Chullin 37 - An Animal that is Close to Its Death
Water and other liquids are not the only agents that prepare foods to accept ritual impurity. The "love" or the "esteem for the holy things" is another. This unusual concept works in the following way: even the frankincense and the wood, which are not edible at all, can accept impurity if they are part of a sacrifice, and certainly food. This is derived from an extra word "meat" in the phrase " and the meat, anybody who is pure can eat the meat ."
If an animal is close to death, then we are afraid that perhaps it died before the shechitah. Therefore, for such an animal we need additional signs of vitality. Rabban Shimon ben Gamliel says that it needs to jerk its foreleg and its hind leg. Rabbi Eliezer says that it is sufficient for it to spout blood. The Sages say that it needs to jerk either a foreleg or a hind leg, or wag its tail. A small animal that merely extended its foreleg but did not retract it is not kosher, since this is just the spasm of the expiration of life.
Art: Frans Snyders - A deer, a fawn and other dead game suspended on hooks