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During the 17th century, between the second half of the 1630s and the years of the 1640s, the town of Leyden became an important center for the experimentation, discussion, and dissemination related to the discovery of blood circulation and the emerging doctrine of digestion. People such as Jean de Walaeus, Franz de le Boë (Sylvius), and Thomas Bartholinus worked there. No less passionate, in this circle, was the activity of the two printers Jean Maire and Franciscus Hackius. In this context, between 1640 and 1643, also René Descartes sojourned in Leyden, living first in the city and then in the castle of Engeest. Two of Descartes' letters to Henricus Regius, and one to Marin Mersenne, date back tothis period. In each letter, the philosopher, solicited by his intelocutors, responds to issues regarding digestion and tied to the discovery of the venae lacteae (lacteal vessels) on thepart of Gaspare Aselli. Referring with all probability to these letters (and to La description du corps humain). Louis De La Forge, in his commentary on L'Home (1664), is the first to mention the assent of Descartes to the discovery of Aselli and the next one of Jean Pecquet (receptaculum chyli). Prompted by La Forge's commentary, the article examines the three letters mentioned above. Proceeding with the examination of Lespassions de l'âme (a work about whose writing it formulates its own hypothesis) and of La description du corps humain, it reaches the conclusion that in Descartes the doctrine of digestion, while playing an important role in his physiology, nevertheless remains on the margin of the discoveries and of the contemporary debate.