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During ancient times and the Middle Ages, knowledge about veins was more accurate than about arteries and the heart. The description of valves in the veins in 1536 induced William Harvey to think that blood flows unidirectionally. This reasoning led him to the discovery of circulation in 1628. Thereafter the venous system received little attention. Venous pressures were first measured by Stephen Hales in 1733. Albrecht von Haller (1707-1788) described the respiratory pump aiding venous return. Introduction of the mercury manometer by Jean Poiseuille (1799-1869) facilitated research on arteries but discouraged venous research by making low pressures more difficult to measure. Pierre Potain (1825-1901) and Sir James Mackenzie (1853-1925) analyzed the venous pulse with tambour recorders. Kenneth Franklin renewed interest in the subject through his book A Monograph on Veins (1937). Development of high-fidelity sensitive instruments during the last two decades finally resolved some fundamental problems concerning the venous system.