Re: Why does it hurt when we touch the iris of our eye but not the sclera?

First things first... I assume you are speaking of touching the cornea, not the actual iris. The cornea is the clear part of the eye that covers the iris and helps to focus light onto the retina. The sclera is the tough white part of the eye that helps protect the inner contents. For a nice description of the anatomy of the eye, check out :

http://www.k12.nf.ca/vista/programdivision/eye_anatomy.html

Now, the really quick answer to your question would be that the cornea has a LOT of pain fibers. Since there are more fibers sending signals to even the slightest of irritations it would hurt more.

The other question that comes up (perhaps this part of what you were asking) is "Why does the cornea have more pain fibers ? What good is it to feel pain ?" I don't have any clear answers on that, but one hypothesis says that pain is your body's way of telling you that "something isn't right". Since even the slightest damage to the cornea can damage vision, it is important to protect the cornea. Since most animals (and people) avoid things that cause pain, they quickly learn to be careful about what gets near their eyes. This however, is only a hypothesis, not a definite fact.