Defining experiment was particularly vexed in the realm of anatomical dissection and vivisection. Was dissection merely descriptive, or something more? Harvey's discovery of the circulation of the blood and Aselli's discovery of the so-called lacteal veins shaped much anatomical research between the late 1620s and the 1650s. While the techniques of dissection and vivisection gained wide use, there was much debate on the validity of the circulation in particular, and its relationship to the lacteal veins. Critics, particularly the French anatomist (...) Jean Riolan, but also the natural philosopher Pierre Gassendi, focused on the lack of causation in Harvey's method and the lack of medical use and not on his use of vivisection. Jean Pecquet's discovery of the thoracic duct in 1651 changed the terms of the debate by definitively connecting the circulation with the lacteals. Riolan's critiques of Pecquet in the 1650s show profoundly differing notions of the purpose of dissection. While Gassendi eventually accepted Harvey's concept of the circulation, Riolan never did. (shrink)