important in the blood that you can _see_ with the microscope; but their
being in the blood is one reason why blood is thicker than water.
Did you ever see a pig or sheep killed? If so, you would be sure to
notice that the blood ran quite fluid from the blood-vessels in the
neck, ran and was spilt like so much water--but that very soon the blood
caught in the pail or spilt on the stones became quite solid, so that
you could pick it up in lumps. Whenever blood is shed from the living
body, within a short time it becomes solid. This becoming solid is
called the =clotting= or =coagulation of blood=.
What makes it clot? Suppose while the blood was running from the pig’s
neck into the butcher’s pail, and while it was still quite fluid, you
were to take a bunch of twigs and keep slowly stirring the blood round
and round in the pail. You would naturally expect that the blood would
soon begin to clot, would get thicker and thicker and more and more
difficult to stir. But it does not; and if you keep on stirring long
enough you will find that it never clots at all. =By continually stirring
it you will prevent its clotting.= Now take out your bundle of twigs: you
will find it covered all over with a thick reddish mass of some soft
sticky substance; and if you pump on the red mass you will be able to
wash away all its red colour, and will have nothing left but a quantity
of white, soft, sticky, stringy material, all entangled and matted
together among the twigs of your bundle. This stringy material is in
reality made up of a number of fine, delicate, soft, elastic threads or
fibres, and is called =fibrin=.
=You see, by stirring, or, as it is frequently called, whipping the blood
with the bundle of twigs, you have taken the fibrin out of the blood,
and so prevented its clotting.=
If you were to take one of the clotted lumps of blood that were spilt on
the ground or a bit of the clot from a pail in which the blood had not
been whipped, and wash it long enough, you would find at last that all
the colour went away from the lump, and you had nothing left but a small
quantity of white stringy substance. This white stringy substance is
fibrin--exactly the same thing you got on your bundle of twigs.
If the blood is carefully caught in a pail, and afterwards not disturbed
at all, it clots into a solid mass. The whole of the blood seems to have
changed into a complete jelly; and if you turn it out of the pail, as
you may do, it keeps its shape, and gives you quite a mould of the pail,
a great trembling red jelly just the shape of the inside of the pail.
But if you were to leave the blood in the pail for a few hours or for a
day, you would find, instead of the large jelly quite filling the pail,
a smaller but firmer jelly covered by or floating in a colourless or
very pale yellow liquid. This smaller, firmer jelly, which in the course
of a day or so would get still firmer and smaller, would in fact go on
shrinking in size, you may still call the =clot=; the clear fluid in which
it is floating is called =serum=.
What has taken place is as follows. Soon after blood is shed there is
formed in it a something which was not present in it before. This
something, which we call =fibrin=, starts as a multitude of fine tender
threads which run in all directions through the mass of blood, forming a
close network everywhere. So the blood is shut up in an immense number
of little chambers formed by the meshes of the fibrin; and it is this
which makes it seem a jelly. But each thread of fibrin as soon as it is
formed begins to shrink, and the blood in each of these little chambers
is squeezed by the shrinking of its walls of fibrin, and tries to make
its way out. The corpuscles get caught in the meshes, but all the rest
of the blood passes between the threads and comes out on the top and
sides of the pail. And this goes on until you have left in the clot very
little besides corpuscles entangled in a network of fibrin, and all the
rest of the blood has been squeezed outside the clot, and is then called
serum. =Serum, then, is blood out of which the corpuscles have been
strained by the process of clotting.=
Now I dare say you are ready to ask the question, If blood clots so
readily when it is shed, why does it not clot inside the body? Why is
our blood ever fluid? This is rather a difficult question to answer.
When blood is shed from the warm body it soon gets cool. But it does not
clot and become solid because it gets cool, as ordinary jelly does. If
you keep it from getting cool it clots all the same, in fact quicker,
and if kept cold enough will not clot at all. Nor does it clot when
shed, because it has become still, and is no longer rushing round
through the blood-vessels. Nor is it because it is exposed to the air.
Perhaps we don’t know exactly why it is, and you will have much to learn
hereafter about the coagulation of blood. All I will say at present is
that as long as the blood is in the body there is something at work to
keep it from clotting. It does clot sometimes in the body, and
blood-vessels get plugged with the clots; but that constitutes a very
dangerous disease.
=24.= Well, blood is thicker than water because it contains solid
corpuscles and fibrin. But even the serum, _i.e._ blood out of which
both fibrin and corpuscles have been taken, is thicker than water.
You know that if you were to take a basinful of pure water and boil it,
it would boil away to nothing. It would all go off in steam. But if you
were to try to boil a basinful of serum, you would find several curious
things happen.
In the first place you would not be able to boil it at all. Before you
got it as hot as boiling water, your serum, which before seemed quite as
liquid as water, only feeling a little sticky if you put your finger in
it, would all become quite solid. You know the difference between a raw
and a boiled egg. The white of the raw egg, though very sticky and ropy,
or viscid as it is called, is still liquid; you will find it hard work
if you try to cut it with a knife. The white of the hard boiled egg, on
the other hand, is quite solid, and you can cut it into ever so thin
slices. It has been “set” by boiling. Well, the serum of blood is in
this respect very like white of egg. In fact they both contain the same
substance, called =albumin=, which has this property of “setting” or
becoming solid when heated nearly to boiling-point. Both the serum of
blood and white of egg even when “set” are wet, _i.e._ contain a great
deal of water. You may dry them in the proper manner into a transparent
horny substance. When quite dry they will readily burn. They are
therefore things which can be oxidized. When burnt they give off
carbonic acid, water, and ammonia; the latter you might easily recognize
by its effect on your nose if you were to burn a piece of dried blood in
a flame. Now, when I say that =albumin= in burning gives off carbonic
acid, water, and ammonia, you know from your Chemistry that it must
contain carbon to form the carbonic acid, hydrogen to form water, and
nitrogen to form ammonia. It need not contain oxygen, for as you know it
could get all the oxygen it wanted from the air; still it does contain
some oxygen. =Albumin, then, is an oxidizable or combustible body made up
of nitrogen, carbon, hydrogen, and oxygen.= It is important you should
remember this; but I will not bother you with how much of each--it is a
very complex substance, built up in a wonderful way, far more complex
than any of the things you had to learn about in your Chemistry Primer.
And this albumin, dissolved in a great deal of water, forms the serum of
blood.
I did not say anything about what fibrin was made of; but it, like
albumin, is made up of nitrogen, carbon, hydrogen, and oxygen. It is not
quite the same thing as albumin, but first cousin to it. There is
another first cousin to both of them, also containing nitrogen, carbon,
hydrogen, and oxygen, which together with a great deal of water forms
muscle; another forms a great part of the red corpuscles; and scattered
all over the body in various places, there are first cousins to albumin,
all containing nitrogen, carbon, hydrogen, and oxygen, all combustible,
and all when burnt giving off carbonic acid, water, and ammonia. All
these first cousins go under one name; they are all called =proteids=.
=25.= Well, then, blood is thicker than water by reason of the proteids in
the corpuscles, in the fibrin, and in the serum, but there is something
else besides. I will not trouble you with the crowd of things of which
there are perhaps just a few grains in a gallon of blood, like the
little pinches of things a cook puts into a savoury dish; though, as you
go on in your studies, you will find that these, like many other little
things in the world, are of great importance.
But I will ask you to remember this. If you take some dried blood and
burn it, though you may burn all the proteids (and some other of the
trifles I spoke of just now) away, you will not be able to burn the
whole blood away. Burn as long as you like, you will always have left a
quantity of what you have learnt from your Chemistry to call =ash, and if
you were to examine this ash you would find it contained ever so many
elements; sulphur, phosphorus, chlorine, potassium, sodium, calcium, and
iron, being the most abundant and most important=.
Blood, then, is a very wonderful fluid: wonderful for being made up of
coloured corpuscles and colourless fluid, wonderful for its fibrin and
power of clotting, wonderful for the many substances, for the proteids,
for the ashes or minerals, for the rest of the things which are locked
up in the corpuscles and in the serum.
But you will not wonder at it when you come to see that the blood is the
great circulating market of the body, in which all the things that are
wanted by all parts, by the muscles, by the brain, by the skin, by the
lungs, liver, and kidney, are bought and sold. What the muscle wants,
it, as we have seen, buys from the blood; what it has done with it sells
back to the blood; and so with every other organ and part. As long as
life lasts this buying and selling is for ever going on, and this is why
the blood is for ever on the move, sweeping restlessly from place to
place, bringing to each part the things it wants, and carrying away
those with which it has done. When the blood ceases to move, the market
is blocked, the buying and selling cease, and all the organs die,
starved for the lack of the things which they want, choked by the
abundance of things for which they have no longer any need.
We have now to learn how the blood is thus kept continually on the move.
HOW THE BLOOD MOVES. § V.
=26.= You have already learnt to recognize the blood-vessels of the
rabbit, and to distinguish two kinds of blood-vessels--the arteries,
which in a dead animal generally contain little or no blood, and have
rather firm stout walls; and the veins, which are generally full of
blood, and have thinner and flabby walls. The arteries when you cut them
generally gape and remain open; the veins fall together and collapse.
The larger the arteries, the stouter and firmer they are, and the
greater the difference between them and the veins.
You have also studied the capillaries in the frog’s foot; you have seen
that they are minute channels, with the thinnest and tenderest walls,
forming a close network in which the smallest arteries end, and from
which the smallest veins begin.
You have moreover been told that all over your own body, in every part,
there are, though you cannot see them, networks of capillaries like
those in the frog’s foot which you can see; that all the arteries of
your body end in capillaries, and all the veins begin in capillaries.
Let me repeat that, one or two structures excepted, there is no part of
your body in which, could you put it under a microscope, you would not
see a small artery branching out and losing itself in a network of
capillaries, out of which, as out of so many roots, a small vein gathers
itself together again.
In some places the network is very close, the capillaries lying closer
together than even in the frog’s foot; in others the network is more
open, and the capillaries wider apart; but everywhere, with a few
exceptions which you will learn by and by, there are capillaries,
arteries, and veins.
Suppose you were a little lone red corpuscle, all by yourself in the
quite empty blood-vessels of a dead body, squeezed in the narrow pathway
of a capillary, say of the biceps muscle of the arm, able to walk
about, and anxious to explore the country in which you found yourself.
There would be two ways in which you might go. Let us first imagine that
you set out in the way which we will call backwards. Squeezing your way
along the narrow passage of the capillary in which you had hardly room
to move, you would at every few steps pass, on your right hand and on
your left, the openings into other capillary channels as small as the
one in which you were. Passing by these you would presently find the
passage widening, you would have more room to move, and the more
openings you passed, the wider and higher would grow the tunnel in which
you were groping your way. The walls of the tunnel would grow thicker at
every step, and their thickness and stoutness would tell you that you
were already in an artery, but the inside would be delightfully smooth.
As you went on you would keep passing the openings into similar tunnels,
but the further you went on, the fewer they would be. Sometimes the
tunnels into which these openings led would be smaller, sometimes
bigger, sometimes of the same size as the one in which you were.
Sometimes one would be so much bigger, that it would seem absurd to say
that it opened into your tunnel. On the contrary, it would appear to you
that you were passing out of a narrow side passage into a great wide
thoroughfare. I dare say you would notice that every time one passage
opened into another the way suddenly grew wider, and then kept about the
same size until it joined the next. Travelling onwards in this way, you
would after a while find yourself in a great wide tunnel, so big that
you, poor little corpuscle, would seem quite lost in it. Had you anyone
to ask, they would tell you it was the main artery of the arm. Toiling
onwards through this, and passing a few but for the most part large
openings, you would suddenly tumble into a space so vast that at first
you would hardly be able to realize that it was the tunnel of an artery
like those in which you had been journeying. This you would learn to be
the =aorta=, the great artery of all; and a little further on you would be
in the heart.
Suppose now you retraced your steps, suppose you returned from the aorta
to the main artery of the arm, and thus back through narrower and
narrower tunnels till you came again to the spot from which you started,
and then tried the other end of the capillary. You would find that that
led you also, in a very similar way, into wider and wider passages. Only
you could not help noticing that though the inside of all the passages
was as smooth as before, the walls were not nearly so thick and stout.
You would learn from this that you were in the veins, and not in the
arteries. You would meet too with something, the like of which you did
not see in the arteries (except perhaps just close to the heart). Every
now and then you would come upon what for all the world looked like one
of those watch-pockets that sometimes are hung at the head of a
bedstead, a watch-pocket with its opening turned the way you were going.
This you would find was called a =valve=, and was made of thin but strong
membrane or skin. Sometimes in the smaller veins you would meet with one
watch-pocket by itself, sometimes with two or even three abreast, and I
dare say you would notice that very frequently, directly you had passed
one of these valves, you came to a spot where one vein joined another.
Well, but for these differences, your journey along the veins would be
very like your journey along the arteries, and at last you would find
yourself in a great vein, whose name you would learn to be the =vena
cava=, or hollow vein (and because, though there is but one aorta, there
are two great “hollow veins,” =the superior vena cava= or =upper hollow
vein=), and from thence your next step would be into the heart again. So
you see, starting from the capillary (you started from a capillary in
the arm, but you might have started from any capillary anywhere),
whether you go along the arteries or whether you go along the veins, you
at last come to the heart.
Before we go on any further we must learn something about the heart.
=27.= Go and ask the butcher for a sheep’s pluck. There will most probably
be one hanging up in his shop. Look at it before he takes it down. The
hook on which it is hanging has been thrust through the windpipe. You
will see that the sheep’s windpipe is, like the rabbit’s, all banded
with rings of cartilage, only very much larger and coarser. Below the
windpipe come the spongy lungs, and between them lies the heart, which
perhaps is covered up with a skin and so not easily seen. Hanging to the
heart and lungs is the great mass of the liver. When you have got the
pluck home, cut away the liver, cut away the skin (pericardium, it is
called) which is covering the heart, if it has not been cut away
already, and lay the lungs out on a table with the heart between them.
You will then have something very much like what
[Illustration:
FIG. 5.--_Heart of Sheep, as seen after Removal from the Body,
lying upon the Two Lungs. The Pericardium has been cut away, but no
other Dissection made._
_R.A._ Auricular appendage of right auricle; _L.A._ auricular appendage
of left auricle; _R.V._ right ventricle; _L.V._ left ventricle; _S.V.C._
superior vena cava; _I.V.C._ inferior vena cava; _P.A._ pulmonary
artery; _Ao_, aorta; _Áó_, innominate branch from aorta dividing into
subclavian and carotid arteries; _L._ lung; _Tr._ trachea. 1, solid cord
often present, the remnant of a once open communication between the
pulmonary artery and aorta. 2, masses of fat at the bases of the
ventricle hiding from view the greater part of the auricles. 3, line of
fat marking the division between the two ventricles. 4, mass of fat
covering the trachea.]
is represented in Fig. 5. If you could look through the front of your
own chest, and see your own heart and lungs in place, you would see
something not so very very different.
If now you handle the heart--and if you want to learn physiology you
must handle things--you will have no great difficulty in finding the
great yellowish tubes marked _Ao_ and _Áó_ in the figure. Your butcher
perhaps may not have cut them across exactly where mine has done, but
that will not prevent your recognizing them. You will notice what thick
stout walls they have, and how they gape where they are cut. _Ao_ is the
=aorta=, and _Áó_ is a great branch of the aorta, going to the head and
neck of one side, perhaps the branch along which we imagined just now
that you, a poor little red blood-corpuscle, were travelling. If you
were to put a wire through _Áó_ you would be able to bring it out
through _Ao_, or _vice versâ_. But what is _P.A._ which looks so much
like the aorta, though you will find that it has no connection with it?
You cannot pass a wire from the aorta into it. It also is an artery, the
=pulmonary[2] artery=. We shall have more to say about it directly.
Now try and find what are marked in the figure as _S.V.C._ and _I.V.C._
You will perhaps have a little difficulty in this; and when you have
found them you will understand why. They are the great veins of the
body. _S.V.C._ is the =superior vena cava=, to form which all the veins
from the head and neck and arms join, the vein in which you were
journeying a little while ago. _I.V.C._ is the =inferior vena cava=, made
out of all the veins from the trunk and the legs. Being veins, they have
thin flabby walls; and their sides fall flat together, so that they seem
nothing more than little folds of skin, and it becomes very hard to find
the passage inside them. But when you have found the opening into them,
you will see that you can stretch them out into quite wide tubes, and
that their walls, though very much thinner than those of the aorta, so
thin indeed that they are almost transparent, are still after a fashion
strong. If you put a penholder or thin rod through either you will find
that they both seem to lead right into the middle of the heart. With a
little care you can pass a rod up _I.V.C._ and bring the end of it out
at the top of _S.V.C._ Of course you will understand that both of these
veins have been cut off short.
=28.= Before we go on any further with the sheep’s heart, let me tell you
something about it, by help of the diagram in Fig. 6, which is meant to
represent the whole circulation. You must remember that this figure is a
=diagram=, and not a picture; it does not represent the way the
blood-vessels are really arranged in your own body. If you had no arms
and no legs, and if you only had a few capillaries at the top of your
head and at the bottom of your body, it might be more like than it is.
In the centre of the figure is the heart. This you will see is
completely divided by an upright partition into two halves, a right half
and a left half. Each half is further marked off, but not completely
divided, into
[Illustration:
FIG. 6.--_Diagram of the Heart and Vessels, with the Course of the
Circulation, viewed from behind so that the proper left of the
Observer corresponds with the left side of the Heart in the
Diagram._
_L.A._ left auricle; _L.V._ left ventricle; _Ao._ aorta; _A_^{1}.
arteries to the upper part of the body; _A_^{2}. arteries to the lower
part of the body; _H.A._ hepatic artery, which supplies the liver with
part of its blood; _V_^{2}. veins of the upper part of the body;
_V_^{2}. veins of the lower part of the body; _V.P._ vena portæ; _H.V._
hepatic vein; _V.C.I._ inferior vena cava; _V.C.S._ superior vena cava;
_R.A._ right auricle; _R.V._ right ventricle; _P.A._ pulmonary artery;
_Lg._ lung; _P.V._ pulmonary vein; _Lct._ lacteals; _Ly._ lymphatics;
_Th.D._ thoracic duct; _Al._ alimentary canal; _Lr._ liver. The arrows
indicate the course of the blood, lymph, and chyle. The vessels which
contain arterial blood have dark contours, while those which carry
venous blood have light contours.]