How were you able to get all those angles? Did you happen to have an actual skull around that you used for reference, or some website/program? (3D Model?)
I own a complete skeleton.
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some more drawings, going ove rface anatomy.
Also drawing face anatomy on skulls,
Those are not the usual portraits one would draw, but rather it's like that I am drawing the base of the skull, try to get this one right just by observation (because the minor proportions of the skull change quite a lot) and I am considering then every muscle before I draw it in.
To add some difficulty I tried with the later ones to get close to the actual people and I also used an increasing difficulty with stronger facial expressions.
This method leads to some quite interesting observations.