The correct description of the circulation of blood was provided by English physician William Harvey (1578–1657).
In the course of many experimental dissections, he established the existence of pulmonary circulation (blood flowing from heart to lungs to heart) and noted the one-way flow of blood.
In his observations about the heart and blood vessels, however, Galen made critical errors that remained virtually unchallenged for 1,400 years.
He mistakenly believed that blood was formed in the liver and was circulated throughout the body by the veins.
Together with Erasistratus, Herophilus established the disciplines of anatomy and physiology (the science that deals with the function of the body’s parts and organs).
In his studies of the heart and blood vessels, Erasistratus came very close to working out the circulatory system of the blood.Vesalius, who founded modern scientific anatomy, noted obvious conflicts between what he saw in his dissections of the human body and what Galen had described.He reasoned that Galen’s errors resulted from only having done animal dissections, which often did not apply to human anatomy.He distinguished the cerebrum (larger portion) from the cerebellum (smaller portion), suggested that the brain was the seat of intelligence, and identified and named several structures of the brain, some of which still carry the names he gave them.He also discovered that nerves originate in the brain and noted the difference between motor nerves (those concerned with motion) and sensory nerves (those related to sensation).He understood that the heart served as a pump and he studied and explained the function of the heart valves.Erasistratus theorized that the arteries and veins both spread from the heart but incorrectly believed that the arteries carried air instead of blood.Bichat found that organs were built up out of different types of simpler structures, and each of these simpler structures could occur in more than one organ.He further noted that different tissues have specific properties and are thereby vulnerable to tissue-specific diseases.The discovery of capillaries (small blood vessels) by Italian anatomist Marcello Malpighi (1628–1694) in 1661 provided the factual evidence to confirm Harvey’s theory of blood circulation.Malpighi discovered the capillaries—the tiny connecting links between the veins and arteries—using the newly invented microscope.