Newly equipped with a skull model sometime in 1995, relatively confident because of regular drawing, I started trying to improve my feeling for human anatomy. Quickly found it a considerable challenge, and the skull sketch a fun exercise for its own sake.
Another early one. Ballpoint pen, smudged with a quick licked thumb.
I started filling pages with a more or less spontaneous, disordered variety of these sketches.
 
(Click image for larger view.)
And more. Human form is endlessly absorbing.