Yosef Karo, Shulchan Aruch — 49 Laws of treifot in the Korkeban…
“ If the needle is found within the thickness of its flesh and cannot be recognized from inside nor outside see above #48. 4 The fact that we rule it kosher when only the outer membrane is punctured, is specifically when it becomes punctured on its own due to illness and not due to a thorn or needle when it's treif as we are then concerned one of the inner organs is punctured and a puncture of any size is significant there. ”
