The heart of animals is the foundation of their life, the sovereign of everything within them, the sun of their microcosm.

William Harvey (1578–1657), the English physician who demonstrated the circulation of the blood, repeatedly emphasized the primacy of the heart in animal life. The imagery in this quotation reflects early modern anatomical and Aristotelian-Galenic traditions that treated the heart as the body’s central “sovereign” organ and source of vital heat. Harvey developed these ideas through extensive dissections and vivisections of animals, especially in the work where he argues that the heart is the first organ to form and the last to die. The “microcosm” metaphor—common in Renaissance natural philosophy—casts the animal body as a small world with the heart as its sun-like center.

The statement presents the heart not merely as a pump but as the organizing principle of animal life: the “foundation” and “sovereign” that governs the internal economy of the body. Calling it the “sun of their microcosm” suggests that, as the sun sustains and orders the cosmos, the heart sustains and coordinates the living organism—distributing warmth, motion, and vitality. In Harvey’s intellectual setting, this language bridges observation and metaphor: it elevates anatomical findings into a philosophical claim about biological hierarchy. The quote also signals Harvey’s broader project of explaining life through structure and motion, with the heart as the central agent.