Aselli became professor of anatomy and surgery at the University of Pavia and practiced at Milan. His discovery of the lacteals (lymph vessels that take up the end products of fat digestion from the intestine) occurred in 1622 during the vivisection of a dog that had been richly fed just prior to the operation. On opening the abdomen, he noticed whitish cords that exuded a creamlike liquid. Upon careful repetition of the experiment, he described these new vessels as venae albae et lacteae (“white and lacteal veins”). He described them in De Lactibus sive Lacteis Venis, published posthumously in 1627, just before the De motu cordis of the English physician William Harvey, who appears to have been unaware of Aselli's work.