I finally found it. The word you're looking for is ecorche.
The Wikipedia entry says:
An écorché (French pronunciation: [ekɔʁʃe]) is a figure drawn, painted, or sculpted showing the muscles of the body without skin. Renaissance architect and theorist, Leon Battista Alberti recommended that when painters intend to depict a nude, they should first arrange the muscles and bones, then depict the overlying skin.
This Wise Geek entry says:
An ecorche or “flayed figure” is a painted, drawn, or sculpted human figure depicted with the skin stripped away, exposing the underlying musculature. Medical texts may use ecorches for illustration so that students can clearly visualize the structures they are studying, but ecorches are most commonly used as references by artists. In some cases, they are also works of fine art in and of themselves, although they can be a bit macabre.
The ecorche appears to date to around the 1400s, when several artists including Leonardo da Vinci started making such figures. Taboos against dissection often made it difficult to access human bodies, whether one was an artist wanting to create more accurate work or a medical student who wanted to learn about the body. da Vinci, along with many other artists, felt that it was important to understand the underlying architecture of the human body when depicting it in artwork, and ecorches assisted artists with this task.