Although it is forbidden to cook meat with milk, not all meat and not all milk are included. Thus, on today’s daf we learn that if someone cooked bones, tendons, horns or hooves in milk, he is not liable. Similarly, if someone cooked meat in whey, he is not liable.
Neither the Gemara nor the rishonim explain why some parts of the animal – bones, tendons, horns and hooves – are not considered meat with regard to this law. One could have easily argued that the passages in the Torah that are the source for the prohibition of cooking meat together with milk repeat the idea “do not seethe a kid in its mother’s milk” without ever mentioning “eating” or “meat.” It would be reasonable to suggest that the prohibition is broad and should not be limited to parts of the animal that are usually eaten.
Some suggest that based on the Rambam (Mishneh Torah, Hilkhot Ma’akhalot Asurot 4:18 and 9:7) and the Gr”a (Shulḥan Arukh Yoreh De’a 87:20) we can understand that the prohibition of milk and meat parallels the prohibition of neveila (an animal that was killed by a predator or died on its own) and tereifa (an animal suffering from a terminal condition), and just as those prohibitions do not include parts of the animal that are not ordinarily eaten, this prohibition does not include those parts, either. The practical ramification of this explanation is that even soft bones would not be forbidden, since they are not ordinarily eaten.
Another approach is to view the prohibition stemming from “do not seethe a kid in its mother’s milk” as limiting this law to things that are ordinarily cooked in order to give them flavor. Things that are usually cooked in order to flavor other things (e.g., bones) would not be prohibited. According to this approach, soft bones, which are eaten by some people, might be considered forbidden. In any case, in his Igrot Moshe, Rav Moshe Feinstein rules that there is no Biblical prohibition against cooking soft bones with milk, arguing that even according to the second approach, most people cook them only to flavor other foods.