Negaim, Chapter 4, Mishnah 6. The concern of this perek is how the simanei tumah operate within the arba'ah mar'os, the four white shades that qualify for nega tzaraas. A shade on its own never renders anybody tamei; it establishes only that the blemish belongs to a category that is capable of becoming tamei. Tumah arrives through one of three simanim: sei'ar lavan, where dark hairs inside the blemish turn white; michyah, a spot of living, healthy flesh within the nega; and pisyon, spreading, which is reckoned only when the first or the second week of hesger comes to a close. The mishnah before us takes up a nega whose shape is irregular: a fine line of the identical shade runs out of the patch, or two little patches are connected to one another by such a line.
A note on the commentators
Several times in this masechta we have wondered about the identity of the "Maharam" quoted by the Tosfos Yom Tov, whether he is the Maharam MiRothenburg. He is. There exists an edition of Mishnayos that prints the Maharam MiRothenburg's commentary on Seder Taharos, and the passages the Tosfos Yom Tov brings appear there letter for letter. Here the Tosfos Yom Tov states it outright, that the Maharam's writings reached him, and that accounts for how steadily he has leaned on them throughout this masechta.
"Baheres" stands for every one of the shades
The mishnah begins: "Baheres kagris," a baheres measuring a gris. The Rambam, the Bartenura and the rest of the mefarshim caution us that the word "baheres" is functioning here as a general term for the entire family of shades: the four whites together with the four further shades that result when they mix with red. It is not confined to baheres in the narrow sense, the white that is lavan kasheleg. Every time these perakim say baheres, understand it as any shade eligible for tumah. And the measure is a gris, the smallest area in which the simanim are able to make a person tamei.
A line the width of two hairs
The mishnah goes on: "vechut yotzei mimenu," a thread of that same shade projects out of the patch. "Im yesh bo rochav" "shtei se'aros," should that thread be as wide as two hairs, then "zokecha lesei'ar lavan ulepisyon," it carries enough halachic weight for sei'ar lavan and for pisyon. Even if not a single white hair is found in the oym, the body of the nega, two white hairs positioned in this narrow line are a full siman tumah, and spreading measured outward from the line is a valid pisyon. Halachically the thread is an extension of the nega.
"Aval lo lemichyah." For michyah, however, it accomplishes nothing. A line two hairs across is incapable of hosting a michyah that would render the man tamei.
The Rav supplies the reason: even when the thread measures the full two hairs, a michyah appearing there does not generate tumah, since a michyah has to be mevutzeres be'emtza hanega, fortified at the heart of the blemish with nega enclosing it on every side, the way a citadel stands in the middle of a city. So thin a line cannot enclose anything.
The Tiferes Yisroel's calculation
The Tiferes Yisroel takes another route. In his view a michyah can indeed be located in the thread, provided the thread is broad enough to encircle it. Do the arithmetic: a michyah is k'adashah, a lentil's size, which comes to two hairs by two hairs. To be genuinely enclosed, it needs two hairs' worth of nega flanking it on one side and two more on the other. That yields a line six hairs wide. Given such a line, the Tiferes Yisroel holds, a michyah within it does count.
He acknowledges himself that the Rav's language does not point this way. The Rav gives the impression that michyah is relevant only in the oym, the main mass of the nega, and never in the thread.
The opening half of the mishnah therefore teaches: a gris-sized baheres with a thread projecting from it. A thread of two hairs' width brings the nega under the halachah of sei'ar lavan (if two dark hairs standing in the line turn white) and under the halachah of pisyon (if the nega expands out of the line), but not under the halachah of michyah.
Two bahoros linked by a thread
The second case: "Shtei bahoros, vechut yotzei mizu lazu." Two patches of the relevant white, with a thread of that shade running between them. Imagine each patch as under a gris, so that neither reaches the shiur alone. "Im yesh berochav shtei se'aros, mitztarfan," a connecting thread two hairs wide joins the two patches into a single measure of a gris. "Ve'im lav, ein mitztarfan": a narrower thread does not join them.
Several practical consequences follow. Even where each patch by itself already covers a gris, the thread still matters enormously, because when they are joined we are dealing with one nega, and when they are not we are dealing with two.
The Rav lays it out. "Mitztarfan, venidonim kenega echad." Given a sufficiently wide thread, both patches are treated as a single nega. Hence "ve'im pasah achas meihem mashehu," if at the close of the first week either one expands by the tiniest bit, "o nolad sei'ar lavan" "be'echad meihen," or a white hair sprouts in either one, "machlit es shteihen," the kohen pronounces the pair tamei as one nega, and the man carries the status of a muchlat on the strength of the two together.
The Rav continues: "ve'im lav, ein mitztarfan," and adds "venidonim kishnei negaim." A thread too thin fails to unite them. Where each patch is below a gris, plainly a thin thread will not manufacture a gris. But even when each patch is a gris in its own right, they remain two independent negaim. Accordingly, writes the Rav, "ve'im pastah achas veha'acheres lo pastah, machlit bazo umasgir bazo," should one spread while the other stayed as it was, the kohen declares muchlat on the one that produced the siman and places the other in hesger. Observe that the Rav here speaks only of pisyon and says nothing about a white hair.
What difference does this make in practice? The Rav writes venafka mina: when the man ultimately becomes tahor, assuming both patches ran the full course of tzaraas, he owes two sets of korbanos. The first nega arrived at its hachlatah and finished its process of taharah ahead of the second, so the two purifications cannot be folded into one. (Had both been declared at one and the same moment, it is not clear from the Rav whether two sets would be required.)
The Tosfos Yom Tov quotes the Rav's wording, that a spreading or a white hair in either patch makes the kohen declare both muchlat, and he notes that the Rosh wrote likewise. He then brings the Kesef Mishneh in Hilchos Tumas Tzaraas, perek 4, who finds a different consequence: if one white hair appears in this patch and one white hair in that patch, then since a thread of two hairs' width fuses them into a single nega, the two solitary hairs also combine and add up to a siman tumah, and the kohen is machlit. A single hair in each patch, and the man is a muchlat.
An aside: pisyon rachok and the Raavad
Here the Tosfos Yom Tov revisits the subject of pisyon rachok, a spread that is not in contact with the nega itself, and this brings him back to the Raavad's shita from the first perek. The mishnah there enumerated shchin and michvah, a boil and a burn, as things that stand between the oym and the pisyon and disqualify it. The Raavad reads that list narrowly: only those items interpose, because those alone are what the mishnah specified, whereas plain healthy skin lying between the nega and the new patch is no barrier at all, and the pisyon remains valid. A number of other Rishonim rule as he does. The Tosfos Yom Tov, even against the weight of many Rishonim, does not budge: healthy skin most certainly interrupts, and a pisyon has to be touching the oym, or in our case the thread, if it is to be a pisyon.
The lesson of the mishnah, then, is to study a nega's contours with care. A thread two hairs wide stretches the nega's boundaries outward, so that white hairs and spreading found along it belong to the nega, and small patches tied together by it count as one; but no thread is ever broad enough to surround a michyah, which must sit entrenched at the very core of the blemish.