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William Harvey's De motu cordis (courtesy of the National Library of Medicine).

of the lungs? If there were two distinct kinds of blood—nutritive blood from the liver distributed by the veins and blood from the heart for the distribution of vital spirits by the arteries—why were the two kinds of blood so similar? Such questions were not unlike those that Harvey's contemporaries were prepared to ask and debate.

Using arguments based on dissection, vivisection, and the works of Aristotle and Galen, Harvey proved that in the adult, all the blood must go through the lungs to get from the right side to the left side of the heart. He proved that the heart is muscular and that its most important movement is contraction, rather than dilation. But his most radical idea was that it was the beat of the heart that produced a continuous circular motion of the blood.

In warm-blooded animals, the systole (contraction) and diastole (expansion) of the heart are so rapid and complex that Harvey at first feared that only God could understand the motion of the heart. He solved this problem by using animals with simpler cardiovascular systems and a slower heartbeat, such as snakes, snails, frogs, and fish. With cold-blooded animals, or dogs bled almost to death, Harvey was able to create model systems that essentially performed in slow motion. When observations and experiments were properly analyzed, it was apparent that the motion of the heart was like that of a piece of machinery in which all the parts seemed to move simultaneously, until one understood the motions of the individual parts.

Harvey also posed a question of child-like simplicity that modern readers find most compelling, because the answer seems to be totally incompatible with Galenic theories. Yet, if this aspect of his work is overemphasized, it tends to remove Harvey from his seventeenth-century context and makes him appear more modern in outlook and approach than is really appropriate. Harvey asked himself: How much blood is sent into the body with each beat of the heart? Even the most cursory calculation proves that the amount of blood pumped out by the human heart per hour exceeds the weight of the entire individual. If the heart pumps out 2 ounces of blood with each beat and beats 72 times per minute, 8640 ounces (2 x 72 x 60), or 540 pounds, of blood is expelled per hour. Whether calculated for humans, sheep, dogs, or cattle, the amount of blood pumped out of the heart in an hour always exceeds the quantity of blood in the whole animal, as demonstrated by exsanguination. Skeptical readers could go to a butcher shop and watch an experienced butcher exsanguinate an ox. By opening an artery in a live animal, the butcher can rapidly remove all the blood.

It is all too easy to assume that these arguments should have provided an immediate deathblow to the Galenic system. However, the kind of evidence that appears most compelling today did not necessarily appeal to Harvey's contemporaries. Arguing from experimental and quantitative data in biology was remarkable in an era when even physicists were more likely to speculate than to weigh and measure. Moreover, opponents of Harvey's work presented what seemed to be quite logical alternatives, at least in light of accepted Galenic theory. For example, some critics argued that the heart attracted only a small amount of blood from the liver, where sanguification (the formation of blood) occurred. This blood foamed and expanded to such a great extent under the influence of the heat of the heart that the heart and arteries appeared to be full. Furthermore, multiplying the putative volume of blood discharged by the heart by the number of beats per minute was meaningless, because it was not necessary to assume that blood was driven from the heart through the arteries with each heartbeat.

Having solved the mechanical problem of the motion of the heart and blood, and demonstrated the true function of the venous valves, Harvey generally avoided arguments about the generation and distribution of the various kinds of spirits. Harvey had demonstrated the errors in Galen's system and had discovered essentially all that could be known about the structure and function of the cardiovascular system without the use of the microscope. Thus, one of the major gaps in Harvey's work was his inability to identify the structures joining the arterial and the venous system. He was forced to close this gap with hypothetical anastomoses or pores in the flesh. As scientists like Marcello Malpighi (1628-1694) extended the limits of anatomical study with the microscope, the capillary network completed the cardiovascular system.

Also unfinished at the time of Harvey's death was a book he planned to publish about his ideas on disease. The manuscript for this book may have been among those destroyed during the Civil War. Because of this loss, Harvey's concept of how knowledge of the circulation might solve questions about disease and medical practice must be constructed by piecing together comments made in his surviving works. De motu cordis promised that the new understanding of the circulation would solve many mysteries in medicine, pathology, and

William Harvey's demonstration of the role of the venous valves in the circulation of the blood.

therapeutics. In later works, Harvey alluded to his ''Medical Observations,'' but no such book was ever published.

Replacing the Galenic system that had so thoroughly, if incorrectly, explained the purpose of the heart, lungs, liver, veins, arteries, and spirits was completely beyond Harvey's technical and theoretical methods and goals. For seventeenth-century physicians, the new theory of the circulation raised more questions than it answered. If Harvey was correct, how could all the vital phenomena that Galenism had dealt with so long and so well be explained? For example, if the tissues did not consume the blood, how did they secure their nourishment? If the blood was not continuously formed from food by the liver, how was it synthesized? If the blood moved in a closed, continuous circle, what was the purpose of the arterial and venous systems and how did the body accomplish the generation and distribution of the vital spirit and the innate heat? If the venous blood did not originate in the liver, which had such a central role in the Galenic system, what was the function of this organ? If vital spirit was not produced by the mixture of air and blood in the lungs or in the left ventricle of the heart, what was the function of respiration? What was the difference between arterial and venous blood if all of the blood was constantly recirculated? If Galen were incorrect about the anatomy and physiology of the human body, what principles would guide medical practice?

Like almost all fundamental discoveries, Harvey's work provoked an avalanche of new questions and a storm of controversy. Many critics were unable or unwilling to understand the implications of Harvey's work. Others found it impossible to give up the old Galenic system that had provided all-encompassing rationalizations for health and disease, diagnosis, and therapeutics. How could medicine be saved if Galen was sacrificed for the sake of Harvey's radical theory? The theory of continuous circulation raised many disturbing questions for which Harvey provided no answers. Such questions stimulated Harvey's admirers to embark on new experimental ventures, while critics denounced his theory as useless, false, impossible, absurd, paradoxical, and harmful.

Well aware of the revolutionary nature of his work, Harvey predicted that no one under 40 would understand it. His work constituted a revolution in science worthy of comparison to that launched by Sir Isaac Newton. Although illness, age, and the loss of precious materials and manuscripts during the Civil War prevented Harvey from accomplishing all his goals, he did live to see his followers establish a new experimental physiology inspired by his ideas and methods. The questions raised by Harvey's work provided the Oxford physiologists—men such as Robert Boyle, Robert Hooke, Richard Lower, John Mayow, and Christopher Wren—with a new research program for attaining a better understanding of the workings of the human body.

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