The heart and blood have been recognized since earliest times as vital elements of life, but the way in which they function was not understood until William Harvey's discovery of the mechanics of the circulatory system in the seventeenth century.

Various Greek physicians had distinguished arteries from veins, but each—Alcmaeon (born c. 535 B.C.) and Praxagoras (born c. 340 B.C.)--thought that arteries carried air, since arteries are usually empty in corpses (the source for most of these investigations). The Alexandrian anatomist Herophilus observed the arterial pulse, which he associated with the heartbeat, and held that arteries contain blood, not air. His fellow Alexandrian, Erasistratus, came close to a concept of the circulatory system, maintaining that both arteries and veins originated from the heart and divided repeatedly throughout the body, ending in fine capillaries. Erasistratus saw the heart as a pump, but he repeated...