Head/Bust with skull inside

thingiverse

The original skull model is likely from this source - http://grabcad.com/library/skull-1. However, I found a similar version on Thingiverse at http://www.thingiverse.com/thing:43591. After downloading it, I refined and tweaked the STL file before printing. The final model can be an entire assembly if using a technology that displays internal details like Polyjet or SLA. Alternatively, you can opt for just the skull or outer head.