The right side is a fairly complex topographical description of the structures. The breakdown describes plane directional changes as well as light effects.

how_to_draw_the_human_head_2 by draw as a maniac, via Flickr

How to Draw a Face - 25 Step by Step Drawings and Video Tutorials

How to Draw Eyes - Structure

How to Draw Eyes - Structure, How to Draw an Eye - Step by Step Tutorial, How to Draw Eyes with thanks to proko, How to draw Face