Hullin 37: At Death's Door (before Shechitah)

A new mishnah! Establishing the category of an animal that is close to death (misukenet) -- in contrast to a treyfa, where it becomes clear that the animal had something wrong internally and would likely have died within the year. This case is an animal that is weak, not damaged internally. Thus, signs of vitality during shechitah become essential, to be certain that the animal didn't die of its own accord just prior to the shechitah. Also, how the Gemara ensures the distinction between a treyfa and an animal that is close to death already, with the prohibited fat (cheilev) as a key point.