The practices of modern medicine have their roots in the 1600s. It was early in the century when the work of English physician William Harvey (1578–1657) demonstrated to the science community that effective medicine depends on knowledge of the body’s structure. From 1597 to 1602 Harvey studied medicine at Padua (Italy) under Italian surgeon Fabricius (or Fabrici; 1537–1619) and went on to perform numerous experiments to learn how blood circulates through the body. In his studies, Harvey discarded the accepted method of studying parts of a problem and then filling in the gaps with theory; instead he aimed to understand the entire circulatory system, studying the pulse and heartbeat, and performing dissections on cadavers. He accurately concluded that the heart pumps blood through the arteries to all parts of the body and that the blood returns through the veins to the heart. Putting his discovery into writing, Harvey published An Anatomical Study of the Motion of the Heart and of the Blood in Animals in 1628.

Another medical development during the 1600s came not at the hands of a physician or surgeon, but rather a naturalist, Antoni van Leeuwenhoek (1632–1723). A surveyor to the court of Holland, van Leeuwenhoek began making his own microscopes and used them to study organisms invisible to the naked eye—he had discovered microorganisms. Leeuwenhoek also observed (but did not name) bacteria, and he accurately described red blood corpuscles, striated muscle fibers, and the lens of the eye. This amateur scientist also disproved the theory of spontaneous generation, the belief that living organisms could be generated by lifeless matter.