Negaim, Chapter 12, Mishnah 5. The subject of our perek is nigei batim, the affliction that appears on the walls of a house. A house falls under this tumah only when its walls were put together out of three components, stones, afar (earth) and eitz (wood), and every one of the walls has to include all three. The required measure of stones has already been established; here the Mishnah supplies the amounts of wood and of earth, and then lists structures and locations that lie outside these halachos entirely.
The measure of wood
"Eitzim kedei litein tachas hashakof": there must be enough timber to be set under the shakof. A shakof (mashkof) is the beam running across the top of a frame, the lintel, and the Bartenura observes that the word covers both the lintel above a doorway and the lintel above a window. That distinction has a practical side. Few houses have entrances piercing all four of their walls, whereas windows on all four sides were altogether common, and since wood is required in every single wall, it is generally a window frame that supplies it.
As for the word itself, Rashi on the Chumash links mashkof to nishkaf, the sense of banging up against, since the door swings upward and meets that beam, which brings it to a halt.
A second measure is then offered. "Kedei la'asos sandal le'achorei hashakof," says Rebbi Yehuda: enough to fashion a sandal on the far side of the shakof. The Bartenura relates that people would fix a block of timber at the back of the lintel in order to protect it, and this block bore the name sandal: just as footwear guards the foot, so this wood guards the beam above the entrance. Presumably Rebbi Yehuda is requiring a smaller quantity than the first view, although how exactly the two amounts compare is not spelled out.
The measure of earth, and the meaning of patzim
"Ve'afar kedei litein bein patzim lechavero": earth sufficient to fill the space between one patz and the next. The Bartenura understands patzim as a term for a fissure or split, and brings the pasuk "hir'ashta eretz petzamta," You made the land quake and tore it open.
The Tiferes Yisroel finds that reading difficult. What would it mean to speak of the stretch running from one fissure to another? Whatever stands between two cracks in a wall is a course of stones or of beams, hardly a spot where earth gets packed in. He suggests instead that patzim denotes not the split itself but the segments produced by the splitting, comparable to "vayevater" at the bris bein habesarim, where the besarim are the resulting halves rather than the act of cutting. So too here, the parts that have been divided off from one another are the patzim, and the earth is pressed into the gap that separates one part from its neighbor.
Structures not subject to this tumah
"Kiros ha'avus vekiros hamechitza einan mitamim benega'im": the walls of a trough and the walls of a mechitza never contract nega'im. The Bartenura takes an avus to be the manger at which livestock eat, and for kiros hamechitza his text reads "shel ginos upardeisos," walls that fence in gardens and orchards (in some editions of the Bartenura a word is lacking here).
The Tiferes Yisroel goes his own way, and he arrives at a remarkable chiddush in the halachos of nigei batim. He reasons that anything standing out in the open away from a dwelling, even if it somewhat resembles a building, such as a fence enclosing a garden or an orchard, is obviously not included, and a manger for animals is ordinarily out of doors in any case. The Mishnah must therefore be describing a manger constructed as part of the house's interior wall, with an opening left in the wall so that the animals can push their heads through and feed from it. A trough of that sort is attached to the wall, and its own sides may well be made of stone, wood and earth, yet it remains free of this tumah. Along the same lines he explains kiros hamechitza as an inner wall, erected merely to separate one space from another and not functioning as one of the house's outer walls.
Out of this he formulates a rule, and he states it openly: a wall that is not built the way the rest of the house's walls are built, meaning not built to protect the house against wind, water and rain, is not subject to tumas nega'im. The implication is far-reaching. In a dwelling of many rooms, quite a few walls face inward only and have no relation to the outside, and by this approach none of them can ever become tamei with a nega. Whether every authority agrees to this is a separate matter.
Yerushalayim and lands beyond Eretz Yisroel
The Mishnah closes with two places: "Yerushalayim vechutz la'aretz," Yerushalayim and anywhere beyond the Land, "ein mitamim benega'im," are not rendered tamei by nega'im. The Bartenura anchors this in the words "beveis eretz achuzaschem," a home situated in the land held by you as your inheritance. Territory outside Eretz Yisroel is not held by you at all, and Yerushalayim, too, was never allotted to the shevatim.
The Tosfos Yom Tov calls attention to the discussion in Yoma about Yerushalayim's status: was the city in fact split between the portions of Yehuda and Binyamin, or was it never distributed at all, belonging jointly to all twelve shevatim without being the property of any one of them? A question then presses: if all twelve tribes share in the city, why should it not qualify as "beis achuzaschem"?
Here the Rambam's words in his Peirush Hamishnayos are worth hearing, for they are simply beautiful. He too maintains that the city was never apportioned among the shevatim, and then he adds an astonishing phrase: it is, rather, a place of avodah to Hashem in which the whole of mankind holds a portion. Yerushalayim is not the exclusive property of Klal Yisroel; every nation on earth has a share in it. This is exactly what we declare whenever we recite Selichos, in the words of the navi that Hashem's house will be known as a place where all the peoples pray. The quality of the Beis Hamikdash spreads out over the entire city, so that Yerushalayim carries meaning for all of humanity. For that very reason it falls outside the category of achuzaschem, and its houses never come under nigei batim.