Chullin, Chapter 7, Mishnah 2. This chapter is devoted to the gid hanasheh, and our mishnah takes up two practical questions: may a Jew hand over a thigh that still contains the forbidden gid, and once a person sets about removing it, how much must come out?
The first ruling: a Jew may hand a non-Jew a complete thigh taken from a properly slaughtered kosher animal, even while the forbidden nerve is still lodged within it. The mishnah's words are "Sholeach adam yarech lenochri", one sends a thigh to a non-Jew, "shegid hanasheh betochah", with the gid hanasheh still inside.
Why there is no concern here
One might have worried about the chain of events that could follow. A Jew notices the gift in the hands of the non-Jew, decides to buy it, and assumes that since it came from a Jew the gid was surely already taken out. He would then eat the thigh with the forbidden gid still in place. The mishnah tells us not to be anxious about that scenario, and it gives the reason: "mipnei shemekomo nikar" - the place of the gid is plainly recognizable.
Simply put, all you have to do is look. When the gid hanasheh has been removed, the thigh bears the marks of the operation: it has been opened up and the nerve extracted. Here the thigh is whole and untouched, with no cut in it at all, so it is obvious to anyone examining it that the gid is still inside. Since the truth is visible on the surface of the meat, there is no mistake to protect against.
How much of the gid must be removed
Now the second ruling. "Hanotel gid hanasheh", when a person extracts the sciatic nerve, "tzarich sheyitol et kulo", the extraction has to be total. Trimming away whatever lies within easy reach will not do; he is required to probe into the flesh and pull out every last strand, including the stretch of nerve buried deep within.
Rebbi Yehuda holds otherwise: "kdei lekayem bo mitzvat netilah", whatever amount discharges the obligation of taking the nerve out is all that is required. In his opinion, a person who has lifted off the exposed stretch of the gid, the part lying visible on the outside of the meat, has already satisfied what the Torah demanded of him, and the section sunk deep in the flesh stays permitted.