we have four chambers,--two upper chambers, one on each side, marked
_R.A._ and _L.A._, these are called the =right and left auricles=; and two
lower chambers, one on each side, marked _R.V._ and _L.V._, these are
called the =right and left ventricles=. The right auricle, _R.A._, opens
in the direction of the arrow into the right ventricle, _R.V._, the
opening being guarded, as we shall see, by a valve. The left auricle,
_L.A._, opens into the left ventricle, _L.V._, the opening being
likewise guarded by a valve; but you have to go quite a roundabout way
to get from either the right auricle or ventricle to the left auricle or
ventricle. Let us see how we can get round the figure. Suppose we begin
with the two tubes marked _V.C.S._ and _V.C.I._, the walls of which are
drawn with thin lines. These both open into the right auricle. They are
the vena cava superior and inferior, which you have just made out in the
sheep’s heart. From the right auricle you pass easily into the right
ventricle; thence, following the arrow, the way is straight into the
tube marked _P.A._ This is the pulmonary artery, the outside of which
you saw in the sheep’s heart (Fig. 5, _P.A._) Travelling along this
pulmonary artery, you come to the lungs, and after passing through
branches not represented in the figure, picking your way through
arteries which continually get smaller and smaller, you find yourself
at last in the capillaries of the lungs. Squeezing your way through
these, you come out into veins, and gradually advancing through larger
and larger veins, you, still following the arrow, find yourself in one
of four large veins (only one of them is represented in the diagram)
which land you in the left auricle. From the left auricle it is but a
jump into the left ventricle. From the left ventricle the way is open,
as indicated by the arrow, into the tube marked _Ao_. This is intended
to represent the aorta, which you have already seen in the sheep’s heart
(Fig. 5, _Ao_). It is here drawn for simplicity’s sake as dividing into
two branches, but you have already been told, and must bear in mind,
that it does not in reality divide in this way, but gives off a good
many branches of various sizes. However, taking the figure as it stands,
suppose we travel along _A_^{2}. Following the arrow, and shooting
through arteries which continually get smaller and smaller, we come at
last to capillaries somewhere, in the skin or in some muscle, or in a
bone, or in the brain, or almost anywhere, in fact, in the upper part of
the body. Out of the capillaries we pass into veins, which, joining
together and so forming larger and larger trunks, bring us at last to
the point from which we started, the superior vena cava, _V.C.S._ If we
had taken the other road, _A_^{2}, we should have passed through
capillaries somewhere in the lower part of the body instead of the
upper, and come back by the vena cava inferior, _V.C.I._, instead of the
vena cava superior. =Starting from the right auricle, whichever way we
took we should always come back to the right auricle again, and in our
journey should always pass through the following things in the following
order: right auricle, right ventricle, pulmonary artery, arteries,
capillaries, and veins of the lungs, pulmonary vein, left auricle, left
ventricle, aorta, arteries, capillaries, and veins somewhere in the
body, and either superior or inferior vena cava.= That is the course of
the circulation. But there is something still to be added. Among the
many large branches, not drawn in the diagram, given off by the aorta to
the lower part of the body, there are two branches which are drawn and
which deserve special notice.
One is a large branch carrying blood to the tube _A.L._, which is meant
in the diagram to stand for the stomach, intestines, and some other
organs. This branch, like all other branches of the aorta, divides into
small arteries, and these into capillaries, which again are gathered up
into veins, forming at last a large vein marked in the diagram _V.P._
and called the =vena portæ= or =portal vein=. Now the remarkable thing is
that this vein does not, like all the other veins, go straight to join
the vena cava, but makes for the liver, where it divides into smaller
and smaller veins, until at last it breaks completely up in the liver
into a set of capillaries again. These capillaries gather once more into
veins, forming at last the large trunk, called the =hepatic[3] vein=,
_H.V._, which does what the portal vein ought to have done but did not;
it opens straight into the vena cava.
The other branch of the aorta of which we are speaking goes straight to
the liver, and is called the =hepatic artery=, _H.A._: there it breaks up
in the liver into small arteries, and then into capillaries, which
mingle with the capillaries of the portal vein, and form one system, out
of which the hepatic veins spring. So you see it makes a great
difference to a red corpuscle which is travelling along the lower part
of the aorta _A_^{2}, whether it takes a turn into the branch going to
the alimentary canal, or whether it goes straight on into, for instance,
a branch going to some part of the leg. In the latter case, having got
through a set of capillaries, it is soon back into the vena cava and on
its road to the heart. But if it takes the turn to the alimentary canal,
it finds after it has passed through the capillaries and got into the
portal vein, that it has still to go through another set of capillaries
in the liver before it can pass through the hepatic vein into the vena
cava.
This then is the course of the circulation. Right side of the heart,
pulmonary artery, capillaries of the lungs, pulmonary vein, left side of
the heart, aorta, capillaries somewhere, sometimes two sets, sometimes
one, vena cava, right side of the heart again. A little corpuscle cannot
get from the right to the left side of the heart without going through
the capillaries of the lungs. It cannot get from the left side of the
heart to the right without going through some capillaries somewhere in
the body, and if it should happen to take the turn to the stomach, it
has to go through two sets of capillaries instead of one.
You see, you really have two circulations, and you have two hearts
joined together into one. If you were very skilful you might split the
heart in half and pull the two sides asunder, and then you would have
one heart receiving all the veins from the body and sending its arteries
(branches of the pulmonary artery) all to the lungs, and another heart
receiving all the veins from the lungs and sending its arteries
(branches of the aorta) all over the body. And you would have two
circulations, one through the lungs, and another through the rest of the
body, both joining each other. Very often two circulations are spoken
of, and because the lungs are so much smaller than the rest of the body,
the circulation through the lungs is called the lesser circulation, that
through the rest of the body the greater circulation.
=29.= I have described the circulation as if the blood always went in one
direction from the right side of the heart to the left, from arteries to
veins, the way the arrows point in the diagram. And so it does. It
cannot go the other way round. =Why does it go that way? Why cannot it go
the other way round?=
The reasons are to be found partly in the heart, partly in the veins.
=In the veins the blood will only pass from the capillaries to the heart.=
Why not from the heart to the capillaries? You remember the little
watch-pocket-like valves, here and there, sometimes singly, sometimes
two or three abreast. =You remember that the mouths of the watch-pockets
were always turned towards the heart.= Now suppose a crowd of little
corpuscles hurrying along a vein towards the heart. When they came to
one of these watch-pocket valves they would simply trample it down flat,
and so pass over it without hardly knowing it was there, and go on
their way as if nothing had happened. But suppose they were journeying
the other way, from the heart to the capillaries. When they came to the
open mouth of a watch-pocket valve, some of them would be sure to run
into the pocket, and then the pocket would bulge out, and the more it
bulged out the more blood would run into it, until at last it would be
so full of blood that it would press close against the top of the vein,
as is shown in Fig. 7 (or, if there were two or three, they would all
meet together), and so quite block the vein up. If you doubt this, make
a watch-pocket out of a piece of silk or cotton, fasten it on to a piece
of brown paper, and roll the paper up into a tube, so that the valve is
nicely inside the tube. If you pour some peas down the tube with the
mouth of the valve looking away from you, they will run through at once;
but if you try to pour them the other way, your tube will soon be
choked, and if you carefully unroll the tube you will find the
watch-pocket crammed full of peas.
[Illustration: FIG. 7.--_Diagrammatic Sections of Veins with Valves._
In the upper, the blood is supposed to be flowing in the direction
of the arrow, towards the heart; in the lower, the reverse way. C,
capillary side; H, heart side.
]
=The valves in the veins, then, let the blood pass easily from the
capillaries to the heart, but won’t let it go the other way.= If you bare
your arm you may see some of the veins in the skin, in which the blood
is running up from the hand towards the shoulder. If with your finger
you press one of these veins back towards the hand it will swell up, and
if you look carefully you may see little knots here and there caused by
the bulging out of the watch-pocket valves. If you press it the other
way, towards the elbow, you will empty it easily, and if with another
finger you prevent the blood getting into it from behind, that is from
the hand, the vein will remain empty a very long time.
The presence of valves in the veins, then, is one reason why the blood
moves in one direction, but other reasons, and these the chief ones, are
to be found in the heart.
Let us now go back to the sheep’s heart.
=30.= You know from the diagram that the two great veins, the superior and
inferior vena cava, open into the right auricle. If you slit up these
two veins in the sheep’s heart, you will find that they end by separate
openings in a small cavity, the inside of which is for the most part
smooth, and the walls of which, made, as you will at once see, of
muscle, are not very thick. This small cavity is the right auricle,
shown in Fig. 8, _R.A._, where the great veins have not been slit up,
but the front of the auricle has been cut away. In this auricle, beside
the openings into the two great veins and another one which belongs to a
vein coming from the heart itself (Fig. 8, _b_) there is quite a large
one, leading straight downwards, into which you
[Illustration: FIG. 8.--_Right Side of the Heart of a Sheep._
_R.A._ cavity of right auricle; _S.V.C._ superior vena cava;
_I.V.C._ inferior vena cava; (a piece of whalebone has been passed
through each of these;) _a_, a piece of whalebone passed from the
auricle to the ventricle through the auriculo-ventricular orifice;
_b_, a piece of whalebone passed into the coronary vein.
_R.V._ cavity of right ventricle; _tv_, _tv_, two flaps of the
tricuspid valve: the third is dimly seen behind them, the _a_,
piece of whalebone, passing between the three. Between the two
flaps, and attached to them by _chordæ tendineæ_, is seen a
papillary muscle, _PP_, cut away from its attachment to that
portion of the wall of the ventricle which has been removed. Above,
the ventricle terminates somewhat like a funnel in the pulmonary
artery, _P.A._ One of the pockets of the semilunar valve, _sv_, is
seen in its entirety, another partially.
1, the wall of the ventricle cut across; 2, the position of the
auriculo-ventricular ring; 3, the wall of the auricle; 4, masses of
fat lodged between the auricle and pulmonary artery.
]
can put your three fingers. This is the opening into the right
ventricle; and you will have no difficulty in putting your fingers from
the auricle into the ventricle and bringing them out again.
[Illustration: FIG. 9.--_The Orifices of the Heart seen from above, the
Auricles and Great Vessels being cut away._
_P.A._ pulmonary artery, with its semilunar valves; _Ao._ aorta,
do.
_R.A.V._ right auriculo-ventricular orifice with the three flaps
(_lv._ 1, 2, 3) of tricuspid valve.
_L.A.V._ left auriculo-ventricular orifice, with _m.v._ 1 and 2,
flaps of mitral valve; _b_, piece of whalebone passed into coronary
vein. On the left part of _L.A.V._ the section of the auricle is
carried through the auricular appendage; hence the toothed
appearance due to the portions in relief cut across.
]
But hold the heart in one hand with the auricle upwards, and try to pour
some water into the ventricle. The first few spoonfuls will go in all
right, and then you will see some thin white skin or membrane come
floating up into the opening and quite block up the entrance from the
auricle into the ventricle; the
[Illustration: FIG. 10.--_View of the Orifices of the Heart from below,
the whole of the Ventricles having been cut away._
_R.A.V._ right auriculo-ventricular orifice surrounded by the three
flaps, _t.v._ 1, _t.v._ 2, _t.v._ 3, of the tricuspid valve; these
are stretched by weights attached to the _chordæ tendineæ_.
_L.A.V._ left auriculo-ventricular orifice surrounded in same way
by the two flaps, _m.v._ 1, _m.v._ 2, of mitral valve; _P.A._ the
orifice of pulmonary artery, the semilunar valves having met and
closed together; _Ao._ the orifice of the aorta with its semilunar
valves. The shaded portion, leading from _R.A.V._ to _P.A._,
represents the funnel seen in Fig. 8.
]
water will immediately fill the auricle and run over. If you look at the
membrane carefully as it comes bulging up, you will notice that it is
made up of three pieces joined together as is shown in Fig. 9 (_lv._ 1,
_lv._ 2, _lv._ 3). These three pieces form the valve between the right
auricle and ventricle, called the =tricuspid=, or three-peaked valve. Why
it is so called you will understand if you lay open the right ventricle
by cutting with a pair of scissors from the auricle into the ventricle
along the side of the heart, or by cutting away the front of the
ventricle as has been done in Fig. 8. You will then see that the valve
is made up of three little triangular flaps, which grow together round
the opening with their points hanging down into the cavity of the
ventricle (Fig. 10, _t.v._) They do not, however, hang quite loosely.
You will notice fastened to the sides of the flaps, thin delicate
threads, the other ends of which are fastened to the sides of the
ventricle, and often to little fleshy projections called papillary
muscles (Fig. 8, _P.P._)
How do these valves act? In this way. When the ventricle is empty, and
blood or water or any other fluid is poured into it from the auricle,
the valves are pushed on one side against the walls of the ventricle,
and thus there is a great wide opening from the auricle into the
ventricle. But as the ventricle fills, the blood or water gets behind
the flaps and floats them up towards the auricle. The more fluid in the
ventricle the higher they float, until when the ventricle is quite full
they all meet together in the middle of the opening between the auricle
and ventricle and completely block it up. But why do they not turn right
over into the auricle, and so open up again the wrong way? Because of
those little threads (the _chordæ tendineæ_, as they are called) which
fasten them to the walls of the ventricle. The flaps float back until
these threads are stretched quite tight, and the threads are just long
enough to let the flaps reach to the middle of the opening, but no
further. The tighter the threads are stretched the closer the flaps fit
together, and the more completely do they block the way from the
ventricle back into the auricle.
=The tricuspid valve, then, lets blood flow easily from the right auricle
into the right ventricle, but prevents it flowing from the ventricle
into the auricle.=
=31.= Now look at the cavity of the ventricle. Its walls are fleshy, that
is muscular, and you will notice that they are much stouter and thicker
than those of the auricle. Besides the opening from the auricle there is
but one other, which is at the top of the ventricle, side by side with
the former. If you put a penholder or your finger through this second
opening, you will find that it leads into the large vessel which you
have already learnt to recognize as the pulmonary artery (Fig. 5,
_P.A._)
Slit up the pulmonary artery from the ventricle with a pair of scissors,
as has been done in Fig. 8, _P.A._ You will notice at once the line
where the red soft flesh of the muscular ventricle leaves off, and the
yellow firmer material of which the artery is made begins. Just at that
line you will see a row of three (perhaps you may have cut one of the
three with your scissors) most beautiful, watch-pocket valves, made on
just the same principle as those in the veins, only larger, and more
exquisitely finished. These are called =semilunar valves=, because each
pocket is of the shape of a half-moon. Lift them up carefully and see
how tender and yet how strong they are. There is no need to tell you the
use of these. You know it at once. =They are to let the blood flow from
the ventricle into the pulmonary artery, and to prevent the blood going
back from the artery into the ventricle.=
On the right side of the heart we have, then, two great valves, the
tricuspid valve between the auricle and the ventricle, and the semilunar
valve between the ventricle and the pulmonary artery. These let the
blood flow easily one way, but not the other. If you doubt this, try it.
Put a tube into either the superior or inferior vena cava of a fresh
heart, tying the other vena cava and another tube into the pulmonary
artery. If with a funnel you pour water into the tube in the vein, it
will run through auricle and ventricle and out through the tube of the
pulmonary artery as easily as possible; but if you try to pour water the
other way down the pulmonary artery, you will find you cannot do it; the
tube gets blocked directly, and only a few drops come back through the
heart into the vein.
Now slit up the pulmonary artery as far as you can, and note when you
cut it how stout and firm are its walls. You will find that it soon
divides into two branches, one for the right lung, one for the left.
Each of these, when it gets to the lung, divides into branches, and
these again into others, as far as you can follow them. You know from
what you have learnt already that these branches end in capillaries all
over the lungs.
=32.= Not far from the two main branches of the pulmonary artery you will
find, covered up perhaps with fat and other matters, some tubes which
you will at once recognize as veins, and if you open any one of these
you will find that you can put a thin rod into it, and that it leads in
one direction to the lungs, and in the other into the left side of the
heart. These are the _pulmonary veins_, and if you slit them right up
you will find they open (by four openings) into a cavity on the left
side of the heart, almost exactly like that cavity on the right side
which we called the right auricle (Fig. 11). This cavity is, in fact,
the =left auricle=; out of it there is an opening into the left ventricle,
very like the opening from the right auricle into the right ventricle.
It too is guarded by flap valves, exactly like the tricuspid valve, only
there are but two flaps instead of three (Fig. 9, _m.v._ 1, _m.v._ 2).
Hence this valve is called the =bicuspid=, or more frequently the =mitral=
valve. Its flaps have little threads by which they are fastened to the
walls of the ventricle, and in fact, except for there being two flaps
instead of three, the mitral valve is exactly like the tricuspid valve,
and acts exactly the same way.
If you cut with a pair of scissors from the auricle into the ventricle,
you will find the left ventricle (Fig. 11) very much like the right
ventricle, only its walls are very much thicker, so much thicker that
the left ventricle takes up the greater part of the heart. You will see
this if you now look at the outside of a fresh heart.
The auricles are so small and so covered up by fat that from the outside
you can hardly see them at all. What you chiefly see are two little
fleshy corners, one of each auricle (Fig. 5, _R.A._ _L.A._), often
called “the auricular appendages.” By far the greater part is taken up
by the ventricles--and if you look you will see a band of fat slanting
across the heart (Fig. 5, 3). This marks the line of the fleshy
division, or =septum= as it is called, between the two ventricles. You
will notice that the point or apex of the heart belongs altogether to
the left ventricle.
[Illustration: FIG. 11.--_Left Side of the Heart of a Sheep (laid
open)._
_P.V._ pulmonary veins opening into the left auricle by four
openings, as shown by the styles or pieces of whalebone placed in
them: _a_, a style passed from auricle into ventricle through the
auriculo-ventricular orifice; _b_, a style passed into the coronary
vein, which, though it has no connection with the left auricle, is,
from its position, necessarily cut across in thus laying open the
auricle.
_M.V._ the two flaps of the mitral valve (drawn somewhat
diagrammatically): _pp_, papillary muscles, belonging as before to
the part of the ventricle cut away; _c_, a style passed from
ventricle in _Ao._ aorta; _Ao_^{2}. branch of aorta (see Fig. 5,
_Áó_); _P.A._ pulmonary artery; _S.V.C._ superior vena cava.
1, wall of ventricle cut across; 2, wall of auricle cut away around
auriculo-ventricular orifice; 3, other portions of auricular wall
cut across; 4, mass of fat around base of ventricle (see Fig. 5,
2).
]
To return to the inside of the left ventricle. Up at the top of the
ventricle, close to the opening from the auricle, there is one other
opening, and only one. If you put your finger into this, you will find
that it leads into a tube which first of all dips under or behind the
pulmonary artery and then comes up and to the front again. This tube is
what you already know as the =aorta=. If you slit it up from the ventricle
(and to do this you must cut through the pulmonary artery), you will
find that on the left side, as on the right, the red fleshy wall of the
ventricle suddenly changes into the yellow firm wall of the artery, and
that just at this line there are three semilunar valves exactly like
those in the pulmonary artery.
=On the left side of the heart, then, we have also two valves, the mitral
between the auricle and the ventricle, and the semilunar between the
ventricle and the aorta. These let the blood pass one way and not the
other. You can easily drive fluid from the pulmonary veins through
auricle and ventricle into the aorta, but you cannot send it back the
other way from the aorta.=
These then are the reasons why the blood will only pass one way, the way
I said it did. There are sets of valves opening one way and shutting the
other. These valves are the tricuspid between the right auricle and
right ventricle, the pulmonary semilunar valves between the right
ventricle and the pulmonary artery, the mitral valve between the left
auricle and the left ventricle, the aortic semilunar valves between the
left ventricle and the aorta, and the valves which are scattered among
the veins of the body. Of these by far the most important are the
valves in the heart: they do the chief work; those in the veins do
little more than help.
=33.= Well, then, we understand now, do we not? why the blood, if it moves
at all, moves in the one way only. There still remains the question, =Why
does the blood move at all?=
You know that during life it does keep moving. You have seen it moving
in the web of a frog’s foot--and whenever any part of the body can be
brought under the microscope, the same rush of red corpuscles through
narrow channels may be seen. You know it moves because when you cut a
blood-vessel the blood runs out. If you cut an artery across, the blood
gushes out from the end which is nearest the heart; if you cut a vein
across, the blood comes most from the end nearest the capillaries. If
you want to stop an artery bleeding, you tie it between the cut and the
heart; if you want to stop a vein bleeding, you tie it between the cut
and the capillaries. You understand now why there is this difference
between a cut artery and a cut vein. And you see that this is by itself
a proof that the blood moves in the arteries from the heart to the
capillaries, and in the veins from the capillaries to the heart.
The blood is not only always moving, but moves very fast. It flies along
the great arteries at perhaps ten inches in a second. Through the little
bit of capillaries along which it has to pass it creeps slowly, but
manages sometimes to go all the way round from vein to vein again in
about half a minute.
It is always moving at this rapid rate, and when it ceases to move, you
die.
=What makes it move?=