Hair, I'd just sit on it for awhile. Take the pic to the vet but I agree with the saddle club members, no heroic actions for a cat that old. Blind and stuck in the house all the time ain't no way for a cat to live.
Thanks to everyone who has offered advice and comfort. I'm still far from settled on this, but I'm leaning towards taking the kittens in for their shots today and having a very frank talk with the vet about our options on whether we need to do something about Cartman. If she thinks there'd be some tangible improvement through some treatment, we'll do that. If she thinks there's not much that is curative that can be reasonably done, we'll make him comfortable as long as we can and we won't let him suffer if we can help it.