William Harvey, "On the Circulation of the Blood" (1628)
William Harvey (1578-1657) argued in his medical treatise On the Motion of the Heart and Blood in Animals (1628) that blood circulation in the human body does not originate in the liver and pass from one side of the heart to the other through pores, but rather that the human heart acts like a pump (or a water bellows, in his famous example). He thus established the field of modern physiology.
As this book alone declares the blood to course and revolve by a new route, very different from the ancient and beaten pathway trodden for so many ages, and illustrated by such a host of learned and distinguished men, I was greatly afraid lest I might be charged with presumption did I lay my work before the public at home, or send it beyond seas for impression, unless I had proposed its subject to you, had confirmed its conclusions by ocular demonstrations in your presence, had replied to your doubts and objections, and secured the assent and support of our distinguished President. For I was most intimately persuaded, that if I could make good proposition before you and our College,…I had less to fear from others…For true philosophers, who are only eager for truth and knowledge, never regard themselves as already so thoroughly informed, but that they welcome further information from whomsoever and from whencesoever it may come; nor are they so narrow-minded as to imagine any of the arts or sciences transmitted to us by the ancients, in such a state of forwardness or completeness, that nothing is left for the ingenuity and industry of others…Neither do they swear such fealty to their mistress Antiquity, that they openly, and in sight of all, deny and desert their friend Truth…
My dear colleagues…I profess both to learn and to teach anatomy, not from books, but from dissections; not from the positions of philosophers, but from the fabric of nature…
From these and other observations of the like kind, I am persuaded it will be found that the motion of the heart is as follows:
First of all, the auricle contracts, and in the course of its contraction throws the blood, (which it contains in ample quantity as the head of the veins, the store-house and cistern of the blood,) into the ventricle, which being filled, the heart raises itself straightway, makes all its fibers tense, contracts the ventricles, and performs a beat, by which beat it immediately sends the blood supplied to it by auricle into the arteries; the right ventricle sending its charge into the lungs by the vessel which is called vena arteriosi, but which, in structure and function, and all things else, is an artery; the left ventricle sending its charge into the aorta, and through this by the arteries to the body at large…
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…But what that remains to be said upon the quantity and source of the blood which thus passes, is of so novel and unheard-of character, 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…Still, the die is cast, and my trust is in my love of truth, and the candor that inheres in cultivated minds. And when I surveyed my mass of evidence…I 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;…I began to think whether there might not be A MOTION, AS IT WERE, IN A CIRCLE. Now this I afterwards found to be true; and I finally saw the blood, forced by the action of the left ventricle into the arteries, was distributed to the body at large…impelled by the right ventricle…through the veins, and so round to the left ventricle in the manner already indicated…
The heart, consequently, is the beginning of life; the sun of the microcosm, even as the sun in his turn might well be designated the heart of the world; for it is the heart by whose virtue and pulse the blood is moved, perfected, made apt to nourish, and is preserved from corruption and coagulation; it is the household divinity which, discharging its function, nourishes, cherishes, quickens the whole body, and is indeed the foundation of life, the source of all action.
Harvey, William. On the Motion of the Heart and Blood in Animals. Willis's translation rev. and edited by Alex. Bowie. Chicago: H. Regnery Co., 1962.