Gemara

(R. Yochanan): One may be Manuy on a Korban Pesach on sinews that ultimately harden [in a mature animal. They are considered like meat.]

(Reish Lakish): One may not be Manuy on them.

R. Yochanan considers their current state. Now they are not hard;

Reish Lakish considers their final state. They will harden.

Question (R. Yirmeyah): Reish Lakish asked R. Yochanan if the skin of the head of a tender calf is Metamei [like food], and he answered that it is not.

This shows that he considers it as if it already hardened!

Answer (R. Avahu): R. Yochanan retracted and said that the Mishnah [from which he learned that we consider the current state] is like an individual. (The Halachah does not follow it.)

Chulin 55b (R. Yochanan): Even if an animal has skin only over the feet, it is Kosher.

Question (Rav Asi - Mishnah): The following skin is considered like meat - the skin on the feet (for it is soft)...

Answer (R. Yochanan): That is not a Stam Mishnah. My text attributes it to R. Shimon [and Chachamim argue];

(Beraisa): If one slaughters an Olah with intent to burn on the Mizbe'ach Chutz li'Zmano (after the allotted time) a k'Zayis of the skin under the tail, one who eats it is Chayav Kares;

R. Shimon ben Yehudah says in the name of R. Shimon, improper intent for any of the following takes effect -- skin on the feet, on the head of a tender calf...

Rishonim

Rambam (Hilchos Ma'achalos Asuros 4:21): Skin of the following is like the flesh, regarding the Isur to eat it, and Tum'ah: people, pigs, a camel's hump that never bore a load and is still soft, the Ervah...

Poskim

Shulchan Aruch (OC 259:7): One may break the seal on an oven, remove food, and seal it again.

Rema: One may not do so at night shortly after putting pots inside, lest they are not yet fully cooked, for sealing the oven causes Bishul.

Question: On Shabbos night, people take fully cooked chicken from a pot and leave some in for the morning. Through great cooking, chicken bones become so soft that they break easily. One can suck and taste the marrow more than the night before. Some eat the soft bones during the day. At night, the bones were not even k'Ma'achal Ben Drusai (a third or half cooked). It was forbidden to return the pot to the fire, for this cooks the bones! In Yerushalayim, some permit and some forbid this.

Igros Moshe (OC 4:76): I never heard anyone ask about this. In Europe, it was very hard to open the oven. If one opened it, it was impossible to seal it, and the food would not be so hot during the day. Here [in the U.S.] people rarely take from the Chullent pot on Shabbos night. Most people put beef in the Chullent. Its bones remain hard even after 24 hours.

Igros Moshe: Does Bishul apply to bones, which are not proper to eat, and do not get truly cooked? Rashi (Shabbos 74b DH Sheva) says that Bishul applies to putting a barrel in an oven to solidify it. The Rambam (Hilchos Shabbos 9:6) says that Bishul applies to softening something hard, or hardening something soft, through fire. However, this need not include softening for the sake of eating something that is not a food or is not destined to be eaten, even if some eat it. Surely, such [a minority of] people do not make it a food, even if it is soft. One who eats bones of Nivlas Ohf Tahor does not become Tamei (Beitzah 7a), even if they are soft and edible. The Rambam forbids, but exempts, since they are not proper to eat. Some skin is considered like meat. We must say that [other] skin is not like meat because it is not proper to eat. In Chulin, R. Yochanan says that a lone opinion considers skin of the feet and of the head of a tender calf to be like meat. Seemingly, the latter is unlike meat because it is destined to harden. We learn about sinews from skin of the head of a tender calf! Skin of the feet is edible even in a big ox! Perhaps it is because it feels like skin. We learn from the Rambam that even after bones become soft through cooking, they are not considered food because it is not common to eat them. Batlah Daito Etzel Kol Adam (we are not concerned for his opinion. The Halachah is based on normal people.) It is like swallowing something other than food. It is not considered eating. Therefore, cooking it is unlike cooking a barrel.

Igros Moshe: Perhaps one is liable for cooking bones enough to soften them and enable breaking them to remove the marrow. However, we find Bishul only for the sake of the matter itself, but not to enable getting what is inside. Making an opening pertains to Binyan. One may break bones with his mouth or a Kli to get the marrow, so it is permitted also through putting in boiling water. Shmiras Shabbos k'Hilchasah (11:18) says to be particular to ensure that bones are fully cooked before returning them to a fire, and that ha'Gaon R. S. Z. Auerbach agreed. If he means that the bones get soft like skin after much cooking, it is totally forbidden, but in practice, this does not occur.

Minchas Shlomo (of ha'Gaon R. S. Z. Auerbach, Ztz"l, 1:6): If chicken bones are not fully cooked and proper to eat, one should not return them to a fire or cover the pot. I agree that if the bones do not become edible, just it enables getting the marrow, there is no problem. The bones are spongy, so presumably when the meat is cooked, the marrow inside the bones is cooked. Through normal weekday cooking one eats the meat and discards all the bones. However, when a Chullent is cooked for many hours, bones of the neck, wings and feet of young birds become soft, and many eat them with the meat. They have a good taste. Meforshim on YD 87:8 bring from Shulchan Gavoha that even if a soft moist bone has the taste of meat and is eaten, one who cooks it with milk and eats it afterwards is exempt. However, many disagree. Further, we cannot learn from Bishul on Shabbos from meat and milk, which depends on being considered meat. One is liable for cooking sinews, even though they have no taste. Shmiras Shabbos k'Hilchasah wrote "one should be particular" because this is like a Chidush, and most people are not careful about it. It would be better to stress that it is totally forbidden if they are not fully cooked.

Minchas Shlomo: I do not understand Igros Moshe's proof that Bishul does not apply to bones. Even if the Isur of Nevelah does not apply to them, this is because they are not meat, but not because they are not proper to eat. A fetal sac is called a mere Pirsha (waste - Chulin 113b), but the Shulchan Aruch forbids it mid'Rabanan. Perhaps Bishul applies to it mid'Oraisa. The Rambam (Hilchos Pesulei ha'Mukdashim 18:23) says that Pigul, Nosar and Tamei apply to a Shilya, like to meat. The Rambam is Mechayev one who eats soft bones of a Pesach, for breaking them, yet one is liable for cooking them, since they become proper to eat. Tzlach (84a) says that through much cooking, even the hardest things become edible. In places where many people eat the bones for pleasure (e.g. in Eretz Yisrael, where slaughtered birds are usually very young, and many bones can be chewed and have good taste), even people who never eat them, perhaps mid'Rabanan they may not return a pot with such bones to the fire.

Minchas Shlomo: I heard that a great Gaon wants to say that one who returns the pot intends only for the food and not for the bones, so this is Pesik Reishei d'Lo Nicha Lei. (Regarding Techumim, we say that a barrel is Batel to the water inside (Eruvin 97b). Here is different, for the bones need to cook. Also, perhaps the secondary is Batel to the primary only in Melachos such as Hotza'ah and trapping, in which the item itself does not change.) For people who chew the bones, it is a Pesik Reishei d'Nicha Lei, or even worse. Chachamim commanded to check one's pockets before going to Reshus ha'Rabim, and not to go out with pebbles or earth in one's shoe, even though it bothers him. It is considered like intent to be Motzi it. Also here, one who returns a pot knows that this will cook bones inside, so he must check that there are not any! Perhaps one may not eat the bones due to Nolad.

Minchas Shlomo: The Rema forbids sealing an oven if there is a Safek lest the food inside is not fully cooked. Here that he does not intend for the bones at all, one may be lenient about a Safek. Meat sellers told me that the chest bone of young calves becomes soft and edible after much cooking, and many buy it in order to put it in the Chullent. When it is soft, they enjoy chewing it. Why didn't the Poskim discuss this?

Minchas Yitzchak (8:25): It is an Isur Torah to return a pot with such bones to the fire. The Mishnah Berurah (318:26) forbids mid'Oraisa cooking what already reached k'Ma'achal Ben Drusai, but relies on Poskim who disagree to permit the food b'Di'eved. These bones did not yet reach k'Ma'achal Ben Drusai, so all forbid returning the pot mid'Oraisa. One may not return food even if it is a Safek if it is fully cooked (Bi'ur Halachah 318 DH Afilu). This is for those who eat them. For those who discard the bones, this depends on the argument about locking a house to guard it when this will trap a deer. The Rashba permits, but the Ran disagreed. The Magen Avraham (316:11) says that one should not be so lenient. I conclude that if it is a Safek that the bones were fully cooked, and he does not normally chew them, he may return them. Also ha'Gaon R. S. Z. Auerbach said so, in any case when many people in the city chew them.

Orchos Shabbos (1, footnote 75): Also ha'Gaon R. Y.S. Elyashiv Ztz"l forbids returning mid'Rabanan for one who does not eat the bones. Ha'Gaon R. N. Karelitz Shlita permits.

Ashrei ha'Ish (2:23:93): Ha'Gaon R. Y.S. Elyashiv Ztz"l holds that if they were cooked four hours on Erev Shabbos, they are k'Ma'achal Ben Drusai. Some say that there is no Bishul after this, so one who does not eat them may return them.