"Leonardo's exercises in anatomy"
Vitruvian Man, by Leonardo
Public DomainVitruvian Man, by Leonardo

Studies of Embryos, by Leonardo
Public DomainStudies of Embryos, by Leonardo
This is a reference to the notebooks of Leonardo di ser Piero da Vinci (1452-1519), the Renaissance genius of art and science known familiarly as Leonardo da Vinci, who made pioneering discoveries in the field of anatomy by drawing nude human figures. His surviving notes run to 13,000 pages, and though they appear to have been written for publication, they never appeared in public.

The two images here are probably the most famous. On the left, Vitruvian Man, and on the right, Leonardo's studies of human embryos. Both of these photographs of Leonardo's notebooks were shot by Luc Viatour.

Microsoft founder Bill Gates owns Leonardo’s 72-page collection of scientific writings, which Gates named the Codex Leicester after he bought it in a 1994 auction for $30.8 million, making it the most expensive book ever sold. Gates has allowed it to be placed on public display once a year around the world.