You must have seen, just now, that the pockets of our dear steward would be rapidly overloaded, were he to keep constantly filling them with the old worn-out materials which the builders rejected, unless he had some means of emptying them as he went along. Accordingly, a wise Providence has furnished the body, on all sides, with clusters of small chambers or cells, in which the blood deposits, as he goes by, all the refuse he has picked up, and which makes its exit from the body sometimes in one way, sometimes in another. Now, the cells of the liver are among these refuse-chambers. One may even consider them as some of the most important ones. When the blood has run its course through the lower compartment, I mean the abdomen, it collects from all directions and rushes into a large canal called the portal vein, which conveys it to the liver. As soon as this canal has entered the liver, it divides and subdivides itself in every direction, like the limbs and branches of a tree diverging from the trunk; and very soon the blood finds itself disseminating through an infinity of small canals or pipes, whose ultimate extremities, a thousand times finer than the finest hairs of your head, communicate with the tiny cells of the liver. There, each of the imperceptible little drops, thus carried into these imperceptibly minute cell-chambers, rids itself—but no one knows how—of a part of the sweepings it has carried along with it. Which done, the little drops thread their way back through other canals as fine as the first, and which go on uniting more and more to each other, like the branches of a tree on their way to the trunk—forming at last one large canal, through which the blood escapes from the liver, once more relieved from its weight of rubbish, and ready to recommence its work.
You are going to ask me, "What is all this to me—this history of the blood and its sweepings? It was the bile you undertook to tell me about, that liquid you spoke of as so necessary for the transformation of the food: we were to get out of the intestinal tubes by the help of the bile, you promised me."
Well, my little impatient minx, it is the history of the bile that I have been relating to you, and what is most remarkable about it is this. You have perhaps heard of those wholesale ragpickers, who makelarge fortunes by collecting out of the mud and dirt of the streets, the many valuable things which have been dropped there? Well, the liver is the master-ragpicker of the body. He fabricates, out of the refuse of the blood, that bile which is so valuable in the economy of the human frame. This bile is neither more nor less than the deposit left by the little drops of blood in the innumerable minute liver-cells. See what an ingenious arrangement, and in what a simple way two objects are effected by one operation!
Now you have learnt the genealogy of the bile, and the double office of the liver, which benefits the blood by what it takes from it, benefits the chyme by what it gives it, and is an economist at the same time—since it only gives back what it has received. This was what I particularly wished to explain to you: the rest you will easily learn.
The bile does not make a long stay in the little cells, it also escapes, by canals similar to those which carry off the blood, after itspurification; and which in a similar way unite by degrees together, until at length they terminate in a single canal, communicating with a little bag placed close against the liver, where the bile accumulates between the periods of digestion—so forming a stock on hand, ready to pour at once into the duodenum when the latter calls for its assistance. The next time the cook cleans out a fowl, ask her to show you the little greenish bladder which she calls the gall and which she takes such care not to burst, because it contains a bitter liquid which, if spilt upon it, would quite ruin the flavor of the fowl. Such, precisely, is the bag which holds the bile. Moreover, it is close by the liver of the fowl that you will find it placed: and you can convince yourself in a moment by it, that the little provision I tell you of is always stored away therein.
We have also within us a multitude of minute electric telegraphs, which transmit intelligence of all that occurs from one part of the body to another, in a more wonderful manner even than the telegraphs of man's making; later we shall see how they work. By their means the little bag by the liver is made aware in the twinkling of an eye of the entrance of the chyme into the duodenum, and forthwith the bile returns for some distance by the canal which brought it, and then branches off into a larger one which opens into the duodenum.
The liver, on getting this intelligence, sets to work more diligently than ever, and the bile flows in streams into the duodenum, where it mixes as it arrives with the current which comes from the pancreas. Thus combined, the two liquids flow over the chyme, which they saturate on all sides; and here, as I have said, the work of the intestinal canal ends. What is serviceable for the blood is separated from the useless refuse, and nothing remains but to get it out of the intestines. It is true that in their character of tubes these are closed on all sides. But do not trouble yourself: a means of escape is prepared.
Before we part, however, I must apologize for something. I have not described to you what the bile consists of, or what kind of refuse the blood leaves in the liver; nevertheless, as you take an interest in this much-neglected book of nature, you ought to know these things.
It is, however, very difficult to lead you by the hand through so many wonder*, where the secrets of nature are all in operation at once, and to explain each as soon as we meet with it. They combine, and progress together like the waves of the sea, where one breath suffices to agitate the whole mass.
When we have talked about the lungs, we will have another word to say about the liver.
LETTER XII. THE CHYLE.
To-day we have to begin by making acquaintance with a new term. I would willingly have spared you this, if I could, for the word is neither a pretty, nor a well-chosen one, but we cannot get on without it.
You are aware now that the learned, unknown sponsors, who gave names to the different parts of the body, bestowed the odd-enough one of chyme on that pasty substance which passes out of the stomach when the cooking is over. We have said quite enough about it, and you know enough of it I am sure. Well! the people seem to have had quite a fancy for the word chyme, for they adopted it again, with only a very slight alteration, when they wanted to specify separately the quintessence of the chyme (the useful part that is), which has to unite with the blood, and which we have been speaking of as the gold of the aliments—this then they called chyle. I give you the name as I received it, but have no responsibility in the matter.
In concluding the last chapter I said we were sure to find there was a plan for extracting the best part of the chyme, viz. the chyle, from the intestinal canal; and a very simple one it is. A complete regiment of those little scavengers lately described, are drawn up in battle-array along the whole length of the small intestine, but especially round about the duodenum. There, a thousand minute pipes pierce in all directions through the coat of the intestine, and suck, like so many constantly open mouths, the drops of chyle as fast as they are formed. They are called chyliferous vessels or chyle-bearers, just as we might call hot-air stoves caloriferous or heat-bearers—from the Latin word fero, which means to carry or bear. I mentioned before that there were, within the intestine, certain elastic valves which obstruct the progress of the chyme, and oblige it to be constantly stopping. There are in fact so many of these, and the skin which lines the intestinal canal is so folded and plaited, that if it were stretched out at full length on a big table, it would cover at least as large a surface as that other skin, with which you are so well acquainted, which entirely clothes the body outside.
Now, the chyliferous vessels we have been speaking of insinuate themselves into all the plaits and folds alluded to, and thus they reach at last the very centre of the chymous paste, and not a single drop of chyle can escape them. They do their work so well, that the separation is effected long before the paste reaches the large intestine; and when that has forced its way through the door which guards the entrance, and which prevents its ever returning again, the chyle is already far off on its mission. It has threaded its way along the little pipes, and, always creeping nearer and nearer, is on the high-road to the heart, where it is anxiously expected.
And what becomes of the rest? There is nothing further to be said about it, but that it shares the fate of everything else which, having answered its purpose in its place, is no longer wanted and must be got rid of. Thus in works where iron-stone smelting is carried on, the refuse that remains after the ore is extracted, though available for road-making or other purposes, is thrown out of the manufactory as a useless incumbrance there.
Our history requires us to follow the fate of that golden aliment the chyle, which is now in a condition to support the life of the body, and every drop of which will turn into blood—the blood which beats at our hearts, nourishes our limbs, and sets at work the fibres of our brain.
I ought to tell you first that the chyle, when it leaves the intestine, is very like milk. It is a white, rather fatty juice, having the appearance, when you look closely at it, of a kind of whey, in which a crowd of globules, or little balls if you prefer it, infinitesimally small, are swimming about. Some people, whose curiosity nothing can check, have put the tips of their tongue to it; so I am able to tell you, if you care for the information, that it has rather a saltish taste.
At this point it is what may be called new-born blood, and to carry on the metaphor, blood whose education has yet to be completed. All the elements of blood are there already, but in confusion and intermingled, so that they cannot yet be recognised. A wonderful fact, and one of which I have no explanation to offer you, because among the many mysteries which are silently going on within us is this, that the education of the new-born blood begins entirely of itself in the vessels which are carrying it along. During their very journey, the confused elements are setting themselves in order and forming into groups. In short the chyle, when it comes out of the chyliferous vessels, is already much more like blood than when it entered them, and yet one cannot account for the change. It is changed, however; its whiteness has already assumed a rosy tinge, and if it is exposed to the air it may be seen turning slightly red, as if to give notice to the observer of what it is about to become.
You know that all our scavengers uniting together deposit their sweepings in one large canal, which is called the thoracic duct. The chyle scavengers arrive there just like the rest, and there our poor friend finds himself confounded for a moment with all the dross of the body, as sometimes happens to men who devote themselves to the public good. But the crisis passes in an instant. A little further off, the thoracic duct pours its whole contents together into a large vein situated close to the heart, and the blood has no difficulty in recognising and appropriating what belongs to him.
Here, my dear little scholar, we conclude the first part of our story. To eat is to nourish oneself; that is, to furnish all parts of the body with the substances necessary to them for the proper performance of their functions. The mouth receives these substances in their crude condition, the intestinal canal prepares them for use, and the blood distributes them.
After the history of the preparation, comes naturally that of the distribution.
The first is called the DIGESTION. It is the history of the chyle, which begins between the thumb and forefinger while as yet invisible, hid in the thousand prisons of our different sorts of food, and ends in the thoracic duct, when, disengaged from all previous bonds, purified and refined by the ordeals of its intestinal life, it leaps into the blood, carrying with it a renewal of life and power.
The second history is that of the CIRCULATION. It is the history of the Blood, that indefatigable traveler, who is constantly circulating or describing a circle (the Latins called it circulus) through the body; by which I mean that it is continually retracing its steps, coming out of the heart to return to it, re-entering it only to leave it again, and so on without intermission, until the hour of death.
The history of the Digestion, which we have just gone through, goes on quietly from one end to the other without any complication.
That of the Circulation, which we are about to begin, is mixed up with another history, from which it cannot be kept separate while the description is going on, although the two histories are in reality quite distinct from each other. The blood describes two circles, to speak correctly: 1st. A wide one, which extends from the extremities of the body to the heart, and back again from the heart to the extremities. 2d. A more contracted one, which goes from the heart to the lungs, and back from the lungs to the heart. Whilst circulating in the lungs, it encounters the air we breathe; and here takes place, between it and the air, one of the most curious transactions imaginable, without which the blood would not be able to nourish the body even for five minutes. This is called RESPIRATION, or the act of breathing.
Digestion, circulation, respiration, the three histories together form but one—that of NUTRITION, or the act of nourishing; in other words, of supporting life. This is what I called eating at first, that I might not mystify you at the beginning with hard words. But now that we are growing learned ourselves, we must accustom ourselves to the terms in use among learned people, especially when they are not more formidable than those I have just taught you.
Our next subject for consideration, then, Will be the circulation; and we will begin with the heart, since that is to the circulation what the stomach is to the digestion—viz., master of the establishment. He is a very important person, this heart, as I hardly need tell you. Even ignorant people speak respectfully of him, and I am sure beforehand that his history will interest you very much.
Do you feel as I do, my dear child? I am quite happy at having brought you thus far on our journey, and at being able to take a rest with you at the gateway of the new country into which we are about to enter, like travelers sitting down upon a boundary frontier. What a distance we have come, since the day when I took you by the hand to conduct you inside this little body, of which you were making use without knowing anything about it! How many things we have learned already, and how many more remain to be learned, of which you have at present no idea! I assure you I should be almost afraid myself of what is before us yet, if I did not rely upon my own strong desire to instruct you, and the tender affection I bear to you. Believe me, the greatest of constraining powers is love; and when I get bewildered in the midst of some difficult explanation which will not come out clearly, I have only to place before me those laughing eyes of yours, where sleeps a soul that must soon awaken to consciousness, in order to make the daylight come into my own!
Must I add, too, that I am not working for you only? We are all placed in this world to help each other, and in striving to bring down light into your intellect, and good sentiments into your heart, I am thinking also of those to whom you, in your turn, may render the same good service hereafter, provided I have the happiness of succeeding now with you. This ought to be so, ought it not? You should resolve to be numbered one day among those who have not lived altogether for themselves, but who have given the world something worth having as they passed through it. To-day's labor will have been well employed if, later on, it turns out that this history of the chyle has not been told you in vain!
LETTER XIII.
THE HEART.
There was once upon a time a banker, a millionaire, who could reckon his wealth not by millions only, but by hundreds of millions and more; who was, in fact, so tremendously rich that he did not know what to do with his money—a difficulty in which nobody had ever been before.
This man took it into his head to build a palace infinitely superior to anything that had hitherto been seen. Marbles, carpets, gildings, silk hangings, pictures, and statues—in fact, the whole mass of common-place luxuries as one sees them even in the grandest royal abodes, fell short of his magnificent pretensions. He was an intelligent man, and thoroughly understood the respect due to his riches; and the common fate of kings seemed to him far too shabby for the entertainment of his dynasty, which he looked upon as very superior to all the families of crowned heads in the world. In consequence he sent to the four quarters of the globe for the most illustrious professors, the most skilful engineers, the cleverest and most ingenious workmen in every department; and giving them unlimited permission as to expenditure? ordered them to adorn his palace with all the wonders of science and human industry.
Science, and human industry, and unlimited means—what will they not accomplish? No wonder that nothing was talked of for a hundred miles around but the magic building—of which, by the way, I do not venture to give you a description, because it would carry me too far away. Let it suffice to say, that never Emperor of China, Caliph of Bagdad, or Great Mogul had such a habitation as our banker, and for a very good reason—he was twenty times as rich as any such gentry as I have named ever were in their lives.
When all was finished one trifling flaw was discovered: the place was not supplied with water. A spring-seeker, who was summoned to the premises, could only discover a small subterranean watercourse, a sort of zigzag pipe, formed by nature, between two beds of clay, in which the rain of the neighborhood collected as in a sort of reservoir. The water was neither very clear nor very plentiful, as you may imagine; and the professor appointed to examine it, having begun by tasting it, made a horrible face, and declared there was no use in proceeding any further; for it had a stagnant flavor which would not be agreeable to my lord.
To the amazement of every body, my lord jumped for joy when he heard this unpleasant news. It was proposed to him to fetch water from a river which flowed a few miles' distance off; but he would hear of nothing of the sort. What he wanted was something new, unexpected, impossible—that was his object throughout. He took a pen and drew up at a sitting the following programme, which caused our poor professors to open their eyes in dismay:—
1st. We will use the water on the premises.
2ndly. It shall flow night day and in all parts of the palace at once.
3rdly. There shall be plenty of it, and it shall be good.
The professors looked at each other for some time without speaking, and the gravest of them, whose fortunes and characters had been long ago established, suggested that they should simply give my lord and his money the slip, and so teach him to make fools of people another time!
But the youngsters, less easily discouraged, cried out against this with one accord. They declared that the honor of science was at stake, and that they ought to return impudence for impudence, by executing to the letter the impertinent programme! At length, after much discussion and many propositions made against all hope, and thrown aside one after the other as impracticable, a sudden inspiration crossed the brain of an engineer who had not yet spoken; and the following is what he proposed:—
What prevented the water from being sweet and fit to drink, was the want of movement and air. What had to be done, therefore, was to erect a pump, but a pump provided with numberless small pipes, extending to the watercourse in all directions, and so arranged that by means of them it should be able to draw up the water from all the corners and windings where it lay stagnating, and then forcing it forward into a pipe terminating in a rose, like that of a watering-pot, whence it should gush out to fall down in fine rain, into a reservoir in the open air. From thence another action of the pump was to bring it back well aerated, to send it once more into a large pipe with numerous lesser ramifications, which should convey it into every corner of the palace.
Up to this point all seemed practicable, but the hardest part had not yet come. The great difficulty was how to supply this enormous consumption with so slender a runnel of water as the one at their disposal. But our engineer had provided for this by a stroke of genius.
Under each of the taps (always kept open), which were dispersed all over the palace, he would place a small cistern, from the bottom of which should go a pipe communicating with the body of the force-pump which drew up the water from the original watercourse. By which means the water which ran from the taps would be taken up again and go back to feed the reservoir in the open air; whence it would again return to supply the taps; and so on and on, the same water continually keeping the game alive, as people call it. Have you not sometimes seen at a circus or theatre a large army represented by a hundred supernumeraries, who file in close columns before the audience, going out at one side of the stage and coming in at the other, following close at each other's heels indefinitely? By a similar artifice the engineer would change his meagre little runnel into an inexhaustible fountain. The water drawn up from the watercourse by each stroke of the pump would fully compensate for what was used in its passage through the palace by the inhabitants. Lastly, as it might sometimes happen that the said inhabitants washed their hands under the taps, the water on its return to the cisterns, was to pass through a series of small filters, in order to cleanse it from any impurity it might have contracted by the way. Always flowing, always limpid, it would soon lose every trace of its original source, and might defy comparison with the water of any river in the world!
A unanimous buzz of congratulations welcomed this plan, at once so simple and so bold, and our professors thought their troubles were over, but they were not at the end of their difficulties yet. When it came to the actual erection of the machine, (naturally a most complicated one, as it had to set a-going a quintuple system of pipes—pipes from the water-course to the pump, pipes from the pump to the reservoir, pipes from the reservoir to the pump, from the pump to the taps, and from the taps to the pump again,)—our banker, who had got amused and excited as they went on, conducted them to a small dark closet, only a few square feet in size, concealed in a corner of the large apartments, and informed them with a laugh that he had no other place to offer them. Besides which, he made them understand that on account of its situation, there could be no question of furnaces or boilers being set up there (he detested equally coal-smoke, fires, and explosions)—nor of workmen employed about the machine (it would not be decent to have them going up and down the front staircase)—nor above all, of the frightful brake-wheels always screeching and grinding, the unwieldy pistons rising and falling with a noise sufficient to give one the headache. He himself slept near the little dark closet, and the slightest noise was fatal to his repose. Having explained all this, the rich man curtly made his bow and retired.
For once our professors owned themselves beaten. They had come forward quite proud of their invention, and now they were received, not with ecstasies of delight, but with fresh demands, more ridiculous even than the first. They were decidedly being mystified, and were preparing in consequence to pack up and begone, furious, and swearing by all their gods that they would never again expose science to see itself disgraced by a purse-proud vulgarian's scorn; when, lo! happily, a good fairy, the special friend of learned men, came passing by that way. She raised her enchanted wand with the tip of her finger, and all at once a little girl dressed in rags appeared in the midst of our astonished professors. Without giving them time to recover themselves, the child put her hand into the little patched waist of her dress, and drew forth a rounded object, about the size of her closed fist from which hung a quantity of tubes spreading in all directions.
"See!" cried she; "here is the machine your banker demands of you."
Picture to yourself a small closed bag, narrowing to a point at the end, and separated within into two very distinct compartments by a fleshy partition which went across the inside from the top to the bottom. Such was the object held up by the little girl. Prom each of these compartments issued a thick tube, ramifying into endless smaller ones; and they were moreover each surmounted by a sort of pouch, into which ran another tube, of the same description as the first. Each of these four portions (the two compartments and their pouches) was in constant but independent motion, distending and contracting alternately; and by carefully examining the noiseless play of this singular machine, (the walls of which were, by the magic power of the fairy, rendered transparent to the bystanders,) the learned assembly were very soon enabled to convince themselves, that it fulfilled all the monstrousconditions exacted of them by the fantastic millionaire.
All was in movement together, I told you; but let us begin at one end. The right-hand compartment and its pouch represented the first pump; the pump employed to draw, by the same stroke, the water from the stagnant channel, and that from the taps. It was perfectly easy to distinguish the two systems of pipes, and how they united together at the small pouch on their arrival. When this was distended, a vacuum was created inside, which was instantly filled by the liquid from the tube which ran into it, (do not ask me why or how; I will explain that presently). When it contracted again, the liquid which had just entered was not able to get back, being prevented from so doing by a very ingenious and simple contrivance, which requires a brief explanation.
Take off the lock from your chamber-door, which opens inside; then, standing outside, push against it with your shoulder, and you will get in without any difficulty. But when you are in, try to push the door open again with your shoulder in order to get outside into the passage, and you will find that you will not be able to pass through, and this simply because it does not open on that side.
Which was exactly what happened to the liquid in the pouch!
The door between the tube and the pouch only opened inwardly, and the liquid finding itself pressed on all sides in proportion as the pouch contracted more and more, and unable to return, was obliged at last to make its way through another similar door which led to the large compartment below. Here the same game recommenced. The compartment which had distended itself to receive it, contracted in its turn, and the liquid finding the road again barred behind it, had no choice but to force its way through the tube which led to the air-reservoir.
Here commenced the work of the second pump,—the pump of the left compartment. The little pouch, when distended, was filled by the liquid from the reservoir, and then forced it forward into the large compartment below, always by means of the same process. This compartment again drove it, by a powerful contraction, into the large conducting tube charged with the office of its general distribution throughout the body. At the end of all which, it returned once more into the right-hand pump as before, to pursue the same course again, &c., &c.
Thus, as you see, the whole mechanism turned upon two little points of detail, of the simplest description possible; namely, first, on the entrance-doors only opening on one side; and secondly, on the elastic covers of the pouches and compartments distending and contracting spontaneously. It was the prettiest thing in the world to see this unpretending-looking little bag working thus, quite naturally, without a suspicion that it was solving a problem which so many men, proud of their science, had given up as hopeless. Certainly here was a machine which made no noise! Once installed in its dark closet, it would have been necessary to place your hand upon it to find out that it moved at all. My lord could certainly sleep beside it without disturbance.
"How much do you want for it?" said they to the poor little beggar girl. "Name your price; have no fear; we will pay you anything you wish."
"I cannot give it to you," replied the child; "I need it too much myself: IT IS MY HEART. Now that you have seen it, make another like it, if you can." And she disappeared.
It is said that the engineer, who longed to see his idea carried out, tried hard to construct a similar machine with gutta-percha and iron wires, and to set it in motion by electricity. But history does not tell us that he succeeded, and we have yet to ask ourselves whether the richest man in the world, aided by the wisest men in the world, could ever provide himself with a miracle of wonder, such as the, ragged child had received as a free gift from the hands of a gracious Creator.
LETTER XIV.
THE ARTERIES.
If you have thoroughly understood the story I last told you, my child, it will have revealed to you the whole mystery of the circulation of the blood, and you are at the present moment wiser than all the learned men of antiquity and the middle ages, for they had none of them the faintest surmise of the truth.
It may, perhaps, seem odd to you that men should have existed for upwards of five thousand years without making inquiry into a matter which so closely concerned them, and which was so easy to find out. Is it not almost incredible that so many hearts should have beaten for so long a period without any of their owners having felt a wish to know exactly why? Yet so it is. The action of the heart and the flow of the blood have not been understood for much more than two hundred years, and the man whose name is attached to this great discovery richly deserves that we should say a few words about him.
He was called Harvey. He was an Englishman; physician to King Charles I., who was beheaded in 1648; and when he first ventured publicly to teach that the blood was constantly circulating from one end of the human body to the other, perpetually returning and retracing its steps, a great scandal was created in the world. He was called a fool,—an impertinent innovator,—a madman. His words shattered old doctrines, and he only received for his reward all the petty annoyances which men are apt to lavish so freely upon any one who tells them something new; because—do you see?—it is so disagreeable to be disturbed in one's habits and preconceived ideas.
Harvey is not the only one in the history of mankind who has committed the sin of being right in defiance of the opinions of his age. It is true posterity takes account afterwards of the labors of genius, and inscribes a fresh name upon her list. But one must pay for this glory in one's lifetime. One cannot have everything at once.
This is an old story, my child, but always new nevertheless; and for my own part it is, I own, one of my pleasures to amuse myself by reflecting how much cause for laughter three-fourths of the great men of the present day are providing for the little girls who shall be alive two centuries hence. Time is a great avenger, and puts many things and men in their proper places.
Let us pause here a moment while we are speaking of Harvey. I should be curious to know what any one of the courtiers of Charles I., bedecked in feathers, ribbons and laces, would have said to the valet who would have placed the excellent Harvey, with his insane invention, above his most gracious majesty, the lord and king of all Great Britain! And yet what is his most gracious majesty to you to-day? What do you owe to him? in what does he interest you? While you can never hear the name of Harvey pronounced without remembering that you are under many obligations to him! A thousand years hence, when society shall have made the great progress which may reasonably be expected, the name of Harvey will be familiar to every one who owns a heart, while that of Charles I. will be only a vanished shadow; a souvenir lost in the maze of history.
Our debt of remembrance paid, let us return to the heart—the little closed bag which labors so prettily. We must now inquire the real names of whatever has figured in our story.
The two great compartments are called ventricles, the two small pouches auricles, and they are also distinguished as being on the right or left side;—right ventricle, left ventricle, right auricle, left auricle.
The inner doors on which depends all the action of the machine, are called valvelets. By-and-bye, when the pump and the steam-engine are explained to you, you will meet again with these treacherous doors, which never allow what has once entered to go back again; but then we shall call them valves.
The air-reservoir, I need scarcely tell you, is the lung, to which the blood goes to put itself in contact with the air.
The subterranean watercourse, of which I hope we have talked long enough, is the small intestine, in which the chyle collects; and the tubes which run into it are, of course, the chyliferous vessels, the only channels by which anything reaches the heart which has not previously gone out from it.
The tubes of distribution, which run out from the machine in all directions, are called with us arteries; the return tubes, which bring back the water to the machine, are called veins.
Finally, we have not exactly the filters employed to clear the water from the impurities contracted as it goes along, for no such thing exists in us. There are in our case the refuse-chambers of which I have already spoken, in connexion with the liver, where the blood disembarrasses itself of any useless materials, and from which it comes out with clean pockets, so to speak, reverting to the comparison of which we have already availed ourselves.
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 vertebræ. 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 _some_thing 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 _in_complete 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.