Download the eye and Mental Ray shaders for Maya 2013 ( Maya Binary ).
Download the Maya Ascii version.
This is an eye that I use for personal art pieces.
Everything is procedural from the size and shape of the cornea to the vein textures. Normally I set the detail lower than what you see above but the detail is there if I need it.
This was inspired by Alex Alvarez's free tutorials: http://www.thegnomonworkshop.com/store/category/167
The backgrounds / light probes are from Paul Debevec.