Studies of the diaphragm and the internal organs; a plan of a fortress at the bend of a river; various architectural studies, including the plan and elevation of a turret; notes on the action of the lungs.

The note along the top edge of the sheet reads ‘On the 9th day of January 1513’. At that time the sixty-year-old Leonardo was based at the family villa of his young assistant, Francesco Melzi, at Vaprio on the river Adda twenty miles (30 km) to the east of Milan, and the sketch at centre left of this sheet records the plan of the castle at Trezzo, a little to the north of Vaprio, which fell to Venetian forces on 5 January 1513. While based at the Villa Melzi, Leonardo painted little, if at all. He designed improvements to the villa, and the architectural sketches here are presumably connected with that work. Leonardo also pursued his scientific studies, primarily his anatomical investigations – though without access to human corpses, he had to use animals, including dogs, birds and oxen, as his subjects.

Leonardo notes at centre right ‘Look at the dead dog, its loins and diaphragm and the movement of its ribs.’ The four oval drawings in the upper half of the sheet thus examine schematically the thorax of the dog. The sketch at upper centre shows the diaphragm lowered as in inspiration, with the heart and lungs briefly outlined. In the drawing at upper left, the diaphragm is raised in a dome, as in expiration, with the branching of the bronchi seen in the lungs, and the oesophagus piercing the diaphragm at centre. The musculo-tendinous attachment of the diaphragm to the ribs is represented by the zig-zag margin, and the ribs with the intercostal muscles are exposed below. The fourth oval drawing, to left of centre, shows the diaphragm and lungs removed to reveal the vertebral column flanked by the ribs and a pair of muscles – possibly the psoas muscles out of position, or iliocostalis shown on the wrong side of the ribcage.

At lower left Leonardo states that movement of both the ribs and diaphragm are necessary for breathing. He understood that the diaphragm is a muscle, and that in its relaxed state the contents of the abdomen push it upwards into ‘the shape and likeness of a very hollow spoon’, as indicated at bottom centre.The two very sketchy anatomical drawings in the centre of the upper half of the sheet distinguish between organs which are peritoneal (suspended within the peritoneal cavity) and those which are retroperitoneal, as the accompanying note explains: ‘The ureters and the kidneys, the spermatic vessels and the diaphragm lie outside the peritoneum, also the great vessels and renal vessels; and the intestines are inside the peritoneum.’

(All the drawings are on the verso of the sheet, as specified in the Windsor inventory; the recto is blank apart from Melzi's inscribed .G.)

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