Our book covers implied lines in the first chapter, and I found this somewhat amusing video where someone draws a frog using only implied lines.  It's interesting because each shape they use could stand alone, but because of the placement you can see the big picture. 

https://www.youtube.com/watch?v=bhyDTyKsnnY 

Mary Stewart states that the reason we connect the lines in our head is because "we have a natural inclination to seek visual unity."  It is a rather interesting concept.