Keilim, Chapter 19, Mishnah 4. This chapter examines the bed and its parts, and in particular the mizran, the woven band of cloth that is wound around the frame of the bed to hold it together. Very often a length of that band remains over and hangs down loosely from the bed. The question that occupied the previous mishnah, and that continues here, is how much of that hanging length is treated as part of the bed itself.
Here the Tannaim disagree. Rabbi Meir maintains that no matter how far the mizran trails downward, it never stops being bed, and so whatever tumah attaches to the bed attaches equally to it. Rabbi Yose sets a boundary: the first ten tefachim of the dangling stretch are still reckoned as bed and take on whatever the bed takes on, while everything beyond that measure is judged on its own. Our mishnah takes this argument and works it out in a practical case.
The Case of the Zav
The mishnah's opening word is "nisa": a person pressed his weight down. The person in question is a zav, and the surfaces involved are "al hamitah ve'al hamizran", the bed and the band coiled around its frame, whose surplus trails loosely below.
A zav is a man who has experienced a particular abnormal discharge that comes from illness. One of the striking features of his tumah is that anything he leans or rests his weight upon becomes an av hatumah through what is called tumat midras, the tumah of being trodden or pressed upon. The object he leaned on does not merely become tamei; it becomes a source of tumah in its own right, at the highest of the levels with which we are dealing here.
Rabbi Meir accordingly rules: "metamei shenayim uposel echad", it passes tumah down two further levels and spoils one beyond them. Trace the sequence. Through the zav's weight the bed acquires the rank of av hatumah, and since Rabbi Meir treats the mizran in its entirety as bed, the band takes on that same rank with it. An av hatumah gives a person, a keli, or foods and liquids the status of a rishon letumah. Foods and liquids touched by that rishon turn into a sheini letumah. And terumah touched by a sheini becomes a shelishi, meaning it is now pasul. Terumah, the portion designated for the kohen, may be eaten by kohanim only in a state of purity, and once tumah has reached it, eating it is no longer permitted. So the band transmits tumah two rungs below itself and ruins a third.
"Divrei Rabbi Meir", this is Rabbi Meir's opinion: the mizran in full carries the rank of av hatumah, the trailing portion included, no matter its length, since to his mind every bit of it is bed. The moment the zav's weight lands on the bed and makes it tamei, the band contracts that tumah along with it.
Rabbi Yose Divides at Ten Tefachim
Now comes Rabbi Yose's voice: "Rabbi Yose omer". He takes up precisely the same picture, "nisa hazav al hamitah" and also "ve'al hamizran": the zav bore down on the bed together with the band tied around it, that strip of woven cloth wound about the frame to keep the bed held together.
His ruling: "asarah tefachim, metamei shenayim uposel echad". The first ten tefachim of the hanging mizran convey tumah for two levels and disqualify one more. This follows directly from his position in the previous mishnah: only those first ten tefachim are considered part of the bed, and so only they, along with the bed, become an av hatumah, with the full chain that flows from an av hatumah.
"Me'asarah velachutz, metamei echad uposel echad". From the ten tefach mark and outward, the slack length passes tumah down one level and spoils one level beyond that. Meaning: that outer stretch never attains the rank of av hatumah at all. It holds the status of a rishon letumah, and only by virtue of touching the inner ten tefachim, which do carry the higher rank. As a rishon it can turn foods and liquids into a sheini, and a sheini renders terumah a shelishi, at which point the terumah may not be eaten.
When the Zav Leans on the Mizran Itself
Rabbi Yose continues with a second scenario: "nisa al hamizran", the zav rested his weight not on the bed but on the hanging mizran alone.
"Me'asarah velifnim, tamei". If he pressed down on the portion within ten tefachim, the stretch nearest the bed, it is tamei. Those first ten tefachim are joined to the bed and count as part of it, so when the zav leans there the mizran becomes tamei, and the bed becomes tamei together with it.
"Me'asarah velachutz, tahor". But if he pressed down on the portion of the mizran beyond those first ten tefachim, out along the loose length that hangs away from the bed, it remains tahor. That outer stretch is not reckoned as part of the bed, and on its own it is not regarded as an object designed to be leaned upon, so the tumah of midras does not take hold there. And since it is treated as detached from the bed for these purposes, the bed itself naturally remains tahor as well.
So the single measure of ten tefachim, according to Rabbi Yose, governs both directions: it determines how far the bed's own status of tumah travels outward along the mizran, and it determines how far in a zav must lean before his weight can render the bed tamei at all.