William Harvey's circulation experiment. Illustrations of an arm with a tourniquet on it, showing the way the valves in the veins control the flow of blood back to the heart. The hand at upper left has stopped the blood just in front of a valve (at O), while another hand has stopped the blood at H. The vein remains empty as the valve prevents the blood from flowing back towards the hand. This artwork is from On The Motion Of The Heart And Blood In Animals (1628) by the English physician William Harvey (1578-1637).