How did William Harvey study the heartbeat in living animals?

William Harvey studied the heartbeat by observing the hearts of various living animals, specifically focusing on cold-blooded creatures and animals near death. Because these subjects had slower heartbeats, Harvey was able to closely examine the mechanics of the organ's motion.1

Prior to his research, it was incorrectly believed that the heart's active phase involved the muscle expanding to draw blood in. Through his observations, Harvey determined that the active phase of the heartbeat occurs when the heart muscles contract, decreasing the organ's internal volume and expelling blood with significant force. These findings were central to his landmark work, Anatomical Exercise on the Motion of the Heart and Blood in Animals, which established the concept of blood circulation.1