Yosef Karo, Shulchan Aruch — 43: Laws of treifot of the spleen
“ And if there's fluid in the spleen even if it's clouded or putrid if the flesh around it is intact it's kosher (N"Y in the name of Morekhai and HG"A and A"L 60:50) . 3 There's an opinion that wrote that whatever is adhered to the belly (Keres, rumen) is called the thick end and nothing else is called that.4 The thickness of a gold dinar, we don't know its (exact) measure, but anyhow it seems it has to be less than half (the spleen's) thickness. ”
