English physician who, by observing the action of the heart in small animals and fishes, proved that heart receives and expels blood during each cycle. Experimentally, he also found valves in the veins, and correctly identified them as restricting the flow of blood in one direction. He developed the first complete theory of the circulation of blood, believing that it was pushed throughout the body by the heart's contractions. He published his observations and interpretations in Exercitatio Anatomica de Motu Cordis et Sanguinis in Animalibus (1628), often abbreviated De Motu Cordis.

Harvey also noted, as earlier anatomists, that fetal circulation short circuits the lungs. He demonstrated that this is because the lungs were collapsed and inactive. Harvey could not explain, however, how blood passed from the arterial to the venous system. The discovery of the connective capillaries would have to await the development of the microscope and the work of Malpighi. He was heavily influenced by the mechanical philosophy in his investigations of the flow of blood through the body. In fact, he used a mechanical analogy with hydraulics. He could not, however, explain why the heart beats. Furthermore, Harvey used quantitative methods to measure the capacity of the ventricles.

Harvey was the first doctor to use quantitative and observation methods simultaneously in his medical investigations. In Exercitationes de Generatione Animalium (On the Generation of Animals, 1651), he was extremely skeptical of spontaneous generation and proposed that all animals originally came from an egg. His experiments with chick embryos were the first to suggest the theory of epigenesis, which views organic development as the production in a cumulative manner of increasingly complex structures from an initially homogeneous material.

Malpighi