Mishnayos Chullin Perek 9 Mishnah 1

All foods that became ritually impure through contact with a source of impurity transmit impurity to other food and liquids only if the impure foods measure an egg-bulk. In that regard, the Sages ruled that even if a piece of meat itself is less than an egg-bulk, the attached hide, even if it is not fit for consumption, joins together with the meat to constitute an egg-bulk. And the same is true of the congealed gravy attached to the meat, although it is not eaten; and likewise the spices added to flavor the meat, although they are not eaten; and the meat residue attached to the hide after flaying; and the bones; and the tendons; and the lower section of the horns, which remains attached to the flesh when the rest of the horn is removed; and the upper section of the hooves, which remains attached to the flesh when the rest of the hoof is removed. All these items join together with the meat to constitute the requisite egg-bulk to impart the impurity of food. Although if any of them was an egg-bulk they would not impart impurity of food, when attached to the meat they complete the measure. But they do not join together to constitute the measure of an olive-bulk required to impart the impurity of animal carcasses. Similarly, there is another item that imparts impurity of food but not impurity of animal carcasses: In the case of one who slaughters a non-kosher animal for a gentile and the animal is still twitching and comes into contact with a source of impurity, the animal becomes impure with impurity of food and imparts impurity of food to other food, but does not impart impurity of animal carcasses until it dies, or until one severs its head. The mishna summarizes: The Torah included certain items to impart impurity of food beyond those which it included to impart impurity of animal carcasses. Rabbi Yehuda says: With regard to the meat residue attached to the hide after flaying that was collected, if there is an olive-bulk of it in one place it imparts impurity of an animal carcass, and one who contracts impurity from it and then eats consecrated foods or enters the Temple is liable to receive karet. By collecting it in one place, the person indicates that he considers it as meat.