Gemara

(Rav Huna): If after Shechitah, a nick was found in the knife, even if the knife was used to break bones the entire day (after the Shechitah), the Shechitah is invalid. Perhaps the nick came when the knife cut the animal's skin, before cutting the Simanim;

(Rav Chisda): The animal is permitted. We assume that the nick came when cutting a bone.

Rav Huna holds like he taught, that an animal is forbidden (to eat) when it is alive. After death, we assume that it is still forbidden unless we know that it was slaughtered properly.

Rav Chisda holds that a bone Vadai (surely) can cause a nick. Perhaps the skin can. We attribute (the nick) to something Vadai, and not to a Safek.

10b: The Halachah follows Rav Huna when the knife was not used to cut bones afterwards. The Halachah follows Rav Chisda when the knife was used to cut bones afterwards.

Inference: Rav Chisda argues even if the knife was not used to cut bones afterwards. He assumes that the knife became blemished through the neckbone (after cutting the Simanim).

Question (Rava - Beraisa): If a Tamei man immersed, and later found a Chatzitzah (obstruction) on his body, even if he worked the whole day (after immersing) with the substance he found on his body, the Tevilah is invalid, unless he is sure that there was no Chatzitzah when he immersed.

He certainly immersed, and it is a Safek if there was a Chatzitzah, and we assume that the Safek occurred!

Answer: There, he was Vadai Tamei. We leave him in his Chazakah (status quo).

Question: Also the animal was forbidden before Shechitah. We should leave it in its Chazakah, and say that it was not slaughtered!

Answer: Regarding the man, there is a Re'usa. (The Chatzitzah suggests that the Tevilah was invalid.)

Question: There is a Re'usa also regarding Shechitah! (The nick suggests that it was invalid.)

Answer: There is no Re'usa in the party in question (the animal). Regarding Tevilah, the Re'usa is on the man himself.

A case occurred, and Rav Yosef forbade 13 animals due to a blemish found in the knife after all were slaughtered.

Opinion #1: This can be even like Rav Chisda. He permitted the first animal, and forbade the rest.

Opinion #2: It must be like Rav Huna; also the first animal was forbidden.

Rav Chisda is lenient to say that the knife became blemished on the neckbone. If so he should be lenient also to say that it became blemished on the neckbone of the last animal, and permit all of them!

Rav Kahana required butchers to check their knives after every Shechitah.

Suggestion: He holds like Rav Huna. If a blemish is found, this disqualifies the previous Shechitah.

Rejection: No, he can hold like Rav Chisda. Even if a blemish is found, we attribute it to the neckbone. The Shechitah was Kosher. One must check the knife to avoid invalid Shechitah of subsequent animals.

Rishonim

Rosh (1:15): The Halachah follows Rav Huna when the knife was not used to cut bones afterwards. We do not attribute the nick to the neckbone, for normally one does not cut it forcefully. However, sometimes after cutting the Simanim and veins, the knife touches the neckbone. The Halachah follows Rav Chisda when the knife was used to cut bones afterwards, and similarly if the blade of the knife fell on hard ground. Therefore, one must check between each Shechitah. If one did not check, and it was found to be blemished, everything slaughtered is Pasul, like Rav Yosef forbade 13 animals, including the first. We are concerned lest it became blemished through the skin.

Ran (3a DH Itmar): If one slaughtered an animal and touched the neckbone, and the knife was found to be blemished, the Shechitah is Kosher. It is as if he cut bones. Presumably one may not rely on this. We rule like Rav Chisda only when he cut bones, so one may be lenient only when he totally cut the bone, but not if he merely touched it. The Ramban says so. Others are more stringent, and disqualify the Shechitah even if he broke the neckbone. This is unlike cutting bones, for the neckbone is soft. And even regarding other bones we are lenient only if he chopped them, but not if he cut through Molich and Mevi (back and forth), the way he cut the neckbone. The Rashba says so.

Question (Tosfos 10a DH Sachin): The Gemara answered that there is no Re'usa in the animal itself. If a Mikveh was found to be deficient, and a Tamei man had immersed in it, he is Tamei. Since it is deficient in front of us, we leave him in his Chezkas Tum'ah (Nidah 2b). We do not say that there is no Re'usa in the man! Here also, we should leave the animal in its Chazakah, and say that it was not slaughtered!

Answers (Tosfos): There is different, for the Mikveh constantly decreases, like the Gemara says there. Alternatively, here is different, for a bone surely blemishes.

Question: What is the difference between a Re'usa in the knife or in the animal?

Answer: The Gemara means that there is a Sefek-Sefeka (two doubts). Perhaps the knife became nicked on a bone. Even if it became nicked on the skin, perhaps the nick did not encounter the Simanim.

Poskim

Shulchan Aruch (YD 18:15): If one slaughtered with a checked knife, and after Shechitah it was used to chop bones, not through Molich and Mevi, and a nick was found in the knife, the Shechitah is Kosher. We assume that the nick came while breaking the bones.

R. Akiva Eiger (YD 18:15): Tosfos answered that perhaps the nick did not encounter the Simanim. He must discuss a blade twice as long as the neck. (If not, the entire blade is needed to slaughter in one Molich and Mevi.) We must say so about one who broke bones (to distinguish from one who immersed and worked with that substance afterwards). If so, if the knife is short, it is forbidden!

Shulchan Aruch (ibid.): The same applies to all similar cases, e.g. it fell on the blade. This is only if we saw it fall on the blade. If there is a Safek, we do not assume that it fell on the blade.

Beis Yosef (DH u'Mah she'Chasav Aval): The Rashba says that if there is a Safek, we do not assume that the knife fell on the blade. This is obvious. Since an animal has Chezkas Isur when it is alive, it remains in Chezkas Isur unless we know that it was slaughtered properly. If we do not know that the knife fell on the blade, we do not know that it was slaughtered properly.

Shach (25): We do not assume that it fell on its blade, because it is a Sefek-Sefeka. Perhaps it did not fall on its blade. Even if it did, perhaps this did not cause a nick. The Rosh (Teshuvah 20:14) says that the ground does not Vadai blemish. This is unlike a knife that was lost, and later it was found blemished (Sa'if 3). There it is Kosher, for there is only one Safek (perhaps he is unaware that he cut bones). Also, here, since the knife is in front of us, we must check it. There was never Chezkas Heter. There, since the knife was lost, there was Chezkas Heter. Therefore, even if the knife was found blemished afterwards, we leave it in its Chazakah. We must distinguish, for Teshuvas ha'Rosh rules like this (Sa'if 3), but in his Pesakim he rules like the Tur and Shulchan Aruch here! The Yam Shel Shlomo (1:18) says like this. The Bach says that the Rosh retracted in his Pesakim from what he wrote in his Teshuvos, and that the Shulchan Aruch and Maharshal overlooked this. This is wrong. Also the Levush, Zivchei Tzedek, and Lechem Chamudos bring both of these. Darchei Moshe brings the law of Teshuvas ha'Rosh.

Shulchan Aruch (ibid.): If one broke the neckbone, we do not say that this blemished the knife.

Taz (12): The Shulchan Aruch is astounding. It understood the Ran to say that others are stringent even if one chopped the neckbone, and therefore forbids this. This is wrong. The Ran mentioned a distinction between chopping and Molich u'Mevi only later, when he says that even if he broke other bones (through Molich and Mevi, we are stringent). Even the third opinion is Machshir if he chopped other bones, and all the more so the second opinion. Surely, initially the Ran discusses breaking the neckbone through Molich and Mevi. He distinguishes from merely touching it. Rather, even if he cut off a piece from it, we do not attribute the blemish to this for two reasons. The neckbone is soft, and it is not prone to blemish knives. Further, even if it were as hard as other bones, since he cut it through Molich and Mevi, this does not blemish the knife. However, if he hit the neckbone forcefully through chopping, surely we attribute the nick to this. This is included in cutting bones, i.e. any bones. Surely the neckbone is included in 'bones'! Chachamim did not say 'except for the neckbone'! Surely we exclude only cutting the neckbone through Molich and Mevi, which is the normal way to cut it after cutting the Simanim. There is a proof from what the Ran said in the name of the Rashba, 'it is not normal for the neckbone to blemish (a knife) through Molich and Mevi.' This implies that if the cut through chopping, we attribute a nick even to the neckbone, for this is common. It is no less than if the blade fell on hard ground, which we permit. All the more so we permit if it chopped the neckbone! The Mordechai (587)`brings that Rivak (Rabbeinu Yehudah bar R. Klunimus) would hit the knife on wood before putting it away, so later he would be able to attribute a nick to this. Surely the neckbone is harder than wood, so we attribute a nick to it, if he chopped! The Tur concludes 'since he cut Derech Shechitah, this is unlike breaking bones.' This connotes if it was not Derech Shechitah, rather, chopping, we attribute the nick to it.

Shach (26): The Shulchan Aruch is difficult. The Poskim connotes that if he chopped, we attribute to the neckbone.

Gra (26): The Shulchan Aruch derives from the fact that we distinguish only when it was used to cut bones.

Pischei Teshuvah (7): If a knife cut the neckbone, and we found a nick too small for a fingernail to get caught, Shevus Yakov (2:53) is Machshir due to a Sefek-Sefeka. Perhaps the Halachah is like the Rosh and those who Machshir such a knife, and perhaps it became nicked on the bone. A case occurred in which the Shochet heard something like 'crack' when the knife encountered the neckbone. Shevus Yakov said that clearly, all agree that this caused the nick, since there are Raglayim l'Davar (circumstantial evidence). Beis Lechem Yehudah questioned the first law, since Shevus Yakov himself (1:46) ruled that Sefek-Sefeka does not help against a Chazakah. He said that the second law applies only if he chopped, but not Derech Molich and Mevi. The Pri Megadim agreed with the first law, and condones one who is lenient like the second law to avoid a big loss.