Capillaries

Capillaries first seen under microscope. Previous unclear was how blood got from arteries to veins (to get back to heart). Harvey's theory suggested that there should be vessels, but they were too small to see.
 Marcello Malpighi Italy 1660 Capillaries

Capillaries first seen under microscope. Previous unclear was how blood got from arteries to veins (to get back to heart). Harvey's theory suggested that there should be vessels, but they were too small to see.