1) THE SINEWS OF THE KORBAN PESACH
OPINIONS: The Gemara infers from the Mishnah that a part of an animal that hardens as the animal matures, but is edible when boiled well, is considered meat (Basar) of the Korban Pesach, and one may fulfill his obligation to eat the Korban Pesach with that part of the animal.
The Beraisa states that soft sinews (Gidin) are also considered meat. However, Rav (83b) says that the sinews of the neck are not considered meat. It is clear that there are two different types of Gidin, one type which is considered meat and another type which is not considered meat.
The Gemara records an argument between Rebbi Yochanan and Reish Lakish with regard to sinews that are soft (and edible) when the animal is young, but become hard (and inedible) when the animal matures ("Gidin she'Sofan l'Hakshos"). Which sinews are these? They cannot be the sinews of the neck, because everyone agrees that the sinews of the neck are not considered meat. They cannot be the soft sinews of the animal, because everyone agrees that those sinews are considered meat, as the Beraisa itself states. Even if those soft sinews will become hard, they are permitted since they can be cooked and softened. About what sinews do Rebbi Yochanan and Reish Lakish argue?
(a) RASHI explains that they argue about the sinews of the neck. Even though Rav says that those sinews are not considered meat, Rebbi Yochanan argues with Rav and says that they are considered meat. Rebbi Yochanan apparently maintains that although they are hard, since they can be softened through boiling they are considered meat, like the ligaments mentioned in the Mishnah.
Why do Rav and Reish Lakish rule that the sinews of the neck are not considered meat, even though they can be softened through boiling? Apparently, Rav and Reish Lakish maintain that cooking the sinews of the neck does not soften them sufficiently, and thus they are not comparable to the Gidin mentioned in the Beraisa which become completely softened through cooking.
(b) TOSFOS (DH Gidin and DH Ho'il) cites the RI who explains that Rebbi Yochanan and Reish Lakish argue about a third type of sinew, the sinew of the spinal column. Although these sinews can be softened by cooking, they still are not considered meat according to Reish Lakish because they do not taste like meat at all. In this respect, they are not similar to the cartilage mentioned in the Mishnah, which not only can be softened by cooking after it hardens, but it also has the taste of meat.
Alternatively, Tosfos suggests that these sinews become hard even while the animal is young. Therefore, the fact that they can be cooked and made soft does not give them the status of "meat." In contrast, the cartilage mentioned in the Mishnah hardens only after the animal matures in age.
Tosfos says that according to both explanations, both types of sinews differ from the sinews of the neck, which are hard even when the animal is young and are not considered to be meat according to anyone, even if they can be softened by cooking.