Here’s all I remember about this: it was a shorter book [150 pages max? Couldn’t have been more]—~20 years old at most? with kinda surreal illustrations. There was some sort of civilization of microscopic people [people??] that lived within this massive person. They might’ve been centered in the kidney/liver. One person has to go on a voyage to the eye, perhaps motivated by the illness go a family member [maybe mother?] and goes to various other body parts before reaching the eye including the nipple. One detail that struck out in particular was that, at the nipple, there’s a little hole through which these people can view the world outside this person they’re stuck in. The book ends with these people having reached the eye—now they can see the world outside of this large person they live in.
I came across your entry while trying to figure out the same story, and I finally cracked it! It's "This Shape We're In," by Jonathan Lethem. It also appears in the collection Men and Cartoons.