TT40, I have the dark ring round my pupil and have been caffeine free for 9 years now, save the odd bite of chocolate! An iridologoist said it was cos my eyes are really orange and green, just the brown is stronger at the edge.

As I don't have orange eyes either (I'm not the Gruffalo!), is there a third idea?

Also, the cornea is dead skin cells, so no blood supply. Just as the skin has living cells a few layers down that replenish your skin constantly, so does the eye. It's called the limbal ring and it prevents the vasularisation of the cornea too (where blood vessels grow over the cornea). The cells in the centre of the cornea take about 2 years to be replaced from the limbal ring. Btw, the sleep in your eyes is the dead cells that slough (pron. sluff) off when you blink and is needed to stop your corneas getting too large! I love eyes, they are fascinating. I was lucky enough to work in eye research for a while (limbal transplants).