Yes, you're right. Just the other day I read a book suggesting that our brains actually fart. I imagine there is loads of research proving otherwise that also dates all the way back to the 70's.
I can tell when a comment has touched a nerve, as you always repeat it back for 'comedic effect'. Are there any books you can recommend that contradict the neuroscientific and therapeutic research dating back to the 70's? No? Thought not.
Interestingly, that phrase, like many others, is suggestive of the connection between the two. But if you want a more definitive answer, I would say the Vagus nerve.
Did you notice on the the Harvard study that multiple studies have suggested that psychological interventions can be as supportive as physiological interventions for gastrointestinal conditions? You can thank me later.