Girolamo Fabrici (1537-1619) studies leg veins and notices that they have valves which only allow blood to flow toward the heart.
Editors. "Observation on venous blood circulation". The Literary Encyclopedia. First published 01 May 2009; last revised 30 November -1.
[http://www.litencyc.com/php/stopics.php?rec=true&UID=6088, accessed 20 April 2015.]