Harvey's discovery of the circulation of the blood is generally regarded as the most important breakthrough in the history of medicine. It is also the starting point of modern physiology. It had long been believed that blood was continually created afresh in the liver, which then sent it out to be absorbed by the body. Harvey, though experimentation, observation, and measurement of blood flow, realized that the circulation was a closed system in which the heart played the central role.

Although Harvey lived to see his theory generally accepted by the medical world, it first met considerable opposition. This third edition of De motu cordis - which is actually only the second complete one-prints the text interspersed with a point-by-point counter-argument by Emilio Parisano, one of Harvey's most vocal opponents. Harvey's professor at Padua, Girolamo Fabrizio [Fabricius], had discovered the valves of the veins but had not understood their purpose. When Harvey wanted to demonstrate that the valves directed the venous blood flow back to the heart, he simply adapted a plate from one of his former professor's works, De venarum ostiolis. This is the only illustration in all editions of De motu cordis.