Curious eyes of the "glass frog"

The "glass frogs" of Central and South American are remarkable for their translucent skin, but I find their eyes equally fascinating.  The photo above is from National Geographic Daily, and the one below from Animal Planet.

I can't tell whether that pattern is vascular, or just pigment.  It must provide some type of survival advantage, presumably for camouflage rather than mating or vision.  Perhaps someone knows...

p.s. - equally cool is the eye of the red-eyed tree frog.