Hullin, Chapter Seven, Mishnah Two
1) One may send to a non-Jew a thigh in which the sciatic nerve has not been removed, because its place is known.
2) When a person removes the sciatic nerve he must remove all of it.
a) Rabbi Judah says: only so much as is necessary to fulfill the mitzvah of removing it.
Section one: We are not concerned lest a Jew buy this thigh with its sciatic nerve, because he will be able to see by looking at the thigh that the entire sciatic nerve is still there.
Section two: When he removes the sciatic nerve, he must remove it all, scraping away at anything that is left.
Rabbi Judah holds that he need not remove the entire thing, rather just enough to fulfill the mitzvah. He need not do any scraping away.