William, 1578-1657. English anatomist, physiologist, and physician who first described the circulation of the blood in 1628. He understood that the interventricular septum is not porous so blood can not pass through it. He demonstrated the volume of blood which passes unidirectionally through a segment of a peripheral vein exceeds the volume of blood within the body, so blood must recirculate. He described the organization of the foetal circulation and the transition to the postnatal organization.