The drawing at upper centre shows the bifurcation of the great vessels, with their ramifications into the iliac vessels surrounding a schematic depiction of the rectum and bladder. Another branch leaves the aorta and travels vertically down behind the rectum – probably the inferior mesenteric artery, greatly simplified. Above the pen drawing are black chalk outlines of the liver, spleen, kidneys and heart, with the right testicular vein travelling obliquely towards its junction with the vena cava.

In the two large drawings below, the ureters descend from the kidneys to the bladder (drawn too large). The rectum curves round behind the bladder, accompanied by a pair of vessels on either side – by comparison with the first drawing, these are probably intended to be the internal iliac vessels, with vesical branches to the bladder. The testicular arteries are seen passing down from the level of the kidneys; these actually leave the aorta itself a little below the renal arteries, but their origin is unclear in the drawing to the left, and in the sketch at upper right, both testicular arteries and veins seem to arise on the corresponding renal vessels. The testes are correctly shown contained within a fluid-filled cavity; the vas deferens rises from the testicle, loops over the top of the pubic bone (in dotted cross-section in the drawing to the left, though drawn too large), and passes into the upper end of the seminal vesicle. The ejaculatory duct runs from the lower end of the seminal vesicle to join the urethra just below the bladder. Leonardo shows the urethra as the sole channel of the penis, and any notion that a ‘spiritual’ component is carried in a second channel from the spinal cord (cf. 919097v) has been abandoned.

(The recto of the sheet, as specified in the Windsor inventory, is blank; all the drawings are on the verso.)

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