עַל־כֵּ֡ן לֹֽא־יֹאכְל֨וּ בְנֵֽי־יִשְׂרָאֵ֜ל אֶת־גִּ֣יד הַנָּשֶׁ֗ה אֲשֶׁר֙ עַל־כַּ֣ף הַיָּרֵ֔ךְ עַ֖ד הַיּ֣וֹם הַזֶּ֑ה כִּ֤י נָגַע֙ בְּכַף־יֶ֣רֶךְ יַעֲקֹ֔ב בְּגִ֖יד הַנָּשֶֽׁה׃
Therefore the children of Yisrael do not eat the sciatic nerve that is on the hollow of the thigh, to this day, because he touched the hollow of Yaakov’s thigh on the sciatic nerve.
Torah Temimah points to Chullin 92a.
כמה חוטין יש בגיד הנשה? תלתא.
How many strands are there in the sciatic nerve? Three.
The Gemara describes the internal structure of the gid.
Derash on the physical description (Bavli).
The sciatic nerve consists of three strands.