EVB2936644: Circulation of the blood from William Harvey's ON THE MOTIONS OF THE HEART AND BLOOD, 1628. Illustrations of a tourniqueted human forearms with pronounced superficial blood veins to indicate circulation. Fig. 3: Blood flow is blocked at two points, O and H. At O, the blood pools in a valve and does not flow toward the hand, but flows toward the heart. Fig. 4: Blood flow is blocked at two points, M and N, with no valve within section / Bridgeman Images