See also the Second Brain-- a fairly detailed book about the elaborate nervous system which runs digestion.

Unfortunately, I lost my copy when I was about halfway through, but I treasure knowing something about the complexity needed to manage storing stomach acid-- stuff which is there to break up proteins-- in the middle of a body built out of protein.

First, the acid isn't made inside cells. There's some chemistry that I didn't understand which makes it possible for the components of stomach acid to combine with each other outside of cells.

And there's a system for adding appropriate amounts of a base to neutralize the acid as the stomach contents head out into the intestines, not to mention a not perfectly reliable valve(?) system for keeping the acid from moving higher in the digestive tract than it should.

Anyway, the book has a history of the development of an understanding that the nerves which run the digestive tract are fairly independent of the brain-- as is commonly the case, it was a hard fight to get the idea across.

I'm not sure how much there is about the connection between the digestive nervous system and emotions, but I gather from the amazon description that there's a conclusion that a lot of digestive problems are from poor regulation of the organs rather than in the organs themselves.