Ohalot, Chapter One, Mishnah Six
1) A person does not defile [as a corpse] until he dies.
2) Even he is cut up or even if he is about to die, he [still]
a) makes levirate marriage obligatory
b) and exempts from levirate marriage,
c) he feeds [his mother] terumah
d) and disqualifies [his mother] from eating terumah.
3) Similarly in the case of cattle or wild animals, they do not defile until they die.
4) If their heads have been cut off, even though they are moving convulsively, they are unclean, like a lizard’s tail, which moves convulsively.
Section one: A living person, even an almost dead person, does not defile until he is actually dead.
Section two: If he is about to die, he is still considered alive. The mishnah lists some consequences (besides his still being pure) for the fact that he still counts as being alive.
If his brother died without offspring, the wife is liable for levirate marriage (yibbum) as long as he is alive.
If his father died, his father’s wife is exempt from yibbum as long as he was alive when his father died.
If he is a kohen and his mother is an Israelite who was widowed from his father (also a kohen), his mother eats terumah as long as he is alive (assuming no other offspring). In other words, since he is alive, her status can still follow that of her offspring.
If he is an Israelite and his mother is a daughter of a kohen widowed from his father (an Israelite) his mother cannot eat terumah as long as he is alive.
Section three: The carcasses of cattle or wild animals that were not slaughtered defile (nevelah). The same is true of animals categorized as “sheratzim.” As is the case of human beings, they do not defile until they are dead.
Section four: Dead animals can at times move, even if their heads have been cut off. Despite this movement, they are still considered dead. After all, once the head is off, they’re not coming back to life. This is similar to the case of a lizard’s tail, which might keep moving, even after it has been cut off.