In 1628, after 20 years of experimenting, William Harvey (1578-1657) finally felt confident enough to publish 'De motu cordis'. In this book Harvey described how the blood circulates through the body: that the blood travels through arteries to the different organs and returns to the heart via veins. Although at that stage he was unable to see them himself he postulated that the blood passes through tiny capillaries allowing the transition from arterial to venous phase. Also, he described the small circulation from heart to lungs and back. He realised that the small and large circulation are separated only by the heart septum which therefore cannot be permeable to blood.