
If you look a little closer, you'll notice an odd square around the top of the head. The effect is more pronounced in other illustrations in the book. You can really see it here:
It turns out that Estienne was dissatisfied with the woodblocks for the book, and had portions of them recut to emphasize crucial anatomical detail. The result is a woodcut inside of a woodcut. He clearly saw the original as a failure of execution--on the part of the woodcut artist.
We are lucky enough to have first editions of both of Estienne and Vesalius. To see them ask for Rare QM21.E82 and Rare QM25.V4.