A historical illustration from "On The Motion Of The Heart And Blood In Animals" (1628) by the English physician William Harvey (1578-1637). This image shows the veins in an arm with a tourniquet on it, illustrating the way the valves in the veins control the flow of blood back to the heart. While the hand at upper left has stopped the blood just in front of a valve (at O), the valve prevents the blood from flowing back towards the hand.