In the late 1870s, Thomas Eakins and his colleague Dr. Keen at the Pennsylvania Academy realized to their dismay that their dissected cadavers were getting a little past their prime.
So they had them cast in plaster. Eakins cast his own hand, too. Later on the plaster casts were converted to bronze.
The tradition of studying skinless cadavers goes back to Leonardo da Vinci. When you remove the dermal layers, the insertion points and the overlapping of the muscles becomes much more apparent.
A body without its skin is called a “flayed figure” or “écorché.” I did this study of an écorché dog from a plate in the book Animal Painting & Anatomy, by Frank Calderon, (London 1936, now Dover)
The French sculptor Jean-Antoine Houdon created this écorché figure as a study aid for artists. Écorchés were in common use at the École des Beaux Arts in the 19th century.
In this painting of a sculptor by Edouard Joseph Dantan, there’s an écorché figure behind the model.
Some art schools and ateliers have returned to using Houdon’s écorchés, and some present-day artists have created new écorché reference sculptures.
A few retail sources carry écorchés, but you should get a good look before buying because some castings are many generations away from the original. Also, consider your intent. A white écorché is better for understanding the planar geometry, while a polychromed, medical-style écorché might be better for studying the muscular anatomy. Example of a polychromed ecorche: link.
Example of white ecorches: link.
"Freedom of Teach" reference figures, link. (Thanks, Bowlin, Mr. Atrocity, and Drew!)
Feel free to pimp other sources in the comments.
Related GurneyJourney post on plaster drawing casts.