Inspired by Homer's description of the ebb and flow of the "wine dark sea," the ancient Greeks conceived a back-and-forth movement of blood. That false notion prevailed for 1500 years until William Harvey proved that the heart pumps blood in one direction through the arteries and it returns through the veins. Here cardiologist Dhun Sethna argues that Harvey's revelation inaugurated modern medicine and paved the way for groundbreaking advances, from intravenous therapy, cardiac imaging, and stent insertions to bypass surgery, dialysis, and heart-lung machines.