I drew these skulls from life but first I constructed them based on rules of proportion. Now I have a bunch of questions for my anatomy teacher Andy (www.andrewameralart.com), because they still don't look quite right. I think in the upper left study I somehow inadvertently moved the top of the zygomatic arch up too high. And the lower one has some unidentifiable problems!
It's strange working with skulls... playing with proportions, both the drawings and the clay model of the skull just look warped and non-human, but as I adjust them they suddenly they start to approach a look I call "skullish", meaning suddenly they start to look like skulls.
These spine studies are after Richer. His anatomy diagrams at first looked cold and a bit boring to me, but the more I copy them, the more I am impressed by the enormous amount of very precise information he packs in: gracefully at that, and with highly economical linework.