William Harvey

by Sir D'Arcy Power

Thus far Harvey’s teaching has been excellent, but now, leaving the highway of fact, he plunges into theory and is at once involved in error. He proceeds, “And now the discussion is brought to this point, that they who inquire into the ways by which the blood reaches the left ventricle of the heart and pulmonary veins from the vena cava will pursue the wisest course if they seek by dissection to discover why, in the larger and more perfect animals of mature age, Nature has rather chosen to make the blood percolate the parenchyma of the lungs, than as in other instances chosen a direct and obvious course—for I assume no other path or mode of transit can be entertained. It must be because the larger and more perfect animals are warmer, and when adult their heat greater, ignited I might say, and requiring to be damped or mitigated, that the blood is sent through the lungs, in order that it may be tempered by the air that is inspired, and prevented from boiling up and so becoming extinguished or something else of the sort. But to determine these matters and explain them satisfactorily were to enter upon a speculation in regard to the office of the lungs and the ends for which they exist. Upon such a subject, as well as upon what pertains to respiration, to the necessity and use of the air, &c., as also to the variety and diversity of organs that exist in the bodies of animals in connection with these matters, although I have made a vast number of observations, I shall not speak till I can more conveniently set them forth in a treatise apart.”

The next chapter is devoted to the description of the manner in which the blood passes through the substance of the lungs from the right ventricle of the heart into the pulmonary veins. It is followed by the glorious eighth chapter, in which Harvey’s style, always impressive and solid, rises into real eloquence, for a great occasion justifies the use of repetitions, of antitheses and an abundance of metaphors. He now quits the method of demonstration and experiment for that of indirect but irrefragable argument. He deals with the quantity of blood passing through the heart from the veins to the arteries, and again brings together all his threads to a nodal point. “Thus far I have spoken of the passage of the blood from the veins into the arteries, and of the manner in which it is transmitted and distributed by the action of the heart; points to which some, moved either by the authority of Galen or Columbus, or the reasonings of others, will give their adhesion. But what remains to be said upon the quantity and source of the blood which thus passes is of a character so novel and unheard of that I not only fear injury to myself from the envy of a few, but I tremble lest I have mankind at large for my enemies, so much doth wont and custom become a second nature. Doctrine once sown strikes deeply its root, and respect for antiquity influences all men. Still the die is cast, and my trust is in my love of truth and the candour of cultivated minds. And sooth to say when I surveyed my mass of evidence, whether derived from vivisections and my various reflections on them, or from the study of the ventricles of the heart and the vessels that enter into and issue from them, the symmetry and the size of these conduits, for Nature doing nothing in vain, would never have given them so large a relative size without a purpose—or from observing the arrangement and intimate structure of the valves in particular and of the other parts of the heart in general, with many things besides, I frequently and seriously bethought me and long revolved in my mind, what might be the quantity of blood which was transmitted, in how short a time its passage might be effected and the like. But not finding it possible that this could be supplied by the juices of the ingested aliment without the veins on the one hand becoming drained, and the arteries on the other getting ruptured through the excessive charge of blood, unless the blood should somehow find its way from the arteries into the veins and so return to the right side of the heart; I began to think whether there might not be a movement, as it were, in a circle. Now this I afterwards found to be true, and I finally saw that the blood, forced by the action of the left ventricle into the arteries, was distributed to the body at large and in several parts in the same manner as it is sent through the lungs impelled by the right ventricle into the pulmonary artery, and that it then passed through the veins and along the vena cava and so round to the left ventricle in the manner already indicated. This movement we may be allowed to call circular.”

Harvey’s great discovery is here formulated in his own words. The lesser or pulmonary circulation was already tolerably well known, owing to the work of Realdus Columbus, the successor of Vesalius in the anatomical chair at Padua, though he had been anticipated by Servetus, who published it at Lyons in 1543 in the “Christianismi Restitutio,” a theological work, containing doctrines for which Calvin caused him to be burnt. But it is more than doubtful whether Harvey knew of this work, as not more than three or four copies of it have escaped the flames which consumed the book and its writer.

Harvey continues his treatise by laying down three propositions to confirm his main point that the blood circulates.

First, that the blood is incessantly transmitted by the action of the heart from the vena cava to the arteries.

Secondly, that the blood under the influence of the arterial pulse enters and is impelled in a continuous, equable, and incessant stream through every part and member of the body, in much larger quantity than is sufficient for nutrition or than the whole mass of fluids could supply.

Thirdly, that the veins return this blood incessantly to the heart. “These points being proved, I conceive it will be manifest that the blood circulates, revolves, is propelled, and then returning from the heart to the extremities, from the extremities to the heart, and thus that it performs a kind of circular movement.”

These propositions Harvey proves to demonstration and in a most masterly manner. He says of the first: “Let us assume either arbitrarily or by experiment, that the quantity of the blood which the left ventricle of the heart will contain when distended to be, say two ounces, three ounces, or one ounce and a half—in the dead body I have found it to hold upwards of two ounces. Let us assume further how much less the heart will hold in the contracted than in the dilated state, and how much blood it will project into the aorta upon each contraction, and all the world allows that with the systole something is always projected ... and let us suppose as approaching the truth that the fourth, or fifth, or sixth, or even but the eighth part of its charge is thrown into the artery at each contraction, this would give either half an ounce, or three drachms, or one drachm of blood as propelled by the heart at each pulse into the aorta, which quantity by reason of the valves at the root of the vessel can by no means return into the ventricle. Now in the course of half an hour the heart will have made more than one thousand beats, in some as many as two, three, or even four thousand. Multiplying the number of drachms by the number of pulses we shall have either one thousand half ounces, or one thousand times three drachms, or a like proportional quantity of blood, according to the amount we assume as propelled with each stroke of the heart, sent from this organ into the artery: a larger quantity in every case than is contained in the whole body. In the same way in the sheep or dog, say that but a single scruple of blood passes with each stroke of the heart, in one half hour we should have one thousand scruples, or about three pounds and a half of blood injected into the aorta, but the body of neither animal contains more than four pounds of blood, a fact which I have myself ascertained in the case of the sheep.”

This is one of the highest efforts of Harvey’s genius. The facts are simple and they are easily ascertained. But the reasoning was absolutely new and the conclusion must remain sound until the end of time, for it is true. It shows too the minute care taken by Harvey not to overstate his case, for he deliberately takes a measurement of the capacity of the ventricles which he knew to be well under the average.