I believe this is actually a the skin one gets after a mummyfication.. this is how the skin of an unmummyfied farao looks now :£ He's amazing to be able to actually create a realistic looking version of it!
the muscles actualy remain, they remove the internal organs, but not the muscles. the mumification would secure that the body would not rot. yet over time the body becomes dried out, and that way you get these.. untasty looking muscles :3