The skeletons are derived from Albinus' engravings. Compare them to these two images from The Art Student's Guide to the Bones and Muscles of the Human Body and Lessons on Foreshortening
The first image is from Bernhard Siegfried Albinus' Tabulae sceleti et musculorum corporis humani, the second is Johann Gottfried Schadow's engraving of bones for art students, after Albinus's work.
Albinus on Anatomy
The Art Student's Guide to the Bones and Muscles of the Human Body: and Lessons on Foreshortening