In 1659 Malpighi returned to Bologna to intercede for his brother Bartolomeo, who had been sentenced to death for killing Sbaraglia’s eldest son, Tommaso, during an argument. Malpighi remained in Bologna as professor of theoretical medicine, and during this time made his first important observations with the microscope.
In a series of letters to Borelli, Malpighi described the fine structure of the lungs. In Galenic medicine, the lungs were considered to be made up of coagulated blood. Not only did Malpighi prove himself to be a follower of the philosophy of mechanics and master of the microscope, but he also managed, by means of various anatomical stratagems, to penetrate inside structures and reveal the subtle mechanism of how they worked.
With his description of alveoli, Malpighi set down the bases for a theory of respiration and provided information that was fundamental to the birth of modern medicine: in his famous work De motu cordis (1628), William Harvey had provided decisive proof against the Galenic system, which stated that blood was produced in the liver and then consumed by the various parts of the body. However, the demonstration that circulation was effectively “closed” was still missing in the Harveyian system of blood circulation. Malpighi revealed that, in the lungs, arteries and veins were linked by a very fine network of capillaries.
Fig. 1: Table from "De pulmonibus observationes anatomicae" (1661). The upper part of the drawing (Part I) shows on the left a frog’s lung seen from the outside, and on the right the lung cut longitudinally. In Part II we can see a microscopic cut of the alveolus of a mammal’s lung, with blood vessels. In order to eliminate blood, air and remains of other tissues from the lung tissue, Malpighi carried out a series of complex anatomical operations. Then he examined the very delicate specimen with a microscope to reveal that the matter in question was an aggregate of membranous alveoli communicating with the last tracheo-bronchial ramifications, embraced by the capillary network. This network established the communication between veins and arteries which up to that point anatomists had been looking for but never found.