Chullin 43 - Seven Teachings of Terefah
There were seven more signs of terefah . They included the teaching of Rav Matna about the ball of the thighbone that popped out of its place in the socket, the teaching of Rachish bar Pappa about an animal that became diseased in one kidney, and so on. However, how could they change the classical number of the eighteen cases ? They did not, rather, those who teach those seven rules combine all cases of punctured organs into one, and then add the seven.
The esophagus has two layers, a red one and a white one. If one was punctured, but the other was not, the animal is not a terefah. The gizzard also has two layers, so the above rule applies. If both layers either in the esophagus or in the gizzard were perforated, but the holes are not aligned with each other - some say that in the case of the esophagus it is kosher, but in the case of gizzard it is terefah, and some say the opposite.
Ulla said, if a thorn is lodged in the esophagus, we do not suspect that it healed (which would still be terefah), rather, it was never punctured.
Art: James Hayllar - The Thorn