Frogs’ lungs and Malpighi’s discovery of pulmonary capillaries

In 1661, Marcello Malpighi (1628-1694), one of the most important 17th-century anatomists and physicians, published two epistles entitled De pulmonibus (On the lungs), both addressed to Giovanni Alfonso Borelli (1608-1679). In the first letter, thanks to the help of the microscope, Malpighi proved that the lungs were not fleshy, but that they had a vesicular structure. Moreover, with these new anatomical findings, he also revised the traditional view on pulmonary function: lungs do not cool the heart, as was previously supposed, but mix the blood. In the second letter, Malpighi decided to perform dissections of frogs. Assisted by his colleague and friend Carlo Fracassati (1630?-1672), he observed the arteriovenous anastomoses in the pulmonary circulation. Magnifying tools and new anatomical procedures allowed him to prove not only this mutual union of arteries and veins, but also that blood moves in opposed directions: thanks to these observations, Malpighi provided a strong evidence supporting Harvey’s theory of blood circulation. However, Borelli, whose collaboration was pivotal, challenged Malpighi’s view on pulmonary function. According to him, lungs do not mix blood, but divide blood particles into their smallest parts. Therefore, their function is not that of mixing, but that of generating blood, due to the triggering action exerted by the air particles. This paper aims at analysing all these issues within Malpighi’s research program and showing the progress achieved by the Italian microscopic anatomy in the second half of 17th century.