During his medical career, Harvey focused much of his research on the mechanics of blood flow in the human body. Most physicians of the 17th-century considered lungs responsible for moving the blood throughout the body. Harvey's famous "Exercitatio Anatomica de Motu Cordis et Sanguinis in Animalibus" commonly was published in Latin at Frankfurt in 1628. At that time, Harvey was 50 years old. The first English translation appeared two decades later. Observing the functioning of the heart in living animals, he was able to see that systole was the active phase of the heart's movement, pumping out the blood by its muscular contraction. Then he saw that the valves in the veins permit the blood to flow only in the direction of the heart and to prove that the blood circulated around the body and returned to the heart.
