The History of a Mouthful of Bread

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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.

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

In the same way, a small amount of air is by no means heavy; but you
can conceive that a great 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!

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