gives me the opportunity of making you understand the action of the lungs by explaining that of the bellows, which is in everybody's hands, but which three-fourths of the people use, without troubling themselves to inquire how it is made or acts.

"A bellows, as you know, is composed of two pieces of board, capable of being separated and brought together again at will, and united by a piece of leather so shaped and arranged that it doubles up when the boards close, the intermediate space forming a firmly-closed box, the size of which increases or diminishes at every movement of the boards.

"We take the bellows down to use it, and there are the boards, lying flat upon each other, the box between them quite small. Is there anything inside, do you think?

"Nothing," you answer; "the bellows is empty."

Do you think so really, my child? Do you think a tumbler is empty, then, when you have drunk out its contents; and that jelly pots are empty when all the jelly is eaten? There are not so many empty things in the world, I assure you, as you suppose. You forget the air—that monster who is always wanting to stretch himself out, and pushes against everything he meets. He is an unceremonious gentleman, who takes possession of every vacant place; as fast as you put a spoonful on your plate, he takes up the room of the jelly which has been removed, and at each mouthful you swallow, he slips into the place of the water which goes away. When you think the glass and pot are empty, they are, in reality, full of air. You cannot see it; but it is there, you may rely upon it.

There is air, then, in the bellows-box, because there is air in every place where there is nothing else to dispute possession with it. The quantity is small in this case, no doubt, because the box is small and cannot hold much.

But now, look! I separate the boards, and the box, which was small, becomes large. For once, then, here is a box which must be partially empty; for it has just, as if by magic, made a space in itself in which positively there cannot be anything, since there was nothing there beforehand.

Ay! but look down at the centre of the upper board. You see a little hole there, do you not, and below the little hole a small piece of leather, which seems to close it up? That is a valve, one of those doors, such as we noticed before in the heart, and such as are to be found, moreover, in most houses, which let people through on one side but not on the other. This one opens when it is pushed from without, but lets nothing out which has once got in. Now, the air outside, as I said before, is always pushing against everything. He pushes as a matter of course, therefore, against the valve, and as there is nothing behind it to resist the pressure, in proportion as room is made inside the box, he enters and fills it with himself.

But presently some one begins to close the bellows, and he finds himself caught between the boards; on which these invite him to begone, with the same sort of politeness displayed by the police, when the hour of departure comes in a place of public exhibition; when, i.e., they spread out on all sides, and force the crowd before them till they have found the road to the door. But the air cannot get back by the way it came in, the door being shut. As, however, it must go out somewhere, whether it likes it or not, it passes through the tube at the end of the box (the nozzle of the bellows), and comes out thence with a rush upon the fire. When it is once gone the bellows can be distended again, and the process be repeated as before indefinitely.

And this is just what goes on inside ourselves. Your chest, my child, is a box which expands and contracts alternately; making a place for the air by the first effort, and then driving it out by the second. It is neither more nor less than a bellows, but of a simpler construction than that used by the cook. The exit pipe serves also for a door of entrance, and there is but one board instead of two.

The exit pipe is the larynx, of which we spoke before, when we were talking of swallowing the wrong way, and which communicates with the air outside, through the nose and mouth at the same time, allowing us to breathe through either one or the other as we like.

As to the board, I said a few words about it when I was describing the liver. It is the diaphragm—that separating partition—that floor which is placed between the two stories or divisions of the body—the belly and the chest.

But here especially the infinite superiority of the works of God over the miserable inventions of man comes out in all its grandeur.

A bellows which was to have the honor of keeping up within us that miraculous fire—the pre-eminently sacred fire—which we call Life, required something more than a common board for its foundation. And accordingly this, of which I am now going to give you a detailed history, is as marvellous as it is admirable. I fancy that when you have read my account, you will no longer turn up your nose at the vile word diaphragm.

Let us first take a peep at the construction of the bellows.

On each side of the vertebral column, from the neck to the loins, spring twelve long bones, one below the other, bent in the form of bows; these are called the ribs. The first seven pairs of ribs rest, and as it were, unite, in front, upon a bone called the sternum, which you can trace with your finger down to the pit of the stomach, at which point the finger sinks in, for there is no more sternum, and the last five ribs on each side no longer unite with those of the opposite one. For which reason they are called false ribs. On the other hand they are joined to each other at the ends by means of a strip or band of a substance sufficiently strong, but at the same time flexible, and somewhat elastic, which is called cartilage or gristle. The next time you see a roasting piece of veal on the table, look well at it, and you will see at the end a white substance which crackles under your teeth; that is gristle.

This forms the framework of our bellows, which you may picture to yourself as a kind of cage, widening towards the bottom and going to a point at the top, for the arches formed by the upper ribs are smaller than the others. The whole terminates in a sort of ring, through which pass, together, the oesophagus and the trachea.

The space between the ribs is occupied by muscles which reach from one to the other, and the whole framework or cage is shut in below by the diaphragm, that marvellous board whose history I have promised to relate.

The diaphragm, as I told you some time ago, is a large muscle, thin and flat, stretched like a cloth between the chest and the abdomen. It is fastened by an infinity of little threads called fibres, to the lower edge of the cage I have just been describing, and it looks at first sight as if it must be incapable of moving, since it is fixed in one invariable manner all round the body.

It moves nevertheless, but not in the same way as the boards of our bellows.

Ask your brother to hold two corners of your pocket-handkerchief; take hold of the other two yourself, and turn the handkerchief so as to face the wind. The four corners remain in their place, do they not? but the middle, inflated by the wind, curves and swells out in front like a ship's sail, which itself is only an immense hand kerchief after all. Then draw the handkerchief tightly towards you, each to your own side, and it will recover itself and become flat again. Loosen it a little and it will curve and swell out again in the middle, and this maneuver you can go through as often as you choose.

Which very maneuver the diaphragm is continually performing, of and by itself.

In its natural position it bulges upwards in the middle, like a cloth swollen out by the wind, and thus occupies a portion of the chest at the expense of the lungs. When air has to be admitted, its fibres tighten and bring it flat again, as you and your brother brought the handkerchief flat just now by tightening it.

The whole space previously occupied by the arch of the diaphragm is thus given up to the lungs, which, being elastic, instantly stretch themselves out to it; while air, running in through the nose and mouth, fills up in proportion the empty place (vacuum) created by the extension of the lungs, exactly as in the case of the bellows.

But soon the fibres of the diaphragm relax. It rises up again into its old position, driving back the lungs as it does so; and the air finding there is now no room for it, goes out by the same way the other came in. I say the other, observe, because the air that goes out is no longer the same as when it came in; and this is the secret of why we breathe; while the up and down movement of the diaphragm is the explanation of how we breathe.

As you perceive, then, the mechanism of these bellows of ours, is of the most simple, and consequently of the most ingenious character, and leaves far behind it anything we have ever imagined.

Are you disappointed? Do you feel inclined to exclaim, "Is this all?" to ask where are the wonders I promised you? to protest that I may talk as I please about the inflating and flattening of a pocket-handkerchief? you can see nothing so marvellous in the matter; nothing worth making your mouth water for.

A little patience, Mademoiselle! Hitherto we have talked only of the machine; but there is a goblin inside it, and our fairy tale is going to begin again.

There are in some families certain old servants who belong to the house, more, it may be said, than their masters, in some ways. They educate the children, and they serve them till death; they live for them alone, and know so well what they have to do, both by day and night, that there is no need to give them any orders. Nay, not only is it unnecessary to give them directions—it is for the most part labor in vain. They are so completely at home in their business, that they will go nobody's way but their own. If you wish them to alter their habits they may obey you for an instant, but it is only to return into the old groove directly after; for they know better than you do what you want.

I was very little when I first read in the story-books of my day, some bitter complaints of the disappearance of this race of old-fashioned servants of the good old times. And you very likely may have seen it said that they are no longer to be met with. Yet there will always be some, depend upon it, in families, who know how to make and to keep them. Good old times or not, they have never been found in any other but these cases.

Still, I have just such a one as I have described—even I who am talking to you—and so has your mamma; and what is more, you have one yourself; and what is more still, everybody else has one. This servant of the good old times, who will never disappear (and this is more than one can promise of any other) is the Diaphragm! When you came into the world, my dear child, and were merely a poor little lump of flesh, without strength, intelligence, or will; incapable of giving any orders whatever to those organs of yours, of whose existence you were not even aware, your diaphragm quietly began his duties, without leave or inquiry from you, and with your first breath your life began. Since which he has always gone on, whether you attended to him or not, and his last effort will be your last sigh.

When you go to sleep, careless of all that is to happen, until you awake again, that servant of yours, indefatigable at his post, labors for you still, and the light breath which half opens your rosy little lips as it passes through them; that light breath which your happy mother watches with such pleasure, is his work. Midnight strikes—one o'clock—two; all around you are buried in sleep—but he is awake still. Were it otherwise—were he to go to sleep when you do, you would never awake again!

This protector of each instant, this faithful guardian of your life, is, nevertheless, subject to you as a servant to his master. Attend to him, and he will obey your orders. You can make him go at a great pace, or slowly, as you choose; or stop him altogether, if the fancy takes you to do so: but this not for long. The servant of the good old times is obstinate in the performance of his duties. He will yield to you in trifles; but do not try to force him over serious matters. I have read somewhere of a desperate young fellow, chained down in a dungeon, who killed himself by holding his breath; but I never quite believed it. Mr. Diaphragm would not allow any one to carry rebellion so far as that.

But we have not finished yet, and you do not yet know how appropriate is the comparison I am making.

Should any misfortune, any grief, any trifling annoyance even, befall his master, a good servant suffers with him, and as much as he does; sometimes even more. Occasionally the master is comforted, while he remains still disturbed.

"And the diaphragm?" you ask.

The diaphragm does precisely the same, my dear child. Yours, especially, shares in all your griefs to such an extent that, truth to say, he is not always quite reasonable. The other day when your mamma did not want to take you into the country with her, he was so sorry for you that he went into perfect convulsions, and you sobbed and sobbed till she was obliged to say, "Come, then, you naughty child;" whereupon you embraced your mamma, and were quite happy again, while he remained still unappeased, and your poor little chest was shaken more than once afterwards by his last convulsions.

Sobbing, you must know, is merely a convulsion—a great shake of the diaphragm—which is the reason of its causing such a heaving of the chest.

It is the same with respect to joy. The joy of the master makes the servant dance, and so the diaphragm too! Its little internal jumps are, then, what we call laughter—a thing you are well acquainted with. Put your hand on your chest next time you laugh (and I hope it will be soon) and you will feel how it dances—thanks to the diaphragm which jumps for joy whenever it finds you in good humor.

Please to observe further, that nothing of all this is done to order. He starts of himself, poor fellow, without waiting to ask if you will ever know anything about it; and, in truth, you have known nothing about it up to the present moment.

What say you to the diaphragm now, my child? Does not the very name please you? You scarcely expected to find there—under your lungs—so good a servant, one so attached to your person, so strongly resembling in all points the best specimens we know among men. And still we have not done. I have reserved as a finale for you a new point of resemblance which will make you open your eyes very wide indeed.

The old servant is sometimes cross and grumbling. If anything is going against his grain in the house he has no scruple in saying so; and his mode of speaking is sometimes rather rude. Nor is it of any use to get impatient and impose silence on him; he will listen to nothing—it is his privilege. But let some unforeseen accident happen to his master, let him see him deeply affected, and in a moment all his anger is over. He sets himself silently to work again, recalled to order twenty times sooner by his master's emotion than by his utmost impatience.

You ask what I am coming to now? My dear child, what I have just told you is the history of the hiccup—the history of the hiccup, neither more nor less.

I must first tell you, however, that the diaphragm keeps up intimate relations with his neighbor below—the stomach. Every time he rises in the breast the stomach rises behind him; and not only the stomach, but also its companions, the intestines. All the officials employed in the business of digestion travel regularly with him; coming down as well as going up in company. Put your hand upon your abdomen and breathe strongly and you will feel the rebound of all the movements of the diaphragm.

Now, when matters are going on wrongly inside, when too much work has been imposed on the officials, or work they dislike, or else when they have been disturbed in their labors, it will sometimes happen that the diaphragm takes part with his comrades in the abdomen. He gets angry then, and shakes his master, who cannot help himself a bit. You must be very well acquainted with these attacks, which are very fatiguing when they last long. One begs pardon and resists him in vain; he does as he pleases, without stopping to listen, turning everything upside down; and do you know the only efficacious plan for calming him at once? It was a constant source of wonder to me when I was little. A sudden fright, a start unexpectedly caused by a friendly hand slipping secretly behind, and laying hold of one, was all-sufficient; disarmed by the agitation you have undergone, the naughty, stubborn muscle forgives you, and you are cured.

Having dwelt so long on the truly wonderful resemblance between the proceedings of two sorts of beings, whom no one that I know of ever thought of comparing together before, I will now, my dear child, give you the key to all these comparisons, which seem so whimsical at first, but are so striking in reality, and which come to my pen of their own accord, as it were, in the midst of the explanations I have undertaken to give you. Many people who would not themselves care for them, will declare that they are too hard for a little girl to follow. But for my own part, I find that the eye can take in a mountain as easily as a fly, and that it is not more difficult to lay hold of great ideas than of little ones. It is short-sighted people, not children, who cannot see far before them. Who made the heavens and the earth? God, your catechism tells you. The same God made both; did he not? We do not acknowledge two. And if it be the self-same God who made everything, the hand of the universal Maker will be found everywhere; and from the highest to the lowest portion of His work the same mind will manifest itself under a thousand different forms. Not only, either, is each man separately, one by one, the work of God. The whole human race, taken in the mass, is also His creation; and the laws by which human society—that great body of the human race—seeks to regulate itself for the preservation of its existence, are undoubtedly the same as those which overruled the organization of our individual bodies. It is not very astonishing, then, if we find, in the life of human society around us, details corresponding with each detail of the life of the human body, or, at any rate, closely resembling them. What would really be astonishing, would be that mankind as a whole should be differently constituted from man as an individual, and that human society should have other appointed conditions of well-being than those of each of its members.

So, while I am on the subject, I should like to advise those who wishto apply themselves to what is called politics—that is to say, social life—to begin their studies of the body social, by studying the body human, first. They will learn more from it than from the newspapers!

But you have nothing to do with all this. For the present, take notice of one thing only; viz., that the hand of the same God has passed over everything, and that there is neither much presumption nor much merit in tracing points of comparison between the different parts of His work. These comparisons are not a mere play of the mind; they really exist ready made in the very foundations of things.

Now let us come down a little from these heights and return to our friends the lungs. I have not spoken about them for some time, and I have not yet told you how they are constructed.

I wish I could show you some, but the cook will do so, if you would like to see them. The lights with which she feeds the cat and the dog are the lungs of some animal.

Take up a piece in your hand, and you will find you have got hold of something light (cooks have not given it its name without a reason), which is also soft, sinks under your finger if you press it, and rises again afterwards like a sponge. In fact, the lung, like the sponge, is composed of an infinity of minute cells, whose elastic sides can be contracted or expanded at will. They are like so many little chambers, into every one of which blood and air keep running hastily, each on its own side, to bid good day to each other, touch hands, and then hurry out as briskly as they came in. Whether the bit of lights the cat is eating, comes from an ox, a pig, or a sheep, you may look at it with perfect confidence; your own lung is precisely like it. You would see nothing different, could you look into your own chest.

So much for the substance of the lungs. As to SHAPE, imagine two large, elongated packets, flat inside, descending right and left, inside the breast, and bearing the heart, suspended between the two, in the middle. The extremity of each packet descends below the heart, and it is in the interval which separates them that the arch of the diaphragm performs its up and down movement.

I have already said that air reaches the lungs through the larynx. The larynx (of which we shall speak further when I have explained another curious thing very valuable to little girls—the voice), the larynx is a tube composed of five pieces of cartilage (you know now what cartilage or gristle is), the firm resisting texture of which keeps it always open. After these five pieces of cartilage, come others, and the tube is continued; but it then takes the name of the trachea; the larynx and trachea constituting the windpipe. At its entrance into the chest, the trachea divides into two branches, which are called bronchial tubes, and which run, one into the right lung, the other into the left. You sometimes hear people talking about bronchitis. It is an inflammation of these bronchial tubes, which are within an inch or two of the lungs. It is necessary, therefore, to be very careful in such circumstances, and do exactly what the doctor prescribes, because— one step further, and the inflammation extends from the bronchial tubes into the lungs themselves, with which it is not safe to play tricks.

Having reached the lungs, the bronchial tubes subdivide into branches, which ramify again in their turn like the boughs of a tree, and the whole ramification terminates in imperceptible little tubes, each of which comes out in one of those little chambers I was talking about just now. And this is the way in which air gets there at all.

The venous blood which leaves the heart, arrives on its side by one large canal, which passes out from the right ventricle, and which is called the pulmonary artery. And, to tell you the truth, while there is no learned man present to be angry with us, it is a very ill-chosen name, because it is venous blood which flows in this so-called artery. But the doctors have decided that all the vessels which run from the heart should be called arteries, and all those which go back to it veins, whatever may be the nature of the blood which they contain. We cannot help it, because they manage all these matters in their own way; but in that case it was scarcely worth their while to talk about arterial and venous blood. It would have been better to have said simply, red blood and black blood.

Be this as it may, venous blood arrives from the right ventricle through the pulmonary artery. This divides itself, like the bronchial tubes, into thousands of little pipes, whose extremities come creeping along the partitions of the little chambers in question.

And here, then, takes place, between the air and the blood, that mysterious intercourse for the account of which I have kept you waiting so long; and at the end of which the black blood becomes red, or, in other words, from venous becomes arterial. I have called it "intercourse," and this is really the proper phrase; for this transformation of the blood is accomplished by means of an exchange. The air gives something to the blood, and the blood gives something to the air—each giving, in exchange, like two people over a bargain in the marketplace.

With your permission, my dear child, we will stop here to-day. We have now got to the charcoal market, and it is a little black.



LETTER XX.

CARBON AND OXYGEN.

Here, then, my dear child, we have arrived at the explanation of that great mystery, WHY we breathe. Keep on the alert, for we are now entering into a region where everything will be new to you.

Here we are at the charcoal market, I said to you just now, and no doubt you concluded that I was beginning another comparison.

But no such thing; there is no question of comparison or simile here; I state the fact itself, pure and simple as it stands: it is a market, for commercial intercourse and exchange are carried on there, as I told you before, and it is a charcoal market, because charcoal is, positively, the essential and chief article of commerce.

You are astonished, I dare say, and are ready to ask me whether I can possibly mean real charcoal, charcoal such as the cook puts into the furnace. Surely, say you, we have nothing like that in our bodies? Surely we don't eat that?

But I answer yes; real, true charcoal, and you do not dislike it; you eat of it even daily; nay, you do not swallow a single mouthful of food which does not contain its proportion of charcoal.

You laugh; but wait a little and listen.

When you are toasting a slice of bread for breakfast, and hold it too near the fire, what happens to it?

It turns quite black, does it not?

When mutton-chops are left too long unturned on the gridiron, what happens to them?

They turn quite black also.

When your brother forgets the apples which he has set to roast, what happens to them?

They turn quite black, as you have seen more than once.

It is always black, then, that these things turn, is it not? and a fine rich charcoaly black, as you may see if you please to observe charcoal closely, for just such is the color of little burnt cakes, over-roasted chestnuts, and potatoes in their skins, which have been dropped into the fire.

But there is a common term by which we can express more accurately the misfortune which has befallen all these various things—slices of bread, mutton-chops, apples, cakes, chestnuts, potatoes, and what-not, when "burnt," "over-toasted," "over-roasted," or "over-baked." We may call them carbonized, or more simply charred or charcoaled; though the word charred is generally used only for burnt wood. But carbon being the principal ingredient of charcoal, and charcoal being one of the purer forms in which we get at carbon, they are almost synonymous terms, and you may call your burnt food carbonized, or charred, or charcoaled, whichever you prefer.

The next question is, how did charcoal or carbon get into the food so as to justify our talking of its being carbonized or charred? Even when we use charcoal stoves for cooking, the charcoal does not jump out and get into the mutton-chops, etc., you may be sure. Then it is clear it must have been in them before they were brought to the fire to be cooked; and such is indeed the case, only its black face escaped notice because it was in such gay-looking company, and kept itself hid behind the others like a needle lost in a match-box. Set fire to the matches, and you will soon have nothing left but the needle, which will then strike your eye at once. And so with our burnt food; the fire has carried off all the other ingredients, and the charcoal is left behind alone, exposed to everybody's view, as if on purpose to teach them that it was always there; in the apples, i.e., the potatoes, mutton-chops, etc., which seemed so tempting when the black rogue was hid, but from which now, when he is there by himself, they turn away in disgust.

Charcoal is, in fact, a much more generally distributed substance than you have been used to suppose, dear child. That which comes from burnt wood is most easily observed, because there is a much larger proportion of charcoal in wood than anywhere else; but there is not a morsel, however small, of any animal or vegetable whatsoever, which does not contain charcoal. In the sugar which you crunch, in the wine which you drink, there is charcoal. I could even find some in the water you wash in if I were to try hard. There is charcoal in the goose-quill which I hold in my hand at this moment, and in the paper on which I am writing, and in the handkerchief on my knee. If I hold them all three in the light of my wax taper, I shall soon see them turn black and betray the presence of our friend. It exists in the wax taper itself, as also in the candle, as also in the oil lamp. If I were to hold a piece of flat glass above their flame, I should collect enough of it to blacken the tip of anybody's nose who presumed to doubt the fact. There is a portion of it in the air; a portion of it in the earth. Where is it not? In short, all the stones of all the buildings in the world are filled with it from top to bottom. Charcoal, under his more scientific and important name of carbon, may be called one of the great lords of the world. His domain is so extensive that one might go round the world without getting out of it; he is even worse than the Marquis of Carabas.

After this you will never, I hope, want to persuade me you do not eatcharcoal; for, indeed, you would be puzzled to escape doing so. Of all the things you see on the dinner-table there is but one in which you will not find it—viz., the salt-cellar; and even while saying this, I mean only, in the salt itself, for as to the salt-cellar, clear and transparent as its glass may be, there is charcoal in it!

Our bodies, therefore, are full of charcoal. Everything that we eat supplies them with enormous quantities of it, which take up their quarters in every corner of our organs. It is one of the principal materials of the vast collection of structures of which I spoke to you in the early part of these letters, and of which the blood, the steward of the body, is the universal master-builder. If you remember, I told you then that these structures fell to pieces of themselves, in proportion as the workmen went on building, and that the blood, which brings fresh materials on its arrival from the lungs and heart, carries away the refuse ones on its return. And, of all these refuse materials, old charcoal is one of those which takes up the most room, as fresh charcoal took up a great deal of room in the new materials. The blood, as he goes back again, has his pockets quite crammed with it, and if he did not try hard to get rid of it as fast as possible, he would be disabled from being of any further use.

Now it is in the lungs that he clears himself of it. He gives it up to the air, which has need of it for a very interesting operation, of which I shall tell you more by and bye; and in return the air gives him something which is quite indispensable to him, for without it he would not dare to return to the organs, as his authority would no longer be recognised.

In the same way, the charcoal-seller goes to market with his charcoal and receives silver in exchange.

If he were to go home without money his wife would receive him with abuse.

But what is the indispensable thing which the blood obtains in his marketing?

Remember its name well: it is OXYGEN.

And we must speak of it with respect, for we are talking here of a very great and powerful personage, very superior even to CARBON. If CARBON be one of the great lords of the world, OXYGEN is its king.

There is a certain substance, my dear child, of which many people, especially little girls, do not even know the name, but which yet constitutes of itself alone a good half of everything we are acquainted with in the world. And this substance is the very thing I have just named to you. It is OXYGEN.

Ascend into the air as high as you can go, viz., to forty miles or so from the ground, as we said before; oxygen forms the fifth part of that vast aerial ocean which surrounds the globe on every side. There it is free—is itself—if I may use the expression; it is in the condition of gas; that is to say, it eludes our sight, though there is no difficulty in ascertaining its presence, when one knows how to set about it.

Go down into the depths of the sea. People think they have good reasons for believing this to be two and a half miles deep on an average, which would give a pretty little sum total of tons for its whole weight, as you will be convinced, if you take the trouble of observing the space it covers on a map of the world;—to say nothing of lakes, rivers, streams, the water in the clouds, the water scattered throughout the interior or on the surface of continents, including that with which you wash your face every morning.

Oxygen enters in the proportion of eight-ninths into the composition of this incalculable mass. Eight-ninths, you understand, which is very near being the whole nine; in every nine pounds of water there are eight pounds of oxygen, the remainder being left for another substance, of which we shall have occasion to speak presently, and which is called hydrogen.

The earth on which you tread is full of oxygen. So far as we have penetrated hitherto into the interior of the globe, we have found king Oxygen everywhere: hidden under a thousand forms, connected with a heap of substances, not one of which could exist without him; imprisoned in a thousand combinations, and always ready to resume his natural condition if his prison-house be destroyed. The whole surface of the earth, plains, hills, mountains, towns, deserts, cultivated fields, everything you would look down upon, if on a clear day you could be carried high enough in a balloon to take in the whole earth at a glance:—all that may be considered as an immense reservoir of oxygen, out of which we should see it escaping in gigantic waves, if some superhuman chemist were to take it into his head to put our poor little globe into a retort of the same kind as chemists use among us. To give you an example; the stones of our fine buildings, in which we have already discovered the presence of carbon, are almost half made up of oxygen. In a stone which weighs 100 lbs. there are 48 lbs. of oxygen, and the first chemist who passes by could make them come out of it if he chose, if he were to use a little trouble and skill.

I enumerated to you last time many of the substances in which carbon is to be found; but as regards oxygen we must give up all attempt at making a list; it would comprehend the whole dictionary. Touch whatever lies under your hand—in your room—in the house—wherever you may go—I will almost defy you to put your finger upon anything—metals excepted—which is not crammed with oxygen. Your very body, to conclude with, would become so small a thing, were the oxygen it contains extracted from it, that you would be perfectly amazed.

So when I told you oxygen was king of the world, I did not say too much, did I? Between ourselves too, it is a great misfortune that people live on so complacently in total ignorance of this all-important material, which is connected with everything, which insinuates itself everywhere, which we make use of every instant of our lives, which may almost be said to be in some sort our very selves, since it constitutes three-fourths of our body, but whose name nevertheless would, I am certain, make many pretty little mouths pout, if one were to utter it in a drawing-room.

This is really the case. Many young ladies who are proud to know who Caractacus was, would be ashamed to know anything about oxygen. There is a foolish notion that women have no business with such subjects, probably because children are supposed not to breathe and mothers are not required to watch over them?

This reminds me that we are on the road to explain respiration, which I had almost forgotten in lifting up this corner of the veil behind which Nature hides her most valuable secrets from the idle and ignorant.

It is oxygen then, which the blood carries off triumphantly from his interview with the air in the cells of the lungs; and, by the way, it is, thanks to this oxygen that it returns from the lungs to the heart, and so from the heart to the organs, with that beautiful rosy tint which distinguishes arterial from venous blood.

Now the blood gives out this oxygen on its road every time it performs the journey, and the perpetual course it performs from the lungs to the organs, and from the organs to the lungs, has for its chief object the perpetual renovation of this previous provision, which is as perpetually consumed.

Do you ask of what use it is? Does the blood leave it at random in our organs, and is it one of the materials with which our steward is constantly providing the little workmen of the body for their various constructions?

No, my dear child. The proverb "One cannot live upon air," is a very true one, although it is equally true that we cannot live without air. Air does not nourish our organs; on the contrary, it consumes them, and what we eat, serves to supply in precisely the same proportion its insatiable appetite. When we leave off eating, from whatever cause, the air does not leave off too. He goes on always just the same, and that is the reason why people who are starved to death are so thin. (The air has consumed the vital parts.)

You did not expect this; but now prepare yourself to go on from one surprise to another. To begin with, I shall have to stop here and explain to you before we go any further—can you guess what? Nay, I am sure you cannot; FIRE.

There is not much connection, you will say, between fire and breathing.

But there you are mistaken. It is precisely the same thing, as I will prove to you next time.



LETTER XXI.

COMBUSTION.

Have you never, my dear child, whilst warming your little feet on the hearth in winter-time, asked yourself, What is fire? that great benefactor of man; fire, without which part of the world would be uninhabitable by us during at least a third of the year; fire, without which we could not bake a morsel of bread, and would have to eat our meat raw; fire, which lights up the night for us, and without which we should have to go to bed when the hens go to roost; fire, which subdues metals, and without which we should have neither iron, nor copper, nor silver, nor anything that is manufactured from those materials; fire, without which, in short, human industry could not rise to much higher results than that of the monkey and of the beaver?

We are all of us, it is true, so much accustomed to fire that we do not pay much attention to it, and have a sort of persuasion that lucifer matches have existed from all eternity. But the first men, who were nearer neighbors to that great discovery whence all others have originated—the first men treated fire with more respect than we do. It was to them one of the mighty things of the world. The ancient Persians made a god of it, and told how Zoroaster, their prophet, went to seek it in heaven, passing thither from the top of the Himalayas, the highest chain of mountains in the known world.

The old Greeks pretended that Prometheus stole it from the gods, to make a present of it to man, which came to nearly the same thing as the Persian account. The Romans had their sacred fire, which the celebrated Vestals were bound to keep lighted, on pain of death to whoever should let it go out. At the present day we do not stand upon such ceremonies, but warm our feet at it quite familiarly, without wishing for anything further. But you would see a terrible revolution in the world if some Prometheus reversed were, some fine morning, to steal it from us, and carry it back to its ancient owners. Every branch of human industry would suddenly stop, as if by enchantment, and in the course of a very few years the poor little framework of human society, of which we are now so proud, would totally change its aspect, and the whole world would be turned topsy-turvy.