no, i didn't meean the transparent aspect.  that's entirely different.  look at your second link and  the last link in my original post.  it shows bumpiness (for lack of a better term).  it's not as if the fibers lay flat. and all of those diagrams, including your cross sections, show the iris inset the entire thickness of the scelera.  in fact, the highest point of the scelera is almost equal to the highest point on the iris in the first cross section (at least by my Firefox MeasureIt extension and my estimation of edges).  maybe i'm misreading that model, but i could have sworn the highest point of scelera was pretty much even with the very lowest point of the iris, and then the iris rose in a donut from there.  and there was absolutely no dip between scelera and iris. 

i'm pretty sure the shading effect i can see is at least mainly from that overhang/depression or whatever you want to call it, but you know much more about these things than i do.  unfortunately, i'm guessing true prism effects is another aspect of those caustics pjz99 was talking about.  i'm guessing you couldn't even test the theory that it had to do with the lens in Poser.  since my only other rendering tools are the Blender internal and Yafaray, i'm guessing i don't personally have a renderer that can test it. at least not well.  i played around with caustics in Yafaray a long time ago and was pretty awful at it.