firm, and with a weight hanging at the other end. You know that every
time the muscle contracted it would pull on the weight and draw it up.
But suppose, instead of hanging a weight on to the muscle, you wrapped
the muscle round a bladder full of water. What would happen then each
time the muscle contracted? Why, evidently it would squeeze the bladder,
and if there were a hole in the bladder some of the water would be
squeezed out. That is just what takes place in the heart. You have
already learnt that the heart is muscular. Each cavity of the heart,
each auricle, and each ventricle is, so to speak, a thin bag with a
number of muscles wrapped round it. In an ordinary muscle of the body,
the bundles of fibres of which the muscle is made up are placed
carefully and regularly side by side. You can see this very well in a
round of boiled beef, which is little more than a mass of great muscles
running in different directions. You know that if you try to cut a thin
slice right across the round, at one part your carving-knife will go
“with the grain” of the meat, _i.e._ you will cut the fibres lengthways;
at another part it will go “against the grain,” _i.e._ you will cut the
fibres crossways. In both parts, the bundles of fibres will run very
regularly. But in the heart the bundles are interlaced with each other
in a very wonderful fashion, so that it is very difficult to make out
the grain. They are so arranged in order that the muscular fibres may
squeeze all parts of each bag at the same time.
Each cavity of the heart, then, auricle or ventricle, is a thin bag
with a network of muscles wrapped round it, and each time the muscles
contract they squeeze the bag and try to drive out whatever is in it.
There are more muscles in the ventricles than in the auricles, and more
in the left ventricle than in the right, for we have already seen how
much thicker the ventricles are than the auricles, and the left
ventricle than the right; and the thickness is all muscle.
And now comes the wonderful fact. These muscles of the auricles and
ventricles are always at work contracting and relaxing, shortening and
lengthening, of their own accord, as long as the heart is alive. The
biceps in your arm contracts only when you make it contract. If you keep
quiet, your arm keeps quiet and your biceps keeps quiet. But your heart
never keeps quiet. Whether you are awake or whether you are asleep,
whether you are running about or lying down quite still, whatever you
are doing or not doing, as long as you are alive your heart keeps on
steadily at work. Every second, or rather oftener, there comes a short
sharp squeeze from the auricles, from both exactly at the same time, and
just as the auricles have finished their squeeze, there comes a great
hug from the ventricles, from both at the same time, but a much stronger
hug from the left than from the right; and then for a brief space there
is perfect quiet. But before the second has quite passed away, the
auricles have begun again, and after them the ventricles once more, and
thus the contracting and relaxing of the walls of the heart’s cavities,
this beat of the heart as it is called, this short snap of the two
auricles, this longer, steadier pull of the two ventricles, have gone on
in your own body since before you were born, and will go on until the
moment comes when friends gathering round your bedside will say that you
are “gone.”
=34. But how does this beat of the heart make the blood move?= Let us see.
Remember that you have, or when you are grown up will have, bottled up
in the closed blood-vessels of your body about 12 lbs. of blood. You
have seen that the heart and the blood-vessels form a system of closed
tubes; the walls are in some places, in the capillaries for instance,
very thin, but they are sound and whole--and though the road is quite
open from the capillaries through the veins, heart, and arteries to the
capillaries again, there is no way out of the tubes except by making a
breach somewhere in the walls.
This closed system of heart and tubes is pretty well filled by the 12
lbs. of blood.
What then must happen each time the heart contracts?
Let us begin with the right ventricle. Suppose it is full of blood. It
contracts. The blood in it, squeezed on all sides, tries to go back into
the right auricle, but the tricuspid flaps have been driven back and
block the way. The more the blood presses on them, the tighter they
become, and the more completely they shut out all possibility of getting
into the auricle.
The way into the pulmonary artery is open, the blood can go there. But
stay, the artery is already full of blood, and so are the capillaries
and veins in the lung. Yes, but the artery will stretch ever so much.
Take a piece of pulmonary artery, and having tied one end, pump or pour
water into the other; you will see how much it will stretch. Into the
pulmonary artery, then, goes the blood, stretching it in order to find
room. As the ventricle squeezes and squeezes, until its walls meet in
the middle, all the blood that was in it finds its way out into the
artery. But the beat of the ventricle soon ceases, the squeeze is over
and gone, and back tumbles the blood into the ventricle, or would
tumble, only the first few drops that shoot backwards are caught by the
watch-pocket semilunar valves. Back fly these valves with a sharp click
(for the things of which we are speaking happen in a fraction of a
second), and all further return is cut off. The blood has been squeezed
out of the ventricle, and is safely lodged in the pulmonary artery.
But the pulmonary artery is ever so much on the stretch. It was fairly
full before it received this fresh lot of blood; now it is over-full--at
least that part of it which is nearest to the heart is over-full. What
happens next? What happens when you stretch a piece of india-rubber and
then let it go? It returns to its former size. The ventricle has
stretched the piece of pulmonary artery near it, beyond the natural
size, and then (when it ceased to contract) has let it go. Accordingly
the piece of pulmonary artery tries to return to its former size, and
since it cannot send the blood back to the ventricle, squeezes it on to
the next piece of the artery nearer the capillaries, stretching that in
turn.
This again in turn sends it on the next piece--and so on right to the
capillaries. The over-full pulmonary artery, stretched to hold more than
it fairly can, empties itself through the capillaries into the pulmonary
veins until it is not more than comfortably full. But the pulmonary
veins also are already full,--what are they to do? To empty the surplus
into the left auricle. Oftener than every second there will come a time
when they can do so.
For at the same time that the right ventricle pumped a quantity of blood
into the pulmonary artery and safely lodged it there, the left ventricle
pumped a like quantity into the aorta, safely lodged it there, and was
left empty itself. But just at that moment the left auricle began to
contract and to squeeze the blood that was in it.
Where could that blood go? It could not go back into the pulmonary
veins, for they were already full, and the blood in them was being
pressed behind by the over-full pulmonary arteries. But it could pass
easily into the empty ventricle--and in it tumbled, the mitral flaps
readily flying back and opening up a wide way. And so the auricle
emptied itself into the ventricle. But now the auricle ceases to
contract--its walls no longer squeeze--it is empty and wants filling,
and so comes the moment when the pulmonary veins can pour into it the
blood which has been driven into them by the over-full pulmonary artery.
Thus the right ventricle drives the blood into the over-full pulmonary
artery, the pulmonary artery overflows into the pulmonary veins, the
pulmonary veins carry the surplus to the empty left auricle, the left
auricle presses it into the empty left ventricle, the left ventricle
pumps it into the aorta--(the stretching of the aorta and of its
branches is what we call the pulse)--the over-full aorta overflows just
as did the pulmonary artery, through the capillaries of the body into
the great venæ cavæ--through these the blood falls into the empty right
auricle, the right auricle drives it into the empty right ventricle,
and the full right ventricle is the point at which we began.
=Thus the alternate contractions of auricles and ventricles, thanks to
the valves in the heart and in the veins, pump the blood, stroke by
stroke, through the wide system of tubes; and thus in every capillary
all over the body we find blood pressed upon behind by over-full
arteries, with a way open to it in front, thanks to the auricles, which
are, once a second or oftener, empty and ready to take up a fresh supply
from the veins.= Thus it comes to pass that every little fragment of your
body is bathed by blood, which a few moments ago was in your heart, and
a few moments before that was in some other part of your frame. Thus it
is that no part of your body can keep itself to itself; the blood makes
all things common as it flies from spot to spot. The red corpuscle that
a minute ago was in your brain, is now perhaps in your liver, and in
another minute may be in a muscle of your arm or in a bone of your leg:
wherever it goes it has something to bring, and something to fetch. A
restless heart is for ever driving a busy blood, which wherever it goes
buys and sells, making perhaps an occasional bargain as it shoots along
the great arteries and great veins, but busiest of all as it lingers in
the narrow pathways of the capillaries.
=35.= When you look down upon a great city from a high place, as upon
London from St. Paul’s, you see stretching below you a network of
streets, the meshes of which are filled with blocks of houses. You can
watch the crowds of men and carts jostling through the streets, but the
work within the houses is hidden from your view. Yet you know that, busy
as seems the street, the turmoil and press which you see there are but
tokens of the real business which is being carried on in the house.
So is it with any piece of the body upon which you look through the
microscope. You can watch the red blood jostling through the network of
capillary streets. But each mesh bounded by red lines is filled with
living flesh, is a block of tiny houses, built of muscle, or of skin, or
of brain, as the case may be. You cannot _see_ much going on there,
however strong your microscope; yet that is where the chief work goes
on. In the city the raw material is carried through the street to the
factory, and the manufactured article may be brought out again into the
street, but the din of the labour is within the factory gates. In the
body the blood within the capillary is a stream of raw material about to
be made muscle, or bone, or brain, and of stuff which, having been
muscle, or bone, or brain, is no longer of any use, and is on its way to
be cast out. The actual making of muscle, or of bone, or of brain, is
carried on, and the work of each is done, outside the blood, in the
little plots of tissue into which no red corpuscle comes.
The capillaries are closed tubes; they keep the red corpuscles in their
place. But their walls are so thin and delicate that they let the watery
plasma of the blood, the colourless fluid in which the corpuscles float,
soak through them into the parts inside the mesh. You probably know that
many things will pass through thin skins and membranes in which no holes
can be found even after the most careful search. If you put peas into a
bladder and tie the neck, the peas will not get out until the bladder is
untied or torn. But if you were to put a solution of sugar or of salt
into the bladder, and place the bladder with its neck tied ever so
tightly in a basin of pure water, you would find that very soon the
water in the basin would begin to taste of sugar or salt--and that
without your being able to discover any hole, however small, in the
bladder. By putting various substances in the bladder, you will find
that solid particles and things which will not dissolve in water keep
inside the bladder, whereas sugar and salt, and many other things which
dissolve in water, will make their way through the bladder into the
water outside, and will keep on passing until the water in the basin is
as strong of sugar or salt as the water in the bladder. This property
which membranes such as a bladder have of letting certain substances
pass through them is called =osmosis=. You will at once see how important
a part it plays in your own body. It is by osmosis chiefly that the raw
nourishing material in the blood gets into the little islets of flesh
lying, as we have seen, in the meshwork of the capillaries. It is by
osmosis chiefly that the worn-out stuff from the same islets gets back
into the blood. It is by osmosis chiefly that food gets out of the
stomach into the blood. It is by osmosis chiefly that the waste,
worn-out matters are drained away from the blood, and so cast out of the
body altogether. By osmosis the blood nourishes and purifies the flesh.
By osmosis the blood is itself nourished and kept pure.
There are two chief things by which the blood, and through the blood
the body, is nourished. These are food and air. The air we have always
with us, we have no need to buy it or toil for it; hence we take it as
we want it, a little at a time, and often. We gather up no store of it;
and cannot bear the lack of it for more than a few moments.
For our food we have to labour; we store it up in our bodies from time
to time, at intervals of hours, in what we call meals, and can go hours
or even days without a fresh supply.
Let us first of all see how the blood, and, through the blood, the body,
is nourished by air.
HOW THE BLOOD IS CHANGED BY AIR: BREATHING. § VI.
=36.= I have already said, perhaps more than once, that our muscles burn,
burn in a wet way without giving light. And when I say our muscles, I
might say our whole body, some parts burning more fiercely than others.
You have learnt from your Chemistry Primer (Art. 2, p. 2) what happens
when a candle is placed in a closed jar of pure air. The oxygen gets
less, carbonic acid comes in its place, and after a while the candle
goes out for want of oxygen to carry on that oxidation which is the
essence of burning. You also know that exactly the same thing would
happen if you were (only you need not do it) to put a bird or a mouse in
the jar instead of a candle. The oxygen would go, carbonic acid would
come, and the little flame of life in the mouse would flicker and go
out, and after a while its body would be cold.
But suppose you were to put a fish or a snail in a jar of pure fresh
water, and cork the jar tight. There seems at first sight to be no air
in the jar. But there is. If you were to take that fresh water, and put
it under an air-pump, you could pump bubbles of air out of it; and if
you were to examine these bubbles you would find them to contain oxygen
and nitrogen, with very little carbonic acid. =The water contains
dissolved air.= After the fish or the snail had been some time in the
jar, you would see its flame of life flicker and die out, just like that
of the bird in air; and if you then pumped the air out of the water you
would find that the oxygen was nearly gone and that carbonic acid had
come in its place.
You see, then, that air can be breathed, as we call it, even when it is
dissolved in water.
Now to return to our muscle. When you were watching the circulation in
the frog’s foot, you could tell the artery from the vein, because in the
artery the blood was flowing _to_ the capillaries, and in the vein
_from_ them. Both artery and vein were rather red, and of about the same
tint of colour. But if you could see in your own body a large artery
going to your biceps muscle, and a large vein coming away from it, you
would be struck at once with the difference of colour between them. The
artery would look bright scarlet, the vein a dark purple; and if you
were to prick both, the blood would gush from the artery in a bright
scarlet jet, and bubble from the vein in a dark purple stream. And
wherever you found an artery and a vein (with a great exception of which
I shall have to speak directly), the blood in the artery would be bright
scarlet, and that in the vein dark purple. Hence we call the bright
scarlet blood which is found in the arteries =arterial blood=, and the
dark purple blood which is found in the veins =venous blood=.
What is the difference between the two? If you were to pump away at some
arterial blood, as you did at the water in which you put your fish, you
would be able to obtain from it some air, or, more correctly, some gas;
a great deal more gas, in fact, than you did from the water. A pint of
blood would yield you half a pint of gas. This gas you would find on
examination not to be air, _i.e._ not made up of a great deal of
nitrogen and the rest oxygen. (Chemical Primer, Art. 9.) There would be
very little nitrogen, but a good deal of oxygen, and still more carbonic
acid.
If you were to pump away at some venous blood you would get about as
much gas, but it would be very different in composition. The little
nitrogen would remain about the same, but the oxygen would be about half
gone, while the carbonic acid would be much increased.
This, then, is one great difference (for there are others) between
venous and arterial blood, that while =both contain, dissolved in them,
oxygen, nitrogen, and carbonic acid, venous blood contains less oxygen
and more carbonic acid than arterial blood=.
=37. In passing through the capillaries on its way to the vein, the blood
in the artery has lost oxygen and gained carbonic acid.= Where has the
oxygen gone to? Whence comes the carbonic acid? To and from the islets
of flesh between the capillaries, to the bloodless muscular fibre or bit
of nerve or skin which the blood-holding capillaries wrap round. The
oxygen has passed from the blood within the capillaries to the flesh
outside; from the flesh outside the carbonic acid has passed to the
blood within the capillaries. And this goes on all over the body.
Everywhere the flesh is breathing blood, is breathing gas dissolved in
the blood, just as a fish breathes water, _i.e._ breathes the air
dissolved in the water.
Goes on everywhere with one great exception. There is one great artery,
with its branches, in which blood is not bright, scarlet, arterial, but
dark, purple, venous. There are certain great veins in which the blood
is not dark, purple, venous, but bright, scarlet, arterial. You know
which they are. The pulmonary artery and the pulmonary veins. The blood
in the pulmonary veins contains more oxygen and less carbonic acid than
the blood in the pulmonary artery. =It has lost carbonic acid and gained
oxygen, as it passed through the capillaries of the lungs.=
=38.= What are the lungs? As you saw them in the rabbit, or as you may see
them in the sheep, they are shrunk and collapsed. We shall presently
learn why. But if you blow into them through the windpipe, which divides
into branches, one for either lung, you can blow them out ever so much
bigger. They are in reality bladders which can be filled with air, but
which, left to themselves, at once empty themselves again.
They are bladders of a peculiar construction. Imagine a thick short bush
or tree crowded with leaves; imagine the trunk and the branches, small
and great, down to the veriest twigs, all hollow; imagine further that
the leaves themselves were little hollow bladders, stuck on to the
smallest hollow twigs, and made of some delicate, but strong and
exceedingly elastic, substance. If you blew down the trunk you might
stretch and swell out all the hollow leaves; when you left off blowing
they would all fall together, and shrink up again.
Around such a framework of hollow branches called =bronchial-tubes=, and
hollow elastic bladders called =air-cells=, is wrapped the intricate
network of pulmonary arteries, veins, and capillaries, in such a way
that each air-cell, each little bladder, is covered by the finest and
most close-set network of capillaries, very much as a child’s
india-rubber ball is covered round with a network of string. Very thin
are the walls of the air-cell, so thin that the blood in the capillary
is separated from the air in the air-cell by the thinnest possible sheet
of finest membrane. As the dark purple blood rushes through the crowded
network, its carbonic acid escapes through this thin membrane, from the
blood into the air, and oxygen slips from the air into the blood.
Thus the dark purple venous blood coming along the pulmonary artery, as
it glides in the pulmonary capillaries along the outside of the inflated
air-cells, by loss of carbonic acid and gain of oxygen is changed into
the bright scarlet blood of the pulmonary veins.
This then is the mystery of our constant need of air. =The flesh of the
body of whatever kind, everywhere all over the body, breathes blood,
making pure arterial blood venous and impure, all over the body except
in the lungs, where the blood itself breathes air, and changes from
impure and venous to pure and arterial.=
=39.= Through the capillaries of the muscle a stream of blood is ever
flowing so long as life lasts and the heart has power to beat; every
instant a fresh supply of bright, pure, arterial blood comes to take the
place of that which has become dark, venous, and impure. Without this
constant renewal of its blood the muscle would be choked, and its vital
flame would flicker and die out.
In the lungs, the air filling the air-cells would if left to itself soon
lose all its oxygen and become loaded with carbonic acid; and the blood
in the capillaries of the lungs would no longer be changed from venous
to arterial, but would travel on to the pulmonary vein as dark and
impure as in the pulmonary artery. =Just as the blood in the muscle must
be constantly renewed, so must the air in the lungs be continually
changed.=
How is this renewal of the air in the lungs brought about?
In the dead rabbit you saw the lungs, shrunk, collapsed, emptied of much
of their air, and lying almost hidden at the back of the chest (Fig. 1,
_G.G._) The cavity of the chest seemed to be a great empty space, hardly
half filled by the lungs and heart. But this is quite an unnatural
condition of the lungs. Take another rabbit, and before you touch the
chest at all, open the abdomen and remove all its contents--stomach,
liver, intestines, &c. You will then get a capital view of the
=diaphragm=, which as you already know forms a complete partition between
the chest and the belly. You will notice that it is arched up towards
the chest, so that the under surface at which you are looking is quite
hollow. If you hold the rabbit up by its hind legs with its head hanging
down, and pour some water into the abdomen, quite a little pool will
gather in the shallow cup of the diaphragm.
In the rabbit the diaphragm is very transparent; you can see right
through it into the chest, and you will have no difficulty in
recognizing the pink lungs shining through it. =You will notice that they
cover almost all the diaphragm--in fact they fill up the whole of the
cavity of the chest that is not occupied by the heart.=
If you seize the diaphragm carefully in the middle with a pair of
forceps, and pull it down towards the abdomen, you will find that you
cannot create a space between the lungs and the diaphragm, but that the
lungs follow the diaphragm, and are quite as close to it when it is
pulled down as when it is drawn up.
=In other words, when the diaphragm is arched up as you find it on
opening the abdomen, the lungs quite fill the chest; and when the
diaphragm is drawn down and the cavity of the chest made bigger, the
lungs swell out so that they still fill up the chest.=
=40.= How do they swell out? By drawing air in through the windpipe. If
you listen, you will perhaps hear the air rush in as you pull the
diaphragm down--and if you tie the windpipe, or quite close up the nose
and mouth, you will find it much harder to pull down the diaphragm,
because no fresh air can get into the lungs.
Now prick a hole through the diaphragm into the cavity of the chest,
without wounding the lungs. You will hear a sudden rush of air, and the
lungs will shrink up almost out of sight. They are no longer close
against the diaphragm as they were before; and if you open the chest you
will find that they have shrunk to the back of the thorax as you saw
them in the first rabbit. The rush of air is partly a rush of air out of
the lungs, and partly a rush of air into the chest between the chest
walls and the outside of the lungs.
But before you lay open the chest, pull the diaphragm up and down as you
did before you made the hole in the diaphragm. You will find that you
have no effect whatever on the lungs. They remain perfectly quiet, and
do not swell up at all. By working the diaphragm up and down, you only
drive air through the hole you have made, =in and out of the cavity of
the chest=, not =in and out of the lungs= as you did before.
We see then that the =chest is an air-tight chamber=, and that the lungs,
when the chest walls are whole, =are always on the stretch=, are on the
stretch even when the diaphragm is arched up as high as it can go.