Tosefta su Genesi 32:38

The [prohibition against eating] the sciatic nerve applies to mixed-breeds, [and] applies to two thighs -- the left thigh and the right thigh. Rabbi Yehuda says, it only applies to one, and logic dictates that it is the right [thigh]. This is the general rule: Whatever has a "spoon of the thigh" (Gen. 32:33), [the prohibition against eating] the sciatic nerve apply to it. Whatever does not have a "spoon of the thigh," [the prohibition against eating] the sciatic nerve does not apply to it. It applies to a fetus, and its fats are forbidden, the words of Rabbi Meir. Rabbi Yehuda says, it does not apply to a fetus. And its fats are permitted, the words of Rabbi Eliezer. And the Sages say, *they are trusted with respect to it (*i.e., butchers are trusted to have removed the sciatic nerve (see Hul. 7:1)) and with respect to [having removed the forbidden] fats. One who sends a cut-up thigh to his friend is required to remove the sciatic nerve [before sending it]. [One who sends a] whole [thigh] does not need to remove the sciatic nerve from it. One who buys a cut-up thigh from the butcher does not need to remove the sciatic nerve from it [since the butcher is trusted to have already removed it], but [one who buys] a whole [thigh] needs to remove the sciatic nerve from it. One who sells a cut-up thigh to an idolater does not need to remove the sciatic nerve from it, as long he does not sell it in front of another Jew, because he (i.e., the other Jew) may come back and buy it from him (i.e., from the idolater, in the belief that the sciatic nerve had been removed). And on account of two things, they said, "We do not sell carrion or tereifah meat to an idolater": (1) because he may return and feed it to a Jew; [and] (2) another thing, is that we are not permitted to cause someone to be mistaken about what he is eating (i.e., the idolater would assume that Jews do not sell carrion or tereifahs and would come to be deceived, cf. Hul. 94a:5). And on account of two things, they said, "A Jew may not say to an idolater, 'Buy meat for me'": On account of [the idolater potentially being sold] carrion-flesh or tereifah. And one other thing: On account of force (i.e., the idolater taking the meat by force rather than paying for it (Hul. 94a:20-94b:1)).

[The laws of the sciatic nerve] apply to [ritually] pure [animals] but do not apply to [ritually] impure [animals]. Rabbi Yehuda says, even to impure [animals], because [the sciatic nerve] was forbidden even before the giving of the Torah [and thus predated the laws of purity and impurity]. They said to Rabbi Yehuda, it does not say, "Therefore the Children of Jacob -- Reuven and Simon [etc.] -- do not eat the sciatic nerve," but rather, "the Children of Israel [do not eat the sciatic nerve]" (Gen. 32:33). Those are the ones who stood before Mount Sinai. Rather, why is it written there (i.e., in Genesis, before the Torah was given)? To make it known why it is forbidden. [The laws forbidding eating the] limb of a live animal apply to impure animals as well as pure animals because it was forbidden to both the Sons of Noah (i.e., Gentiles, who are not liable in laws of purity and impurity) as well as to [the Children of] Israel. And [one who eats the sciatic nerve] may be held liable both on account of it being a limb from a live animal and on account of it being from an impure animal [in the event that it was indeed taken from a live and/or impure animal], the words of Rabbi Yehuda. And Rabbi Eliezer and the Sages say, one cannot be held liable except on account of it being a limb from a live animal, as it is said (Deut. 12:23), "You shall not eat the life with the flesh," and thus it is forbidden [to eat the sciatic nerve of a live animal]. On account of "You shall not eat the life with the flesh" they render him liable. This comes to exclude an impure animal, as to which all of it [and not just the sciatic nerve] is prohibited. One who eats the limb of a live animal transgresses a negative commandment. [Relatedly, with respect to one who eats] the fats (as opposed to the flesh) of a live animal, he is liable for transgressing two negative commandments, (1) on account of its [forbidden] fats and (2) on account of it [being from a] live animal. [With respect to] the blood of a live animal, he is liable for transgressing two negative commandments, (1) on account of [the prohibition against eating] blood (Deut. 12:23), and (2) on account of it [being from a] live animal. [The prohibition against eating a] "limb of a live animal" may render one liable on account of [the animal being] carrion. [One who eats the] blood of consecrated animals is liable on account of two negative commandments, (1) on account of [the prohibition against eating] blood, and (2) on account of [the prohibition against a non-priest eating] consecrated animals, the words of Rabbi Yehuda. And the Sages say, he is only liable for transgressing one negative commandment. [One who eats] the fat-tail of consecrated animals (see Lev. 9:18-20) is liable on account of two negative commandments, (1) on account of [eating the] fat-tail, and (2) on account of [eating from] consecrated animals. Rabbi Yehuda renders him liable for transgressing three [negative commandments]: (1) on account of [eating forbidden] fats, (2) on account of consecrated animals, and (3) on account of the fats of consecrated animals.