Recto: A note on how to draw the lungs, with an illustrative sketch in black chalk, this being a tracing from the verso to show the organs from behind. To the right, a black chalk drawing of a left thigh and leg seen from the back.

Verso: Two schematic drawings of the heart and pulmonary vessels, one with liver and spleen; a diagram of trachea and intra-pulmonary bronchi, and one of the great vessels, heart, liver, spleen and male genito-urinary system; notes. Melzi's inscribed .P.

Despite the confidence of the drawings, this representation of the cardiovascular system is based primarily on tradition, which continued to dominate Leonardo’s depictions until after 1510 – in fact the layout of the major organs and great vessels is essentially the same as 912597, drawn some twenty years earlier. Several of the structures have been depicted so as to fulfil their supposed function, and it is clear that Leonardo’s first-hand knowledge of the heart was still no more than rudimentary.

The vena cava is shown as arising from the liver as two essentially separate vessels, suggesting that the drawing should be dated before RCIN 919028r, in which Leonardo demonstrated that the heart is the centre of the venous system as well as the arterial. Above the liver, the inferior vena cava joins directly to the superior vena cava, and at this junction are branches to the right lung and to the heart. In the drawing at centre left the vena cava and aorta are omitted entirely, and we see two branching systems from the right lung passing directly to the right ventricle of the heart, and two from the left lung passing to the left ventricle (there is no indication of the atria). At this time Leonardo thought that these branching systems carried air, not blood, as described at lower right:

"The wind reflected from the lung to the heart cannot enter the heart unless there is an outlet. Therefore two passages are necessary: one which, when the lung sends out air through the trachea, at the same time sends air into the concavity of the heart; and a second passage through which air issues from the heart and returns ... through the said trachea, the passage of the lungs."

Among other errors one might note the bovine aortic arch, a recurring feature of Leonardo’s heart studies; the vessels connecting the liver and spleen, which may be diagrammatic but even so are not correct; and the testicular arteries coursing posterior to the renal vessels. There are some accurate observations, such as the form of the seminal vesicles (studied in more detail on the companion sheet, 919098v), and the termination of the left testicular vein at the left renal vein; but Leonardo’s great achievements in his subsequent studies of the heart are not even hinted at.

Text from M. Clayton and R. Philo, Leonardo da Vinci: Anatomist, London 2012.