Illustration of the lungs of a frog from Marcello Malpighi's book On the Lungs, Bologna, 1661. The left lung is viewed from the outside and shows the network of capillaries supplying the lungs. The right lung is sectioned and shows the view of the capillaries from the inside. Malpighi's observation that blood flowed through a complex network of capillaries was an important step in the understanding of respiration. His observations were made using the newly invented microscope. An Italian physiologist, Malpighi (1628-1694) is considered to be the father of microscopy.