Ohalos, Chapter 2, Mishnah 5. Our chapter has been occupied with the various things that carry tumas meis and with the measures the Chachomim fixed for each one. This mishnah gathers a group of them together and teaches a single principle about all of them: they defile only when the full shiur is there. Take away a bit, and there is nothing left to be metamei.
The mishnah opens: "Eilu mitamin b'tumas meis," these are the items that convey tumas meis, "v'im chasru, tehorin," but if they are lacking, if part of the measure is gone and the shiur is no longer intact, they are tohor. Then comes the list.
The list
- "K'zayis basar min hameis": an olive's volume of flesh from a meis.
- "K'zayis netzel": an olive's volume of netzel, the liquid that seeps out of the flesh of a meis as it decomposes.
- "M'lo tarvad rakav": a large spoonful of rakav, the dry dust that a decomposed meis leaves behind.
- "Revi'is dam": a revi'is halog of blood from a meis.
- "V'etzem k'se'orah": a piece of bone the size of a barley grain.
- "V'eiver min hachai she'chaser atzmo": a limb severed from a living person whose bone is incomplete.
Why specifically a k'zayis of flesh, and why specifically a revi'is of blood? The Bartenura brings the explanation of Chazal. These two shiurim are drawn from the very beginning of a human being: at the point when the fetus first takes shape, its body is the size of a zayis and it holds a revi'is of blood. That formation is the smallest thing that can be called a person, and it sets the measure for tumas meis. And keep in mind how small a k'zayis really is; anyone who has ever looked at an olive knows we are not speaking of much.
Eiver min hachai
The last case in the list is different in character. An eiver min hachai is a complete limb torn away from the body of a living person, and such a limb carries tumah even though the person himself is very much alive. But the limb must be whole in order to have that status. The mishnah therefore says "she'chaser atzmo": if its bone is missing a piece, the limb is tohor.
The Bartenura sharpens the point. Even the largest limb of the body, if a portion of its bone has been lost, no longer defiles as an eiver min hachai. Missing flesh, however, is not the same. A limb whose flesh is partly gone still retains its status and is metamei, and only if so little flesh remains that it could no longer be described as flesh sufficient "leha'alos arucha," enough that the limb would have healed over had it stayed attached to the living body, does it lose its tumah.
The thread running through the entire mishnah is that a shiur set by the Chachomim is a whole; it is not a quantity that can be trimmed and still function. A k'zayis of flesh, a k'zayis of netzel, a spoonful of rakav, a revi'is of blood, a barley grain of bone, a complete limb from a living person: each defiles when it is intact, and each is tohor the moment it is chaser.