Yosef Karo, Shulchan Aruch — 46: Laws of treifot of the intestines
“ In a beast it doesn't seal. In a bird all its fat seals.Rema: And it only seals in a part where the fat is adhered to the part it belongs to. But if it seals a hole in a different part it's not called sealing and it's treif. (B"Y in the name of Tosafot, Rosh and Rashba) 2 If the intestines prolapsed then went back in by themselves it's kosher. But if they were pushed back in, if they were inverted, even though they're not punctured it's treif, for it's impossible to revert them to how they were after they're inverted therefore it can't survive. ”
