William Harvey and Capillaries

Circulation of the blood is so familiar that one can hardly imagine a time when it was not fully understood. Indeed, the ancients knew about the pulse and the flow of blood. They recognized, too, the vital importance of the heartbeat and nourishment. Yet the concept of a complete blood circuit emerged only in the early 1600s, largely owing to investigations by William Harvey (Figure 23.1). Harvey has since earned renown as one of biology’s great heroes. But what guided Harvey to his landmark discovery? According to many popular accounts, Harvey’s genius was reflected in his remarkable ability to deduce circulation without being able to observe the capillaries that ultimately close the circuit between arteries and veins. Moreover, Harvey’s reasoning was so powerful, they contend, that he was able to confidently predict the presence of the tiny blood vessels without ever seeing them. Only later did others confirm his insightful prediction. That triumph, tragically too late for Harvey himself to appreciate, seems to vividly demonstrate the importance of deduction and prediction in Harvey’s work—and in science generally. However, these stories do not measure up to historical evidence very well. Nonetheless, the widespread error is itself telling. Probing the erroneous stories more deeply, one can gain an even deeper appreciation of scientific myth-conceptions and how they foster misconceptions about the nature of science (essay 21). Most important to understanding Harvey’s discovery, perhaps, is his adoption of the renewed spirit of experimentation in the early 1600s: an eagerness to tinker with and actively probe nature (essay 1). Rather than read books, he dissected animals. He cut open fish, frogs, and other creatures to observe their beating hearts. His unexpected observations led him to new conclusions, which he published in 1628 in De Motu Cordis et Sanguinis, or On the Motion of the Heart and the Blood.