in the raws is a txt file named tissue_template_default. It has all the comon body materials such as bone, cartilage, flesh, fat, nails, hair, skin, well, a lot. Most have healing rates of either 100 or 1000, with 100 being very fast and 1000 being slow. While I think nerves and such should not usually heal, we all know that in many cases partial function returns after time. A long time often, but it's still recovery of a sort. A value of 10.000 or so, or more, would simulate that a bit. My problem with this bit of raws is about something else though.
I usually add healing rates to some of the materials for the following reason. Wounds can be healed etc etc, but some materials in the game don't heal by themself. Cartilage, nerves, some are entirely reasonable. Another one that makes for a barrel of fun is nails. As you probably know, dwarves with open wounds can get infections. Since nails don't heal, any dwarf who had a completely smashed foot will be fine, except if he also broke a nail. The nail won't heal, he'll be trailing an infected foot for the rest of his life. In fort mode this is annoying and if the dwarf is not too strong he may succumb, but he'll be geting regular medical treatment. Good for the doctors. In adventure mode it basically means the end of your adventures. There are no treatments, no doctor cutting away infected parts, no washing with soap to prevent or kill the infection. Someone some time ago put it quite well: The worst enemy of an adventurer; the infected toenail. Don't worry about the dragon, if all else fails you can run away from it. You can't even cut off your own toe (I just realized that might be worth a try, but do I want to make a new adventurer to perform a sort of surgery on my previous one in order to amputate his left pinky toe with a sword or axe?)
Similarly, animals in fort mode with smashed hoofs, tusks or horns are doomed. They're not even treated and the damage never heals, unlike in real life.
I think, btw, that if you edit healing rates in the save raw, you don't have to generate a new world, but I'm not quite certain