"movement of the heart"
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In 1628, William Harvey published De Motu Cordis ("On the Motion of the Heart") and overturned 1,500 years of received wisdom. He proved that the heart is a muscle, a pump, and the engine of a single unified circulatory system. Blood doesn't seep slowly through the body as Galen had claimed; it flows continuously, driven by the force of each contraction and returned through veins lined with one-way valves.
Harvey also observed that the hearts of athletes and working animals were visibly larger and stronger than those of the sedentary: an early glimpse of what we now call cardiac adaptation. The harder you train, the more capable your heart becomes.