Hullin, Chapter Seven, Mishnah Six
In this interesting mishnah, the sages argue whether the prohibition of the sciatic nerve applies to non-kosher (unclean) animals.
1) It applies to clean animals but not to unclean.
a) Rabbi Judah says, even to unclean animals.
2) Rabbi Judah said: was not the sciatic nerve prohibited from the time of the sons of Jacob, and at that time unclean animals were still permitted to them?
a) They replied, this law was ordained at Sinai but was written in its proper place.
Section one: According to the first opinion, if one eats the sciatic nerve of an unclean animal, a donkey, for instance, he is liable for eating an unclean animal, but he is not liable for having eaten the sciatic nerve. Rabbi Judah disagrees and holds that he has transgressed two prohibitions: 1) eating an unclean animal; 2) eating the sciatic nerve.
Section two: Rabbi Judahs argument is based on the unique placement of the sciatic nerve prohibition in the Torah. In the Torah, the prohibition is given in Genesis, before the rest of the laws are given at Sinai. Before the laws were given, all animals were permitted to all of humanity, including the children of Israel. Thus the sciatic nerve was prohibited at a time when unclean animals were permitted. The special nature of this prohibition stays in place, according to Rabbi Judah, even after the Torah was given on Sinai.
The other rabbis argue that this law was actually given on Sinai as well. From the time of Jacob until the giving of the Torah there was no prohibition of the sciatic nerve. When the law was placed in the Torah it was put into the context of the story of Jacob, for the reason for this prohibition lies there. Thus it only applies to animal that were permitted after Sinaiclean animals.