The transistions of lightning and the way you spreaded your darkening/ligthening is very good. But his skull is simply incorrect, it may be beceause you dont know the structure of the skull and you drawed it of pure logical guessing, and not of knowledge. It would be nice that when it occurs, you start up Google, then you can mimic it. For a heart i did, i didnt know the veins, i would've drawed the classical heart ( the one made of one triangle and 2 balls ) if i wouldn't opened Google and typed heart.
My skull sentence would sum up the rest of his anatomical aspect you changed in any ways. There's a nice choice of color, they're not too saturated, bright or dark. But i would suggest some variations, they're fairly easy to do.. the human skin's global color is tan, but it have shades of red, blue and green. On this skin, or on your next, try to introduce something more than your global color that you shaded.