Marcello Malpighi was a professor of anatomy at the University of Bologna. He is considered one of the greatest anatomists of the 17th century, the founding father of microscopic anatomy, and the first histologist.
He studied several areas including the development of the chick, silkwarm embryology, and microscopic analysis of the skin, kidneys, liver, and spleen. Malpighi made important discoveries about the structure of plants and animals with the use of microscope.
In 1660, he discovered the capillary system in the lung of a frog using a microscope. He communicated his findings to his friend Giovanni Borelli in two letters entitled, "De pulmonibus observationes anatomicae" (Anatomical observations on the lungs). The discovery of this network connecting the venous and arterial systems was a milestone and solidified Harvey's concept of blood circulation.
The text of "De pulmonibus observationes anatomicae" was included in Malpighi's "Opera omnia" which appeared in 1687. In "De pulmonibus", Malpighi wrote:
"The blood is forced and scattered by the pulse through the arteries into a network. As the blood stream, thus repeatedly divided, is carried round in a sinuous manner, its colour fades. It is thus distributed until it approaches the walls, receiving branches of the veins. While the heart is still beating two movements in opposite directions can be seen, making the circulation of the blood evident. The same may be seen even better in the mesentery.
With unaided vision I might have believed that the blood escaped into an empty space and was recollected again by gaping vessel, but for the tortuous and scattered movement of the blood in different directions and its union again into definite vessel. Such is the branching character of these vessels as they proceed on the one side from the artery and again o the other side to the vein that there appears to be a network made up of continuation of the two vessels. Hence it appears to the senses that the scattered blood flows along tortuous vessels, and is not poured out into spaces, but always continues in tubules, and that its dispersion is due to the multiple winding of the vessels."
He demonstrated his discovery with two illustrations of historical significance which are displayed here.
Figure 1 shows a dried lung. Figure 2 shows a cross-section view demonstrating the interior vesicles and sinuses with portion of the interstitial tissue. Figure 3 shows the disposition of the trachea, its division and the pulmonary vessels.
Figure 1 shows lungs of frog with trachea attached: A: Larynx, C: place of the heart, D: external part of the lung, E: prolonged rete of the cells, F: prolongation of the pulmonary artery, H: prolongation of the pulmonary vein Figure 2 demonstrates lung capillaries of a frog ( microscopic view): A: interior part of the cell, C: trunk of pulmonary artery with its branches ending in a network, D: trunk of pulmonary vein with its branches over the slopes of the walls.
Singer C. The discovery of the circulation of the blood. London, Bell, 1922
Malpighi M. De pulmonibus observationes anatomicae, Bologna, 1661. Translated by James Young, M.D., Proc Roy Soc Med 1929-1930; (part1)-23:7-11