Gemara

(Rav Yosef): If a hole in the lung is a Tereifah, even if the lung sticks to the flesh between the ribs, it is Tereifah!

(Beraisa): If a man's Ever was punctured, he is a Petzu'a Daka (he may not marry). If the hole was sealed, he is Kosher. This is a disqualification that can become permitted.

Suggestion: 'This excludes our case (the sealing does not make the animal Kosher).

Rejection (Ravina): No. It excludes a scab that forms on a hole in the lung. That does not help.

54a (Mishnah): If the kidneys were removed (it is Kosher.)

55b: If the kidney shrank, if it shrank to the size of a bean in a small animal, or to the size of a grape in a large animal, it is Tereifah.

Chulin 68b (Rabah): A Treifah never becomes permitted.

Rishonim

Rambam (Hilchos Shechitah 8:26): Even though a removed or missing kidney is Kosher, if a kidney is too small, i.e. the size of a bean in a small animal, or the size of a grape in a large animal, it is Tereifah.

Rosh (3:45): The Torah forbids a small kidney. 'If the kidney shrank' connotes that it was big and shrunk. If it was naturally small, it is Kosher.

Yam Shel Shlomo (3:88): This is like the Rashba in the name of the Ramban.

Ha'Emek She'elah (152, p.232): A Beraisa says that a Petzu'a Daka can become Kosher, and this is a Pesul that can become permitted. Rav Yosef thought that this excludes a hole in the lung, and later the lung sticks to the flesh. Ravina rejected this. Rather, it excludes a scab that forms on a hole in the lung. What is the difference between sticking to the flesh or a scab? According to the Rosh, Rav Yosef thought that a cure b'Yedei Shamayim does not help for Tereifos. Ravina answered that it helps. We exclude a sealing by a scab that will later come off. It is not a good sealing. This implies that if it were a good sealing, it would be Machshir. The problem is not that once we called it Tereifah, it cannot become Kosher. Also Rashi holds that a Tereifah can become Kosher, but the only good sealing is a hole in the lung that later sticks to the flesh. This is unlike the Shach and Taz, who say that Rashi holds like the Ran, that it was never Tereifah. The She'altos derives that a Pesul that became Kosher b'Yedei Adam is Kosher, for the Gemara did not exclude this from this is a Pesul that can become permitted.

Ran (Chulin 11a DH v'Ika): Since a hole in the lung can get healed, why is it Tereifah before it heals? A bruised kidney is not a Tereifah, because we can heal it. We say that a Tereifah is something that cannot heal. It seems that indeed, any Tereifah cannot live. Some holes can never heal. Other holes can heal; they were never Tereifah. If we knew well enough, we would have said from the beginning that it is Kosher. Since we do not know, we forbid.

Rashba (Teshuvah 1:98): Whatever Chachamim called a Vadai Tereifah can never be permitted. The Gemara asked about cases in which Tereifos were cured and lived, and it answered that they did not live 12 months. If one lives, this proves that the injury is not a Tereifah, or it was a miracle.

Mishmeres ha'Bayis (35 Sof Amud a): The general rule is that a Tereifah does not become Kosher again. If every scab heals a hole, many Tereifos would return to be Kosher! Rather, letter of the law, a hole that can be sealed is not a Tereifah. We have a Safek, because sometimes it does not seal well. Until it was sealed well, it was a Safek. Rav Yosef called this a Pesul that returns to be Kosher. Ravina told him that a true Tereifah never heals.

Mishbetzos Zahav (YD 33:4 b'Sof, DH Od): If a scab formed on a hole (in the Veshet), it can live for 1000 years. Twelve months shows that a Safek Tereifah was really Kosher. If a Tereifah developed a scab, it can live, but it is still Tereifah, like the Rashba (98) wrote. Once it was Tereifah, it never becomes Kosher. If there was a Safek whether the Veshet was punctured, and it lived 12 months and we find a scab, it is Tereifah.

Poskim

Shulchan Aruch (YD 44:6): Some say that it is Kosher when the kidneys were removed only by hand, or it was born that way, but if it shrunk to less than a Shi'ur due to illness, it was already forbidden, and it does not become Kosher when they shrink further and dissipate.

Beis Yosef (DH v'Chosav): This is from the Kolbo, based on the Ra'avad.

Yam Shel Shlomo (3:88): The Kolbo says that the Ra'avad permits if it was naturally small, and forbids if it shrank due to illness. We must say that it is Kosher without kidneys when it was born that way, but if this was through illness, once they shrank to less than the Shi'ur it became forbidden, and it cannot become Kosher again when they dissipate. One can check this through the kidney sac. If it is full of Chelev, this shows that it was created this way. If it is empty, this shows that there were kidneys, and they dissipated. If so, one must check even if it is missing only one kidney, for even one shrunk kidney forbids! This is wrong. The Mishnah is Machshir removed kidneys. This connotes through man, or through illness. The Rambam (8:24) explicitly says that when removal of a limb is Tereifah, this is when it was cut, but not if it was born that way. The same applies when a missing limb is Kosher. This is whether it was due to man or sickness, unless it is stricken. Even those who argue with the Rambam cannot say that a removed limb is Kosher only if it was born that way, but not through illness. Even though a Tereifah can become Kosher again, since it is not through man, this is not a problem. A lesion in the lung becomes Kosher if it becomes tangled with flesh. Only through man it cannot become Kosher, like we say about a leg cut at the juncture of the sinews.

Taz (13): I agree with the Kolbo. Indeed, in some places we find that a Tereifah becomes Kosher. There is no source to say so here. Perhaps 'removed kidneys' refers to from birth, but not from illness, for then it was once Tereifah. This is unlike a lesion that became tangled (with flesh). Surely, what becomes healthy again is not Tereifah. If we would know that every punctured lung will later be tangled, we would not call it Tereifah. Since we do not know this, we consider it Tereifah. However, a shrunk kidney is destined to dissipate. If it will return to Kashrus, why should we call it Tereifah? If you will say that perhaps a cure will stop it from rotting more, all the more so it is not Tereifah! Rather, a shrunk kidney is Tereifah because it will totally rot away.

Shach (15): Surely the Kolbo agrees that if a man totally removed it at once it is Kosher, like the Shulchan Aruch and Drishah (2) say. Rather, he teaches that after it shrank, it does not become Kosher again. This is unlike a hole in the lung that became tangled. There, the hole caused the Tereifah, and now it is filled. Also, we hold that a sealing does not help for the lung (39:18,36:3). When the wall covers it is different, for it was covered from the beginning.

Beis Meir: 'It was covered from the beginning' means that it was never Tereifah, for it was initially sealed. This is like the Shach said above (36:6). However, 46b proves unlike this (it says that a scab does not Machshir).

Machatzis ha'Shekel (15): The Kolbo is Machshir only when it was created without a kidney. He connotes that if man removed it, it is Tereifah. The Maharshal disagrees, for the Mishnah is Machshir if the kidneys were removed. This cannot refer only to when it was born this way! 'It was covered from the beginning', i.e. the beginning of the Heter was already there, just the Heter is complete only when it is tangled with flesh.

Pri Chodosh (13): Chulin 68b (which says that a Treifah never becomes permitted) is no proof for the Kolbo. We find that a punctured lung or liver (41:8) can become Kosher. We must say that there is no Heter through man, but it can become fixed through itself. Perhaps when it is small, it is painful (and will cause death), but when it dissipates, the pain ceases! I hold like the Maharshal; the Shach and Taz hold like the Kolbo.