Records the discovery of the lacteal vessels. While performing vivisection on a dog that had recently fed, Aselli noticed a network of vessels in the mesentery and along the peritoneal surface of the intestine. The vessels released a whitish fluid similar to milk when incised, so Aselli called them lacteas, sive albas venas. He made a systematic study of these vessels in different species of animals, noting the chronological relationship between their engorgement and the animal's last meal, and erroneously conjectured that the vessels led to the liver; it was not until Jean Pecquet's discovery of the thoracic duct and its continuity with the lacteal vessels that the process of absorption was clearly established.

De lactibus sive lacteis venis was posthumously published in Milan at the press of Giambattista Bidelli through the efforts of Nicolas Fabry de Peiresc. The work contains a beautiful engraved title page and a portrait of Aselli by the Milanese painter and engraver Cesare Bassano. The four folding chiaroscuro woodcuts in this work, printed in black, red and two shades of brown, were the first color-printed illustrations in a medical or anatomical work. They are unsigned and authorship of these has not been established.