Let us now return to the blood which nourishes the organs.
LETTER XVII.
ARTERIAL AND VENOUS BLOOD.
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.