Negaim, Chapter 10, Mishnah 10. This mishnah brings the whole subject of nesakim to its conclusion. A nesek is one of the four types of nega that can come upon a person, and unlike the others it has nothing to do with a baheres at all: it is a loss of hair in patches, and it applies only in the two hairy regions, the scalp and the bearded area of the face.
Spreading that purifies
The mishnah opens: "Mi shehayah bo nesek kagris venitak kol rosho", one who had a nesek the size of a gris, and then the nesek spread until it took over his entire head and all the hair of the head fell out. Such a person is tahor.
The logic here mirrors a rule familiar from the baheres: prichah. When a baheres erupts and whitens a person from head to foot, that total takeover is itself grounds for taharah rather than tumah. Nesakim follow the identical pattern. Once the bald patch has consumed the entire scalp, the man goes out tahor. And the limitation is identical as well: just as prichah purifies only where tumah was already in force, either by a kohen's outright declaration of hechlet or by a period of hesger, so too the wholesale takeover of the head purifies only a person whose tumah had already been established.
The head and the beard: two separate regions
Next the mishnah turns to the two places where a nesek can appear at all, "Harosh vehazakan", the scalp and the bearded part of the face. Regarding this purifying takeover, the mishnah rules that they are "ein me'akvin", neither one detains the other, "zeh es zeh". In other words, the two hairy regions are judged separately. Should the nesek claim the whole scalp and leave it bare, that counts as a complete takeover and the man is tahor, and it makes no difference that his beard remained untouched. Turn the case around and the ruling is the same: a beard emptied of all its hair brings taharah even while the scalp still has its hair. For this purpose each area stands alone, a nega unto itself. So holds Rebbi Yehudah, and the Rambam rules this way in practice.
Rebbi Shimon's kal vachomer
A dissenting voice follows. "Rebbi Shimon omer": to his mind the two areas are indeed "me'akvin zeh es zeh", each one holding up the other. A scalp stripped bare while the beard keeps its hair yields no taharah, and neither does a beard stripped bare while the scalp keeps its hair.
Rebbi Shimon argues from a kal vachomer. He begins "vedin hu", it follows by reason: consider "mah im or hapanim", the skin of the face, together with "ve'or habasar", the skin of the body. For prichah of a baheres to be complete, the whiteness must reach even the hairless stretches of the face, the brow and the area of the cheekbones. Leave those unwhitened and the prichah does not count; they hold up the process. Yet observe that this is true although "sheyeish davar acher", another entity, namely the beard, is "mafsik beineihem", standing between them as an interruption. We never suggest that the spread may simply halt at that dividing line. Now compare, says Rebbi Shimon, "harosh vehazakan", the scalp and the beard, "she'ein davar acher mafsik", with nothing whatever dividing them, "eino din sheya'akvu", is it not all the more reasonable that they detain "zeh es zeh"? If two regions kept apart by an interruption must nevertheless wait for each other, then two regions lying flush against one another surely must, and the baldness ought to be required in both. Such is Rebbi Shimon's reasoning, though the accepted halachah remains that of Rebbi Yehudah.
No combining and no spreading between them
The mishnah returns to "Harosh vehazakan" with a further ruling: they are "ein mitztarfin", they do not join, "zeh im zeh". Think of michveh and shechin, two wounds that resemble each other closely and are distinguished only by the agent that caused them; precisely because they are distinct nega'im, half a gris of the one adjoining half a gris of the other will not add up to a full measure. The scalp and the beard work the same way. A chatzi gris of nesek at the edge of the scalp touching a chatzi gris of nesek at the edge of the beard remains two half measures and never becomes one gris of nesek.
Finally, "Ve'ein posin mizeh lazeh": expansion from the one area into the other is not counted as expansion. Picture a nesek of a full gris in the beard, situated right where the beard meets the hair of the head, which then grows so that additional hairs drop out on the scalp side. That extension is no pisyon at all. Reverse the direction, a nesek on the scalp reaching down into the beard, and the ruling does not change.
Where is the beard?
The mishnah signs off by mapping out the region in question. "Eizehu zakan?", what counts as the beard? "Min haperak shel lechi", from the hinge where the jawbone is joined, situated just under the ear, "ad pikah shel gargeres", as far as the small knob of the windpipe, the protrusion commonly known as the Adam's apple. That entire span, descending from the jaw's joint to the knob at the throat, is the zone in which a nesek of the beard can be contracted.
And so the halachos of nesakim come full circle around a single theme, hair. This nega announces itself when hair vacates a patch of the scalp or beard, and its taharah arrives, remarkably enough, when the hair vacates the region altogether.