Kaiju Anatomy 101 (UPDATED)
I don't know who'd have a scalpel big enough to look inside of these guys, but here are two full-color anatomical paintings of Godzilla and Hedorah (a.k.a. The Smog Monster).
I found these over at the Monster Brains blog, although the original artist has yet to be identified. I'm guessing that King Ghidorah probably stepped on him before he could sign his work.
Update, 10/7/11: I just found some additional anatomical drawings of kaiju over at the Pink Tentacle site. This set includes another drawing of Godzilla, as well as pictures of Anguirus, Gamera, and Mothra larva.