Halachah: From here we can extrapolate that, if during the Shechitah the part of the esophagus that has been Shechted is found to be punctured, the animal is T'reifah

Proof: ... since the Gemara here would have forbidden it due to the sides, if not for the fact that the cut opens wide.

Clarification (Part 1): The Halachah at the end of the first week is - that he is Tamei if it spreads, and even if it did not, he is Tamei too, if there are two white hairs to begin with ...

Clarification (Part 2): ... but if, at the end of the week, neither is there a white hair, nor did it spread, then he is Tahor.

Clarification (Part 3): But other forms of Nega'im require a second week of quarantine.

Question: The Lashon is a little difficult however; It ought not to have said ''Metam'in', but rather 'Metaharin', as opposed to other kinds of Nega'im, which require a second week of quarantine.

Answer: That is precisely what the Gemara means ... It is Metamei specifically with white hairs or spreading; otherwise, if neither occurs, even though the Nega did not become weaker, he is Tahor.

Clarification: It is possible that even Rebbi Yossi in Perek Kirah (Shabbos 39.), who considers it Toldos ha'Or, since it passes in front of the entrance to Gehinom, will concede here that it has the Din of 'Sh'chin and not of Michvah, since the fire of Gehinom is a Heavenly one.

Implied Question: Even though at the end of Perek Eilu Devarim (Pesachim (73a) the Gemara declares him Chayav regarding Shabbos, in a case where someone Shechts on Shabbos outside the Azarah to Avodas-Kochavim, because of the element of Tikun - that he removed the animal from the status of Eiver min ha'Chai?

Answer (Part 1): ... Shabbos is different, in that we hold 'Meleches Machsheves Asrah Torah', and as long as there is the slightest Tikun, it falls into the category of 'Meleches Machsheves' (since that is what he had in mind when he performed the Melachah), despite the fact the overall damage exceeds that of the Tikun.

Answer (Part 2): ... but as far as Avodah-Zarah is concerned, once the overall damage exceeds that of the Tikun, it is not considered Hana'ah.

Clarification (Part 1): The Mishnah at the end of Maseches Avodah-Zarah (78:), which permits sticking a knife ten times into hard earth, without needing to make 'Libun' is speaking with regard to cutting cold things, as the Gemara there explicitly states ...

Clarification (Part 2): ... and the Gemara here, establishes that the knife requires 'Libun' in fire in order to satisfy the opinion that considers the location of the Shechitah boiling.

Question: The Gemara here seems to hold that a knife requires 'Libun', and so is implied by the Tosefta in Avodah-Zarah (Perek 9), where it rules that spit-rods, metal grills and knives require 'Libun' in fire; Whereas from the Gemara in Perek Kol Sha'ah (30:) it seems that 'Hag'alah' (in boiling water) will suffice, as the Gemara asks there 'What do we do with knives on Pesach?' And it concludes that 'both things require Hag'alah (and not 'Libun').

Answer Explanation #1: Rabeinu Tam therefore explains that Chametz is different, inasmuch as it absorbed Heteira (what was permitted [in similar vein to the Gemara's distinction in Maseches Avodah-Zarah, 76a]).

Implied Question: Even though at the time when it exudes, it is Isura ...

Answer: ... at the time that it absorbed it was Heteira.

Implied Question: Seeing as Rav Ashi himself made this distinction in Maseches Avodah-Zarah, (76a) why in Pesachim 30b) did he state that he would make new knives for Pesach (Why did he not rely on his own ruling in Avodah-Zarah?

Answer: Because he was strict with himself.

Conclusion: It appears that what the Gemara conclusion 've'Hilch'sa Idi ve'Idi be'Roschin' are, in fact, the concluding words of Rav Ashi.

Explanation #2: Furthermore, says Rabeinu Tam, the Tosefta here is speaking about large knives with which one roasts meat, and which are therefore used with fire - which explains why they require Libun.

Proof: And this is borne out by the fact that the Tana lists them together with spit-rods and metal grills; whereas the Pesach knives refers to small ones.

Halachah: The Rivam rules that the used knives that one purchases from Nochrim require Libun, seeing as they tend to use them in attending to their lamps, which contain Cheilev, as well as for turning over the meat that is roasting on the coals and cutting it.

Refutation (Part 1): Rabeinu Tam however rules that, since that is not their main used main use, we do not contend with the fact that they may have happened to use them for that purpose.

Refutation (Part 2): And besides, even if they did happen to use them with fire, Hag'alah will suffice - since whatever eventually exudes through Hag'alah with boiling water, will exude after the first Hag'alah.

Refutation Overruled: This is not a good S'vara however, since it is possible that once a knife has been used with fire, it never exudes the Isur completely through Hag'alah, without Libun, only it keeps on exuding a little each time it is boiled in water ...

Precedent: ... similar to earthenware, which cannot be Kashered through Hag'alah, where we do not say that they exude whatever they have absorbed after the first Hag'alah, but rather that they never exude from it completely.

Clarification (Part 1): We need to establish this where one knows that it was a bas Yoma (used within twenty-four hours [and which therefore gives a good taste]) ...

Proof (Part 1): ... Otherwise, why would Rav require peeling, bearing in mind that S'tam vessels belonging to Nochrim are assumed not to be B'nei-Yomon'.

Proof (Part 2): And in connection with a mouse that fell into beer in the last Perek of Avodah-Zarah (68b) the Gemara is not sure as to whether Rav holds 'Nosein Ta'am li'Fegam (something that gives a bad taste) is permitted or forbidden.

Proof #1 (Part 1): And we have a proof that vessels belonging to Nochrim are assumed to be not B'nei Yoman from oil of a Nochri, which the Mishnah in Perek Ein Ma'amidin (Avodah-Zarah 38b) forbids; and Shmuel there attributes this to the exuding of the vessels which renders them forbidden. However, the Gemara concludes there that Rebbi Yehudah Nesi'ah and his Beis-Din took a count and permitted it, because they held 'Nosein Ta'am li'Fegam is permitted, and the 'Ta'am li'Fegam' to which they refer is based on the fact that the vessels are not B'nei Yoman'.

Alternative Explanation: It is not possible to explain that even if the fat was actually visible it would be considered Nosein Ta'am li'Fegam into the oil ...

Refutation: ... because then there would be a Kashya on our Mishnah, since there (in Avodah-Zarah) the Gemara permits honey because 'What is there to worry about? - if it is because of a mixture, it smells putrid; whereas if it is because of what vessels belonging to Nochrim have absorbed, "Nosein Ta'am li'Fegam" is permitted!' And by oil it is forbidden.

Proof #1 (Part 2): The reason must therefore be because with regard to honey, even if there is fat that is visible, it gives a bad taste to the honey, whereas by oil, the bad taste is due to the fact that it is not ben Yomo.

Proof #2: Furthermore, the Gemara there explains that the reason that a grain of Chiltis (a sharp spice) is forbidden is because they (the Nochrim) cut it with a knife, even though Nosein Ta'am li'Fegam is permitted, nevertheless, due to its sharpness, it transforms the taste into a good one. Now if S'tam vessels of Nochrim were 'B'nei Yoman', that reason would be superfluous.

Implied Halachah #1: As a result, the Rivam instituted that, after the Nikur (the removal) of the Cheilev from the animal, one should peel all the areas where the knife cut, since, due to 'Duchka de'Sakina' (the pressure of the knife), the animal absorbed some of the Cheilev.

Implied Halachah #2: Similarly, when bringing meat from the Nochri butcheries, he necessitated peeling all the areas where the Nochri cut with his chopping-knife or knife ...

Question: ... and one needs to justify our Minhag to be lenient in this matter.

Answer: Perhaps it is because it is only by the neck of the animal, which is slightly hot, that Duchka de'Sakina is effective.

Proof #1 (Part 1): ... since the Gemara will shortly extrapolate from Eiver min ha'Chai that it also absorbs Heter, but counters that it only absorbs it when it becomes hot. Now, according to the Lashon that attributes it to Duchka de'Sakina, the Gemara ought to have asked that the Heter too, it only absorbs due to Duchka de'Sakina, in which case it could not refute it in that way ...

Proof 1# (Part 2): ... unless we say that even when it absorbs via 'Duchka de'Sakina', that is only at the end of the Shechitah, when it is hot.

Proof #2: Furthermore, later in Perek Kol ha'Basar (111b) the Gemara will issue a ruling forbidding a radish that one cut with a meaty knife together with a Kutach (a preserve containing milk) - because, due to the sharpness of the radish, the knife exudes what it is has absorbed and the radish absorbs it - but not on account of Duchka de'Sakina!

Inference: This implies that it is unanimously Kasher for use with cold, specifically that is, to Shecht with it another time

Question: ... even though the knife is dirty with a lot of blood ...

Answer: What difference does it make, seeing as the neck of the animal anyway contains a lot of blood from the animal itself, which it does not absorb because the 'Simanim are busy exuding blood'.

Qualification: But to cut hot things is forbidden, even after Shechting a kasher animal.

Proof: Indeed the Gemara in 'Kol ha'Basar (111b) specifically states 'A knife with which one Shechted may not be used for cutting hot things'.

Implied Question: Why does the Gemara at the end of Maseches Avodah-Zarah (76b) require sticking a knife of Avodas-Kochavim into virgin soil ten times, before using it to cut a radish?

Answer #1: It is not comparable to the knife of T'reifah in our Sugya, which is only used casually, whereas the knife of Avodas-Kochavim, which is used to cut Isur regularly, the fat sticks to it and does not leave it without sticking it into earth.

Answer #2: Rabeinu Tam explains that it is because there the knife is used to cut hot things, whereas here the neck is cold.

Question: In Perek Kol ha'Basar (Daf 111b) Shmuel forbids using a knife with which one Shechted to cut hot things, even if one subsequently washes it?

Proof #1: ... as is implied a little later, where it continues 'To cut cold things, some say that it (the knife) requires washing (not the meat, as Rashi explains, since the Gemara uses the word 'Ba'i' [with an 'Alef' at the end]) denoting the feminine form.

Proof #2: Moreover, the Gemara is coming to teach us how to rectify the knife, like it says 'One may not use it to cut hot things' and similarly 'to cut with it cold things'.

Answer: The case there is different, since the Isur there is only that of blood, and blood, which slides of the surface on which on which it finds itself is not absorbed so much.

Reservation: It is nevertheless forbidden to us it for cutting hot things or even cold things without washing it (or at least wiping it (according to those who do not require washing)

Precedent: ... like we find with regard to liver that is lying on top of a piece of meat, which is forbidden Lechatchilah (and we do not say that it merely slides off), since the liver contains a lot of blood.

Clarification: And the next time that he wants to cut meat with it, he should remember to Kasher it.

Implied Question: And on the knife with which he Shechted the Gemara does not ask why he cannot Shecht and then cut with it ...

Answer: ... because one tends to protect one's Shechitah-knife from getting spoilt.

Proof: And regarding Shabbos too, the Gemara considers it like the peg of a plow, which is Muktzah because of its preciousness - 'Muktzeh Machmas Chasaron Kis'), because one does not use it for other things so that it should not become spoilt.

Clarification: This refers to immediately after it is cut, before the flesh has become cold; but once it becomes cold, it is unnecessary ...

Source: As the Pasuk writes in Shemini "And they placed the fat pieces on top of the breasts".