Chullin, Chapter 7, Mishnah 3. This entire chapter is devoted to the gid hanasheh, the sciatic nerve that the Torah forbids us to eat. Having dealt until now with which animals and which parts are included in the prohibition, the mishnah now turns to the punishment: how much must a person eat before he is liable, and how many sets of lashes he receives.
The mishnah opens: "Ha'ochel migid hanasheh kezayit," a person who consumes from this sinew an amount equal to an olive in volume, "sofeig arba'im," is flogged with forty. Through that act of eating he has breached a Torah prohibition, and since the ordinary shiur for eating in the Torah's prohibitions is an olive's bulk, malkut is incurred.
When less than a kezayit is still enough
"Achalo ve'ein bo kezayit," he ate the sinew in its entirety, yet the sinew was a small one and its volume fell short of an olive: "chayav," he is liable to lashes all the same. What he swallowed was a complete biryah, a whole created thing, rather than a fragment of something forbidden.
Here is a wide-ranging rule worth filing away. As a rule, lashes for eating something the Torah forbids require an olive's bulk. An item that constitutes a complete unit exactly as it was created, however, is governed by a different standard. Consider a parallel case: someone who swallows a whole insect, intact and undamaged, is flogged even though the creature never amounted to an olive's volume, and the reason is precisely that an entire biryah went down.
Two thighs, two prohibitions
"Achal mizeh kezayit umizeh kezayit," he ate an olive's bulk from the sinew of one leg and an olive's bulk from the sinew of the other: "sofeig shemonim," he is flogged with eighty. Each leg carries its own forbidden sinew, so two distinct prohibitions have been violated, and each one draws its own forty.
"Rabbi Yehudah omer," Rabbi Yehudah takes a different position: "eino sofeig ella arba'im," only a single set of forty is administered, since in his view the Torah forbids the sinew of the right leg alone. His proof lies in the wording of the pasuk, which speaks of the sinew "asher al kaf hayarech," resting on the hollow (kaf literally means a spoon or palm) of the thigh. The heh hayedi'ah, the definite article, indicates a single specified thigh and not the pair of them. Once the verse has narrowed itself to one, we take it to mean the animal's right side. On this reading rests Rabbi Yehudah's argument against the Tanna Kamma.