.Mark, demonstration with charcoal.
A note about the process here. I show these elaborate line drawings that end up getting completely covered over by the tonal work. This is done intentionally as an exercise for students. Invariably, beginners carefully render parts of the anatomy but are far less careful about relating those parts to each other. (You've seen those drawings that look like a bumpy sack of walnuts.) In class we start with careful construction drawings that force students to properly fit each part of the anatomy into the whole head. Having done plenty of the bumpy, poorly constructed heads myself, I forced myself to use this method and got much improved results. That being said, for my current tonal drawings I do the line drawing quickly and lightly. Not nearly as refined as you see here but with the idea in mind. I hope these drawings make clear how the construction lines can help wrap the light and shadow around the whole head for a solid three dimensional effect.
You can see more demos if you scroll down to August of 2006.