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.