Chullin, Chapter 7, Mishnah 1. A fresh subject opens here: the issur of the gid hanasheh, the nerve running along the thigh (the sciatic nerve). Its roots reach back to Yaakov Avinu's night struggle with the angel, from which he emerged with a wrenched hip. On account of that episode the Torah commands, "Al kein," therefore, that Bnei Yisrael are not to eat this nerve, the one "asher al kaf hayarech," which sits upon the spoon shaped hollow of the thigh. Our chapter unpacks the laws of that prohibition, and the opening mishnah charts its full range: where it holds, in which creatures, and in which circumstances.
Where the prohibition applies
The mishnah begins: "Gid hanasheh noheig," the prohibition is operative, "ba'aretz uv'chutz la'aretz," within Eretz Yisrael and beyond its borders alike. "Bifnei habayit veshelo bifnei habayit": it held while the Beit Hamikdash stood, and it holds equally in a period when it does not stand.
"B'chullin uvimukdashin." Both non-sacred animals slaughtered for the table and consecrated animals fall under it. The practical outcome for korbanot: an offering may not go up in flames on the mizbei'ach while that nerve is still inside it, and it has to be extracted beforehand. The pesukim establish that only what is fit for eating may be brought upon the mizbei'ach; since this nerve is off limits for eating, it must come out before the animal is consumed by the fire.
Which animals, and which side
"Venoheig b'veheima uv'chaya": both the domesticated animal and the wild animal are subject to it. "B'yarech shel yamin," in the right thigh, "uv'yarech shel smol," and in the left as well. Neither leg is exempt; the nerve of each one is forbidden.
"V'eino noheig b'of," the issur has no application to a bird, "mipnei she'ein lo kaf," since a bird possesses no kaf. In a beheimah, and in a human being too, the flesh covering the hip joint swells into a rounded hollow resembling the bowl of a spoon, and the Torah anchored the prohibition precisely to that formation. In a bird the flesh at that spot lies flat, so its thigh never enters the issur at all.
A fetus found in a slaughtered animal
"Venoheig bashalil." A shalil is an offspring found inside the mother after her shechitah, and here we are speaking of one that is alive and has completed its nine months. Such a fetus has a nerve of its own, and that nerve is forbidden. So rules Rebbe Meir, who maintains that a living, full term offspring discovered in the womb requires shechitah of its own and is not permitted agav imo, by way of its mother. Since it stands as an animal in its own right, its nerve carries the issur.
"Rebbe Yehuda omeir": Rebbe Yehuda takes the opposite position, "eino noheig bashalil," the issur does not reach the fetus's nerve. In his view, the Torah's heter for whatever is discovered inside a properly slaughtered animal extends even to a living, full term offspring within her. It is reckoned as part of the mother rather than as a separate beast, so nothing needs to be removed from its thigh.
The surrounding fat
"V'chelbo mutar." The fat around the nerve is permitted to be eaten. The Torah forbade only the nerve itself, not the fat encasing it. The Bartenura notes, however, that Klal Yisrael took upon themselves a stricter practice here: "Nahagu bo issur," they treat that fat as forbidden and do not eat it. In his words, "Yisrael kedoshim nahagu bo issur."
Are butchers believed?
"V'ein hatabachim ne'emanin," butchers carry no credibility, "al gid hanasheh, divrei Rebbe Meir." In Rebbe Meir's opinion, a butcher who claims to have pulled the nerve out of the thigh cannot be relied upon. The extraction is intricate work that eats up time, while the butcher is under pressure to keep meat moving to his customers, so there is genuine reason to fear a rushed job. The buyer must therefore examine the meat himself and verify that nothing of the nerve was left behind.
"Vachachamim omrim": the Sages disagree. "Ne'emanin alav," butchers are trusted regarding this nerve, "v'al hacheilev," and regarding the forbidden fats as well. A butcher's declaration that he did the removal is accepted, since we have no grounds for suspecting that he would lie about it.