Gallbladder:Intestine fistula from gallstones?

[WashMe]

P. 335 FA 2011, bottom of the "Gallstones" section.

2 questions:

(1) how (anatomically) does the gallbladder form a fistula with the intestine, and what is the mechanism?

(2) how does a stone obstructing the ileocecal valve lead to air in the biliary tree?

Thanks!

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[turkeyjerky]

a big gallstone just kinda erodes into the intestine (via inflammation and ischemia and $hit), then, because there is a fistula connecting the small intestine (which has gas in its lumen) and the gallbladder (which normally does not have gas) you can see air in the biliary tree, which in the correct setting is diagnostic of gallstone lieus.