Basically, you have the human systems (and not just the skeleton and muscles, you have digestive, circulatory, etc), and you can add any of them to the skeleton (or no skeleton).
The great part about it is that it’s not just a texture, the muscles are actually models and are all separated, so you can add and remove parts from the body, and also make them transparent or highlight them. This is so helpful because you can see the origins of the muscles and the insertions to the skeleton in 3D.
I just typed 3d anatomy on google and I found it 
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