Bagginsbill:

I am a nurse practitioner, and a cardiac specialist. I do on occassion closely examine the eyes.

You are very correct, and I am impressed by the level of your research on modeling the eye. I should have known you would be as careful of detail in this as you are in everything else.

The limbus is the narrow juctional area between the sclera and the cornea. As a transitional zone it has some of the white of the sclera and some of the clearness of the cornea. Older people (and some other folks as well) tend to have bloodshot eyes, but I have never seen a reddened limbus, even when the sclera was bruised and blood red.

Arcus senilis occurs underneath the cornea at the outer border of the iris and can be a very narrow band or wider. It can also be an incomplete circle. The inner edge of the limbus (on close up inspection) will be a dark very thin circle around the outer edge of the color of the arcus.

http://www.thaiclinic.com/boardimg/no_fate-arcussenilis.jpg
http://webeye.ophth.uiowa.edu/eyeforum/atlas/photos/arcus-senilis-OS-India-1966-301X274.jpg

Sometimes the color of the arcus and the limbus will almost merge.

http://classes.kumc.edu/som/aamc/icm851/images/photoimages/eye80.jpg

The picture that lesbentley shared reminded me of arcus senilis, except the band of color was dark and not light. I though I read that this might be caused by not having a texture on the eye covering to simulate the limbus.

http://www.renderosity.com/mod/forumpro/media/folder_9/file_448973.png

It seemed to me that if a light color could be placed in that band, it would look like arcus cornealis and might be useful for close-up renders of older people. Can that effect be captured?

Sorry if I was simplistic.

LMK

Probably edited for spelling, grammer, punctuation, or typos.