William Harvey

by Sir D'Arcy Power

This part of his argument is ended with an appeal to practical experience. “The truth, indeed, presents itself obviously before us when we consider what happens in the dissection of living animals: the great artery need not be divided, but a very small branch only (as Galen even proves in regard to man), to have the whole of the blood in the body, as well that of the veins as of the arteries, drained away in the course of no long time—some half hour or less. Butchers are well aware of the fact and can bear witness to it; for, cutting the throat of an ox and so dividing the vessels of the neck, in less than a quarter of an hour they have all the vessels bloodless—the whole mass of blood has escaped. The same thing also occasionally occurs with great rapidity in performing amputations and removing tumours in the human subject.... Moreover it appears ... that the more frequently or forcibly the arteries pulsate, the more speedily will the body be exhausted of its blood during hæmorrhage. Hence also it happens that in fainting fits and in states of alarm when the heart beats more languidly and less forcibly, hæmorrhages are diminished and arrested.

“Still further, it is from this, that after death, when the heart has ceased to beat, it is impossible by dividing either the jugular or the femoral veins and arteries by any effort to force out more than one-half of the whole mass of the blood. Neither could the butcher ever bleed the carcass effectually did he neglect to cut the throat of the ox which he has knocked on the head and stunned before the heart had ceased beating.”

Harvey continues to push his argument to a logical conclusion in the succeeding chapters of his Treatise partly by argument and partly by adducing fresh experimental evidence. But if any one shall here object that a large quantity may pass through (the heart) and yet no necessity be found for a circulation, that all may come from the meat and drink consumed, and quote as an illustration the abundant supply of milk in the mammæ—for a cow will give three, four, and even seven gallons a day, and a woman two or three pints whilst nursing a child or twins, which must manifestly be derived from the food consumed; it may be answered, that the heart by computation does as much and more in the course of an hour or two.

“And if not yet convinced he shall still insist, that when an artery is divided, a preternatural route is, as it were, opened, and that so the blood escapes in torrents, but that the same thing does not happen in the healthy and uninjured body when no outlet is made ... it may be answered, that ... in serpents and several fish by tying the veins some way below the heart, you will perceive a space between the ligature and the heart speedily to become empty, so that unless you would deny the evidence of your senses, you must needs admit the return of the blood to the heart.... If, on the contrary, the artery instead of the vein be compressed or tied, you will observe the part between the obstacle and the heart and the heart itself to become inordinately distended, to assume a deep purple or even livid colour, and at length to be so much oppressed with blood that you will believe it about to be choked; but the obstacle removed, all things immediately return to their natural state in colour, size, and impulse.”

Harvey next proceeds to demonstrate his second proposition. He shows that the blood enters a limb by the arteries and leaves it by the veins; that the arteries are the vessels carrying the blood from the heart, and the veins the returning channels of the blood to the heart; that in the limbs and the extreme parts of the body the blood passes either immediately by anastomosis or mediately by the pores of the flesh.

Harvey is here hampered by the conditions of the age in which he lived, yet it is here that he shows himself far superior to his contemporaries as well as to the most enlightened of his predecessors. His lens was not sufficiently powerful to show him the capillary blood-vessels, and he had therefore no real knowledge of the way by which the blood passed from the arterioles into the venules. On the other hand, he did not repeat the mistake made by Aristotle, and reiterated by Cesalpino in 1571 that the blood passed from the smallest arteries into “capillamenta,” the νεῦρα of Aristotle.

Later commentators have given to Cesalpino the credit due to Harvey by translating “capillamenta” into our term capillaries. But this process of “reading into” the writings of man what he never knew is one of the commonest pitfalls of defective scholarship.

Harvey attempted to solve the problem of the capillary circulation by an appeal to clinical evidence, which soon led him into inaccuracies, as when he says that the fainting often seen in cases of blood-letting is due to the “cold blood rising upwards to the heart, for fainting often supervenes in robust subjects, and mostly at the moment of undoing the fillet, as the vulgar say from the ‘turning of the blood.’”

This Chapter XI. is an important one. Harvey takes the operation of bleeding as one which is familiar to every class of his readers, and he uses the various phenomena which attend the application of a ligature to the arm to clinch his arguments as to the existence of the circulation of the blood. He introduces incidentally his surgical and pathological knowledge, quoting, amongst other instances, the fact that if the blood supply to a tumour or organ be stopped, “the tissues deprived of nutriment and heat dwindle, die, and finally drop off.” He also introduces some pathological results from personal experience, for he says:—“Thrown from a carriage upon one occasion, I struck my forehead a blow upon the place where a twig of the artery advances from the temple, and immediately, within the time when twenty beats could have been made, I felt a tumour the size of an egg developed, without either heat or any great pain; the near vicinity of the artery had caused the blood to be effused into the bruised part with unusual force and velocity.”