Chapter Nine on experimental evidence
Chapter Fourteen Conclusion of Harvey's 1628 work.
 But lest anyone say that we cheat and merely make plausible assertions without a basis and advance new views without just cause, there are three suppositions which come up for confirmation. If these are stated, then I think the truth which I advocate automatically follows and the fact is plain to all.
The first supposition is that the blood is continuously and uninterruptedly transmitted by the beat of the heart from the vena cava into the arteries in such amount that it cannot be supplied from the ingesta, and thus in such wise that the whole mass of the blood passes across from the vena cava into the arteries within a short space of time.
The second supposition is that the blood is continuously, evenly, and uninterruptedly driven by the beat of the arteries into every member and part, entering each in far greater amount than is sufficient for its nutrition or can be supplied to it [without such rapid circular movement] by the whole mass of the blood.
The third supposition, similarly, is that the veins themselves are constantly returning this blood from each and every member to the region of the heart.
With these suppositions thus stated, I think it will be manifest that the blood goes round and is returned, is driven forward and flows back, from the heart to the extremities, and thence back again to the heart, and so executes a sort of circular movement.
Let us estimate, either theoretically or by actual testing,  how much blood the left ventricle holds in its dilated state, that is, when it is full. Say this is two or three or one and a half ounces—I have found over two in a cadaver. Let us similarly estimate how much less the heart holds in its contracted state, in other words, what is the degree of its contraction; how much the ventricle's capacity is reduced in its contraction or contractions; how much blood it extrudes into the aorta—that it always extrudes some has been shown in Chapter Three above, and is agreed by all on the evidence of the structure of the valves. From all this let us feel that we may, by a reasonable inference, declare the amount ejected into the artery to be a quarter or a fifth or a sixth, or at the least an eighth, part of the dilated ventricle's content.
In man, then, let us take the amount that is extruded by the individual beats, and that cannot return into the heart because of the barrier set in its way by the valves, as half an ounce, or three drachms, or at least one drachm. In half an hour the heart makes over a thousand beats; indeed, in some individuals, and on occasion, two, three, or four thousand. If you multiply the drachms per beat by the number of beats you will see that in half an hour either a thousand times three drachms or times two drachms, or five hundred ounces, or other such proportionate quantity of blood has been passed through the heart into the arteries, that is, in all cases blood in greater amount than can be found in the whole of the body. Similarly in the sheep or the dog. Let us take it that one scruple passes in a single contraction of the heart; then in half an hour a thousand scruples, or three and a half pounds of blood, do so. In a body of this size, as I have found in the sheep, there is often not more than four pounds of blood.
In the above sort of way, by calculating the amount of blood transmitted [at each heart beat] and by making a  count of the beats, let us convince ourselves that the [whole amount of the blood mass goes through the heart from the veins to the arteries and similarly makes the pulmonary transit.
Even if this may take more than half an hour or an hour or a day for its accomplishnnent, it does nevertheless show that the beat of the heart is continuously driving through that organ more blood than the ingested food can supply, or all the veins together at any given time contain.
Nor should it be said that the heart in its contraction varies between extruding a virtual nothing and an imaginary something, such a view having been already confuted, and in addition being contrary to sense and to reason. For, if the ventricles must fill up again with blood in the heart's diastole, they must always extrude blood in their systole, and that in no niggardly fashion (since the blood channels are not small or the degree of the heart's contraction minute), whether the fraction be taken as a third or a sixth or an eighth of the original volume. The ratio of the blood that escapes expulsion to that previously contained in the ventricle and refilling it in its diastole must likewise correspond to that obtaining between the capacity of the ventricle in its contracted state and the capacity of the same chamber in its dilated state. And, as in its dilatation it cannot be refilled with a zero or imaginary charge of blood, so in its contraction it never expels a zero or imaginary amount, but always a positive one proportionate to the degree of that contraction. Whence it is to be concluded that, if the heart in one beat in man, the sheep, or the ox, emits one drachm, and there are a thousand beats in half an hour, ten pounds five ounces have been transmitted in the same time; if in one beat it emits two drachms, the total is twenty pounds ten ounces; if in one beat half an ounce, the total  is forty one pounds eight ounces; finally, if it is an ounce at each beat, eighty three pounds four ounces have been transfused in half an hour from the veins into the arteries. But how much is extruded at each heart beat in the individual pulsations, and when more and when less, and for what reason, will perhaps be made clearer by me later as a result of numerous observations.
Meanwhile this much I know, and may I remind all men of it, that the blood makes the passage at times in greater, at times in lesser, amount; and that its circuit is effected now more rapidly and now more slowly according to temperament, age, external and internal causes, things natural and non-natural, sleep, rest, feeding, exercises, mental affections, and the like. But indeed, if the blood passes in even minimal amount through the lungs and the heart, it is carried to the arteries and to the whole of the body in far richer amount than can be supplied from the ingestion of foodstuffs or, in general, without it making a circuitous return.
This we see clearly when we watch the dissection of living animals. For not only when the aorta is opened but also (as Galen establishes in respect of the human subject) if even a very small artery is divided, the whole mass of the blood, within the space of half an hour or less, will be withdrawn from the veins as well as the arteries throughout the body. Butchers, likewise, can produce adequate confirmation of this truth. For, if the neck arteries are cut in the slaughtering of an ox, they drain off the whole mass of the blood, and empty all the vessels, in under a quarter of an hour. In the amputation of limbs or the excision of tumours, I have sometimes found a rapid occurrence of the same end-effect.
The force of this argument is not lessened by allegations that the venous outflow of blood equals or exceeds the arterial one when an animal's throat is cut or human  limbs are amputated. For the opposite is in fact what happens. The veins, since they collapse, and have no intrinsic power to force blood out of themselves, and since (as will appear later), the situation of their valves is an obstacle, produce very little blood. The arteries, on the other hand, spout out relatively freely and abundantly a rushing torrent of blood. But you should put the matter to the test in a sheep or a dog by leaving the vein intact and incising the neck artery alone. This will give you a striking impression of the vehemence, force, and rapidity with which the body as a whole loses all its blood, leaving both veins and arteries empty. From what I have said above, it is clear that the blood which the arteries receive can only reach them by being passed through the heart. If, therefore, you ligate the aorta near the root of the heart, and open the neck-artery, or some other one, you will without doubt see nothing but empty arteries and full veins.
From my supposition you will see plainly why, in dissections, so much blood is found in the veins, but little in the arteries; why much in the right ventricle, little in the left one. It was, perhaps, this difference which gave the ancients occasion for doubt and led them to think that, during the life of the animal, spirits alone were contained in those cavities [i.e. in the arteries]. The real reason is, perhaps, that blood can pass from the veins into the arteries in no other way than through the heart itself and the lungs. When these latter have stopped moving at the end of expiration, the blood is prevented from flowing out of the small branches of the artery-like vein into the vein-like artery and thence into the left ventricle of the heart (in the embryo we earlier noted that it had been prevented because the lungs were motionless and so did not open and close successively the minute connections of the vessels and the invisible  porosities). As, however, the heart does not cease to move when the lungs do, but continues thereafter to beat and to survive, the left ventricle and the arteries go on discharging blood systematically round the body and into the veins, but receive no replenishments through the lungs and are thus virtually emptied. But even this is some encouragement to belief in my contention, since no other reason can be adduced for the phenomenon save that which I state as deriving from my supposition.
In addition it is clear from the supposition that the more often or more strongly the arteries pulsate, the more rapidly will the body be exhausted of blood in haemorrhages. It is also clear that, whenever the heart beats more languidly, more feebly, and without vigour, any haemorrhage is allayed and held in check.
Further, the supposition explains why in a cadaver, after the heart has stopped beating, you will be unable by any effort you make to withdraw from the opened neck or leg veins and arteries more than a half of the blood mass. Nor will a butcher be able to bleed an ox completely, after stunning it with a blow on the head, unless he has cut its throat before the heart has ceased to beat.
Finally, my supposition allows me to surmise why no one has yet made a correct statement about the site, mechanism, and causation of the anastomosis of the veins and the arteries. I am now busy with that research.
 May I now be permitted to summarize my view about the circuit of the blood, and to make it generally known!
Since calculations and visual demonstrations have confirmed all my suppositions, to wit, that the blood is passed through the lungs and the heart by the pulsation of the ventricles, is forcibly ejected to all parts of the body, therein steals into the veins and the porosities of the flesh, flows back everywhere through those very veins from the circumference to the centre, from small veins into larger ones, and thence comes at last into the vena cava and to the auricle of the heart; all this, too, in such amount and with so large a flux and reflux— from the heart out to the periphery, and back from the periphery to the heart—that it cannot be supplied from the ingesta, and is also in much greater bulk than would suffice for nutrition.
I am obliged to conclude that in animals the blood is driven round a circuit with an unceasing, circular sort of movement, that this is an activity or function of the heart which it carries out by virtue of its pulsation, and that in sum it constitutes the sole reason for that heart's pulsatile movement.