Discovery of the capillary circulation. Malpighi demonstrated that the pulmonary tissues are vesicular in nature and showed that the trachea ends in bronchial filaments. His De pulmonibus includes his demonstration of the capillary anastomosis between arteries and veins. This book, which is very rare, consists of two letters to Borelli describing observations made through the microscope on the lung of a living frog. In the second letter Malpighi described small channels connecting arteries with veins, the capillaries. This was the first proof that blood circulation occurred within a closed hydraulic system. The second edition was published as an appendix to Thomas Bartholin’s De pulmonum substantia et motu diatribe, 1663. It is republished in his Opera omnia, Lugduni Batavorum, 1687, ii, 331. A facsimile was published in Milan in 1958; English translation by J. Young in Proc. roy. Soc. Med., 1929-30, Sect. Hist. Med., 23, 1-11. See No. 915