Negaim, Chapter 8, Mishnah 6. This chapter deals with pericha, the case in which the baheres spreads until it covers the metzora completely. When such a spreading follows a ruling of tumah it purifies him, and when it follows a ruling of tahara it renders him tamei. But for any of this to apply, the covering must be total. Our mishnah asks which parts of the body must be included in that reckoning and which parts may be left out.
Here is the rule the mishnah lays out: "Kol haro'uy litamei benega habaheres me'akev es hapericha, vechol she'eino ro'uy litamei benega habaheres eino me'akev es hapericha." Wherever the skin is capable of contracting the tumah of a baheres, that spot obstructs the pericha so long as the whiteness has not reached it. Wherever the skin has no such capacity, leaving it uncovered creates no obstruction whatsoever.
"Fit to become tamei through baheres" means fit to carry the ordinary simanei tumah of a baheres or of the other shades of nega on the skin: pisyon, two white hairs, or a michya, a patch of healthy live flesh the size of an adashah, whether these appear at the outset or at the end of the first or second week of hesger. Skin of that kind counts, and so long as such skin is still uncovered, the pericha is incomplete.
Which places are not fit? The hair-bearing areas of the head and the beard, which are governed by the laws of nesek rather than baheres and are therefore removed from the din of baheres altogether. Likewise a shechin (a boil), a michvah (a burn) and a keidach (a subset of the burn), so long as they are mordin, that is, still raw, moist and open sores. These are the domain of an entirely different nega with its own halachos, and a baheres appearing on them has no legal effect there.
The mishnah gives an example. "Keitzad?" How does this work in practice? The whiteness has covered the man completely, "porchah bekulo", except that it has not reached his scalp or his beard, and it has not reached a shechin, a michvah or a keidach while these are still open and raw. None of those areas carries any susceptibility to the tumah of a baheres; they stand outside its laws entirely (mufka). They therefore obstruct nothing at all, the pericha is reckoned as full, and it takes effect upon all the remaining skin.
Now the mishnah adds a striking ruling: "Chazar harosh vehazakon venikrechu, hashechin vehamichvah vehakeidach vena'asu tzoravos, tehorim." If afterwards the head and the beard lose their hair, so that those areas are once again places of ordinary skin susceptible to baheres, or the boil, burn and sore form a membrane and begin to heal (tzoravos), so that they too become susceptible, the man nevertheless remains tahor, even though the baheres does not cover those places. Since the pericha was a valid, complete pericha at the moment it took effect (those areas being excluded then), the later change does not undo it. This is unlike the rashei evarim of the previous mishnah, which reverse the status when they are uncovered; here the pericha stands as a good pericha.
The closing case runs in the opposite direction. The whiteness spread over the man, yet a bit of ordinary skin the size of half an adashah remained uncovered alongside the head and the beard, or alongside the boil, the burn and the sore. That remnant is true or basar, capable of bearing the tumah of a baheres, and it therefore obstructs the pericha, so that this spreading accomplished nothing from the outset. Afterwards the scalp and the beard shed their hair, and the three wounds skin over and are reckoned as healed (tzoravos). Now the mishnah rules that even though the baheres by this point has covered that bit of living flesh too, the man remains tamei, and he will not be tahor "ad shetifrach bekulo", until the spreading takes in every part of him.
The Rav explains why. The original spreading was not a pericha at all, since half an adashah of skin near the head was left over. And the spreading of now, which covers that half an adashah, is likewise not reckoned a pericha, because in the meantime the head and beard have gone bald and the wounds have turned to tzoravos, and all of these are now like ordinary skin. It does not help unless the baheres spreads over them too.
The two halves of the mishnah come out as a single, clear rule. Had the half an adashah been covered at the outset, so that the baheres embraced every place then susceptible to its tumah, the pericha would have been a valid one, and no later change (baldness, healing wounds) would spoil it. But since the first spreading was defective and never took effect, nothing is settled, and the man's status is judged by the situation as it stands at each moment. Now the wounds have healed, the head and beard are bare, and every one of those areas has joined the ranks of skin susceptible to baheres. He is not tahor until the baheres covers the half an adashah that was missing before, together with the newly healed shechin, michvah and keidach and the newly bald areas of the scalp and face. Only a covering that leaves nothing out is a pericha.