In the heart itself two distinct motions are observed—first of the auricles, and then of the ventricles. These alternate contractions and dilatations can have but one result, namely, to force the blood from the auricle to the ventricle, and from the ventricle, on the right side, by the pulmonary artery to the lungs, and on the left side by the aorta to the system.
These considerations suggest to the mind of Harvey the idea of the circulation. "I began to think," he says, "whether there might not be a motion, as it were, in a circle." This is next established by proving the three following propositions:—
(1) The blood is incessantly transmitted by the action of the heart from the vena cava to the arteries in such quantity that it cannot be supplied from the ingesta, and in such wise that the whole mass must very quickly pass through the organ;
(2) 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 were sufficient for nutrition, or than the whole mass of fluids could supply;
(3) The veins in like manner return this blood incessantly to the heart from all parts and members of the body.
As to the first proposition Harvey says, "Did the heart eject but two drachms of blood on each contraction, and the beats in half an hour were a thousand, the quantity expelled in that time would amount to twenty pounds and ten ounces; and were the quantity an ounce, it would be as much as eighty pounds and four ounces. Such quantities, it is certain, could not be supplied by any possible amount of meat and drink consumed within the time specified. It is the same blood, consequently, that is now flowing out by the arteries, now returning by the veins; and it is simply matter of necessity that the blood should perform a circuit, or return to the place from whence it went forth."
Demonstration of the second proposition—that the blood enters a limb by the arteries and returns from it by the veins—is afforded by the effects of a ligature. For if the upper part of the arm be tightly bound, the arteries below will not pulsate, while those above will throb violently. The hand under such circumstances will retain its natural colour and appearance, although, if the bandage be kept on for a minute or two, it will begin to look livid and to fall in temperature. But if the bandage be now slackened a little, the hand and the arm will immediately become suffused, and the superficial veins show themselves tumid and knotted, the pulse at the wrist in the same instant beginning to beat as it did before the application of the bandage. The tight bandage not only compresses the veins, but the arteries also, so that blood cannot flow through either. The slacker ligature obstructs the veins only, for the arteries lie deeper and have firmer coats. "Seeing, then," says Harvey, "that the moderately tight ligature renders the veins turgid, and the whole hand full of blood, I ask, Whence is this? Does the blood accumulate below the ligature coming through the veins, or through the arteries, or passing by certain secret pores? Through the veins it cannot come; still less can it come by any system of invisible pores; it must needs, then, arrive by the arteries."
The third position to be proved is that the veins return the blood to the heart from all parts of the body. That such is the case might be inferred from the presence and disposition of the valves in the veins; for the office of the valves is by no means explained by the theory that they are to hinder the blood from flowing into inferior parts by gravitation, since the valves do not always look upwards, but always towards the trunks of the veins, invariably towards the seat of the heart. The action of the valves is then demonstrated experimentally on the arm bound as for blood-letting. The point of a finger being kept on a vein, the blood from the space above may be streaked upwards till it passes the valve, when that portion of the vein between the valve and the point of pressure will not only be emptied of its contents, but will remain empty as long as the pressure is continued. If the pressure be now removed, the empty part of the vein will fill instantly and look as turgid as before.
Other confirmatory evidence is then added, e.g. the absorption of animal poisons and of medicines applied externally, the muscular structure of the heart and the necessary working of its valves.
William Harvey, the illustrious physiologist, anatomist, and physician, to whom this discovery is due, was the eldest son of a Kentish yeoman, and was born in April, 1578. At the age of ten he entered the Canterbury Grammar School, where he appears to have remained for some years. At sixteen he passed to Caius-Gonvil College, Cambridge, and three years afterwards took his B.A. degree and quitted the university. Like most students of medicine of that day, he found it necessary to seek the principal part of his professional education abroad. He travelled to Italy, selected Padua as his place of study, and there continued to reside for four years, having as one of his teachers the famous Fabricius of Aquapendente. On his return to England, in 1602, he took his doctor's degree at Cambridge, and entered on the practice of his profession.