49 The Circulation of Blood 1628 IT TOOK ROUGHLY 2,000 years of medical sleuthing to unlock the secrets of the circulatory system. Aristotle started the search, hypothesizing that the liver was the source of blood. But it wasn't until the 16th century that physicians began uncovering enough clues about arteries, veins and the heart to propose new theories and to challenge professional doctrine. Ignoring the threat of ostracism, British physician William Harvey spent 20 years researching the circulatory system and writing An Anatomical Study of the Motion of the Heart and of the Blood in Animals, published in 1628. For the first time, he demonstrated that the heart controls circulation. His conclusions were met with scorn. But his description of how blood flows away from the heart in arteries, then back through veins--still valid nearly 400 years later--remains one of the most significant medical discoveries of the millennium, a testament to observation, accurate description and mathematical proof.