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.