Human anatomy was Leonardo's most important scientific pursuit. He dissected some thirty corpses himself, intending to publish a treatise on the subject (contrary to popular belief, dissection was permitted by the Church). In the winter of 1507-8 he conducted an autopsy on an old man in a hospital in Florence. This double-sided sheet comes from the notebook compiled following that dissection. The main study here highlights the great vessels, the aorta and vena cava. At top left Leonardo attempts to resolve an ancient debate about the centre of the venous system - whether the heart of the liver - by appeal to the analogy of a plant. In the notes, written in his habitual mirror-writing, Leonardo states that the heart is the equivalent of a seed or nut, whereas the vessels ramifying in the liver are analogous to roots. As a plant has its origins in the seed, so the vessels originate in the heart, with the liver simply the source of nourishment. The other side of the sheet shows Leonardo, unusually, struggling to make sense of his dissection notes. The vessels connecting the liver, spleen and kidneys to the aorta and vena cava are repeatedly drawn, and are thus rather confused in the upper two studies. Only in the third drawing does Leonardo arrive at a clear arrangement, though it is only partially correct, and drawn with a regularity quite unlike that actually found on opening a body. The liver is small and the spleen enlarged, indicative of cirrhosis of the liver and portal hypertension in his elderly subject. While Leonardo's anatomical studies were far from perfect, his understanding of the heart and of the muscles and bones surpassed that of any contemporary. But his treatise on anatomy was never written, and Leonardo's astonishing investigations were to languish unappreciated for centuries.