Negaim, Chapter 10, Mishnah 8. This chapter deals with the nesek, the nega that appears on the head or in the beard where hair normally grows. Two signs render a nesek tamei: se'ar tsahov, golden hair, and pisayon, spreading. And there is a sign that keeps it tahor: healthy black hair standing within the nesek. Our mishnah continues a discussion begun just before it. There the case was two nesakim lying beside one another with a strip of hair between them. Here the geometry changes: one nesek sits inside the other, a circle within a larger circle, and what separates them is a ring of hair running all the way around the inner circle.
Setting the scene
The opening words describe two nesakim, one of them positioned "zeh lifnim mizeh," one deeper in and one further out, with a "shitah" of hair dividing them, hair that is "mafsekes beineihem." A shitah is a line or row of hair; in the configuration before us that line curves around and closes on itself, ringing the inner nesek and cutting it off from the nesek beyond.
The Rav fills in the dimensions. At the center sits a nesek of a gris. Wrapped around it is the band of hair. Beyond the band lies the second nesek, which also has its required measure and which loops around the band on all sides, a belt of nesek whose thickness is that of an adashah, a lentil. That outer ring is sized precisely so that it can enclose the hairs that are mevutzaros.
Two expressions should be clear before we proceed. Hair that was growing there beforehand and simply stayed in place once the nesek appeared is termed mishur, remnant hair. It functions as a sign of taharah only when it is mevutzar, that is, hemmed in: the nesek wraps around it from every direction, and it stands off from the kamas harosh, the head's untouched growth, by a distance of at least two hairs.
The Meleches Shlomo points out that in our arrangement the outer nesek is clean of tumah immediately, without anything further needing to happen. The reason is that the band of hair sitting inside it counts, as far as the outer nesek is concerned, as mishur that is fully hemmed in on all sides. The Rav reads the mishnah along a different line, but the ruling would seem to go with the Meleches Shlomo, since what stands before us is remnant hair located within the body of the outer nesek and not along its rim.
When the band is broken in one spot
"Nifratz b'makom echad, tamei." Should the encircling hair be interrupted at one point, the inner nesek stays tamei. An interruption means an opening wide enough to hold two hairs, turning the closed loop into an open curve.
Why does this leave the inner nesek tamei? The Rav explains that black hair does stand next to it, but that hair no longer qualifies as mevutzar: once the loop has been opened at one point, the row is not hemmed in and therefore cannot purify the nesek it surrounds. The outer nesek, on the other hand, is tahor, since from its vantage point there are two black hairs sitting enclosed inside it. The Rav's wording also allows the reading of the Meleches Shlomo, that the outer nesek was already tahor at the outset, back when the band of hair stood intact in the middle of it.
When the band is broken in two spots
Two interruptions in the ring produce a ruling of tahor, and here the logic is simple. The length of hair that now has an opening two hairs wide flanking it on both ends is unquestionably hemmed in, with nesek all around it. That purifies the inner nesek too.
The Rav arranges it by degrees: the inner nesek is tahor, and the outer one even more obviously so, because the pair of nesakim have merged into one nesek. Standing inside that single nesek is what is left of the row, hair of the mishur that predates the nega, and within it two black hairs that are mevutzaros, and it is they that bring taharah.
How wide does each opening have to be? The mishnah asks, "Kamah t'hei hapirtzah?" and answers, "makom shtei sa'aros," enough room for two hairs, and this on both of the two ends.
One opening that measures a gris
The mishnah continues: "Nifratz b'makom echad kagris, tahor." A solitary break in the ring, provided it is a full gris across, yields taharah. The Rav shows how this fits with the earlier line: the ruling of tamei for a break at a single point applies when that break falls short of a gris. Once it reaches a gris, though, a single break suffices, because with one nesek lodged inside the other the two now count as one nesek, and any hair left over is thereby hemmed in within it.
The Tiferes Yom Tov asks the question that leaps out. In the case taught immediately before, two nesakim resting alongside each other, even a gris-wide break brings no taharah, and the hairs of the separating strip are not considered mevutzaros. What accounts for the difference? There, at the far side of the strip, the hair continues into the kamas harosh, the growth on the head that the nesek has not invaded. That linkage keeps it from being hemmed in. (The Maharam supplies the definition: kamas harosh means the hair of the head lying outside the nega.)
In our case, with one nesek nested inside the other, the gris-wide break does something different: it fuses the two nesakim into one entity. The hair that survives is then located within that single expanded nesek. It does not have to occupy the exact middle; it is sufficient that it is not on the border. Being hemmed in, it is mevutzaros, and the ruling is tahor.