CIRCULATION OF BLOOD IN THE TADPOLE

MOST HONOURABLE GENTLEMEN, etc.

My last, most respectful letter to you was of the 24th of last month, in which I treated of the sting of the gnat, namely, that this sting, taken out of its case, consists of four distinct stings.

Herewith I again send you some of my trifling observations.

In our country we have two species of frogs. The first, which we used to have in great abundance round about our town, is commonly called frog. Of late years, however, these frogs have been very rare. I suppose because our sluggish water‑courses have lately beers filled, as it were, with a kind of noxious little fish (hitherto unknown, so far as I am aware), called stickle‑backs, which devoured the frogs when they were still tadpoles.

I was greatly pleased to see very distinctly the circulation of the blood [in the tadpole, which was driven on from the parts that were nearest to the body to those on the outside, thus causing an uninterrupted, very rapid circulation. This circulation was not regular in its movement, but at very short intervals it was continually brought about anew with sudden impulses, and before there was another sudden impulse we might (in case we had not observed a continual increase in the rapidity) have thought that a stoppage in the circulation would follow. But scarcely had the blood begun to move more slowly when there was again a sudden impulse of the blood, so that there was an uninterrupted current; and trying accurately to measure the very short time in which each impulse took place, 1 found that in the time wanted to count rapidly to a hundred, there were as many as a hundred sudden impulses. From this I concluded that as often as these sudden impulses occurred, the blood was driven from the heart.

In another place I saw that three of the thinnest arteries, each running in a curve, all met together in one point and there formed a blood vessel or vein, and consequently this blood vessel was as wide as the three arteries mentioned. These three distinct vessels with their somewhat circular course, in which the circulation took place, were so small that a grain of sand could have covered them.

Such blood vessels running across each other I often noticed before when I tried to discover the junction of arteries and veins in other animals, but I was quite certain that the return circuit of the blood does not take place in the large vessels, but in the smallest or thinnest; for if it were otherwise, I conclude that all the parts of the body could not be fed. And as these discoveries seemed inscrutable to me, I gave up my investigations on this head for some years. If now we see clearly with our eyes that the passing of the blood from the arteries into the veins in the tadpoles only takes place in such blood vessels as are so thin that only one corpuscle can be driven through at one time, we may conclude that the same thing takes place in the same way in our bodies, as well as in that of all animals. And this being so, it is impossible for us to discover the passing of the blood from the arteries into the veins in our bodies or that of other animals, first, because a single globule of blood being in a vein. has no colour; secondly, because the blood does not move in the blood vessels when we make this investigation.

I have said before that the corpuscles or globules that make the blood red are so small that ten hundred thousand of them arc not so big as a grain of coarse sand; and so we can easily imagine how very small the blood vessels are in which the circulation of the blood takes place.

The observations told here have not been made once, but they have been resumed repeatedly, giving me much pleasure, and every time on different tadpoles, and the result has almost always been the same. But it is remarkable that in the very small vessels mentioned above and placed furthest from the heart, as there at the end of the tail, the impulse was not by far so sudden and strong as in the vessels nearest to the heart. And though the uninterrupted current could be clearly observed, it could be distinctly seen that at each impulse from the heart the current was a little quicker.

When I looked along the length of the tail and at the thickest part of it, I could clearly see that on either side of the bone there was a large artery, through which the blood was carried to the extremity of the tail, and which on its way sent out several small branches.

When I looked at the part of the tail beside these arteries on the outside, I discovered there two large veins, which carried the blood back again to the heart, and, moreover, I saw that blood was driven into this large vein from several small veins. In short, I saw here the circulation of the blood to my perfect satisfaction, because there was nothing, though ever so slight, that caused me any doubt.

Also, I observed the young frogs when they had changed from tadpoles into frogs, and I also discovered in them a very large number of small blood vessels, which, continually running in curves, formed the vessels called "arteries and veins" (capillaries), from which it was perfectly clear to me that the arteries and veins are one and the same continuous blood vessels. But I saw them clearest of all and most of all at the end of the projecting parts of the legs, which we may call fingers, and of which the frog has four on each fore‑leg and five on each hind‑leg.

These blood vessels, called "arteries and veins" (being nevertheless identical), were exceedingly numerous at the ends of these fingers, and each ran in a curve, which made it impossible to follow the particular course of each vessel. All these vessels were so small or thin that no more than one corpuscle could pass through it at a time. But when I examined these fingers about the first or second joint; I found the blood vessels there, which we call arteries and veins, bigger, so big even that the blood in these vessels had a red colour.