Negaim, Chapter 9, Mishnah 1. This perek opens an entirely new category of nega: not a white spot that appears on ordinary healthy skin, but one that appears on the site of a wound or a burn. The two names for these sites are shechin and michvah, and before we can learn the halachah of the Mishnah we have to know exactly what they are.
The Tiferes Yisroel, in his Mar'eh Kohen, sets this out based on the Rambam, and his formulation is more concise and clearer than the Rambam's own.
What is a shechin
A shechin is skin of a person that became damaged from within, by some internal toxin ("eresh shebegufo") that attacked the flesh at that spot, or from without, by being struck there with a piece of wood or with a stone or the like. The blow damaged the skin, and the skin then "nischamem betiv'o," it became hot and inflamed by its own nature, an expression Rashi uses constantly in Chumash.
What is a michvah
A michvah is skin of a person that became damaged by burning: he was burned by fire itself, or by glowing coals (gochclos), or by a piece of red hot iron (barzel meluban), or by hot ash (remetz), or he was scalded by boiling water, or by anything else that was heated by a fire that a human being kindled, as opposed, for example, to the hot springs of Chamei Teveriah.
Shechin and michvah are extremely similar to one another. As we will see further on in this perek, they do not combine with each other, and essentially they differ only in the cause of the injury: whether it came from a man-made fire or from something else.
Three stages of the wound
Now comes the crucial point. Neither a shechin nor a michvah carries tumas negaim at every stage of its existence.
- While the injury is still raw and open, it bears the name mordim, a term already encountered in earlier perakim. In such a condition the parshah of tumas negaim does not touch it at all.
- At the other end, once healing has finished, even if tzalekes (scar tissue) is left behind in its place, the case has departed from nigei shechin umichvah entirely. Should a baheres arise there later on, it is treated as nigei bosor, a nega of ordinary flesh, with every law that attaches to it, no different from the skin of anyone else.
- The subject of our perek lies between these two. Healing has begun, and a delicate covering has grown over the injury, no thicker than the peel of a garlic clove (klipas hashum). If a baheres measuring a gris emerges upon that covering, whether in one of the eight whitenesses (the four principal mar'os together with the reddish blends taught in earlier perakim) or in some mixture of them, that is what we call a nega of shechin umichvah.
So the entire tumah of shechin and michvah is confined to that initial healing stage: a baheres that forms on top of the thin membrane covering the wound or the burn.
The halachah of the Mishnah
"Hashechin vehamichvah mitam'im beshavua echos." When a cohen sees such a baheres on the membrane of a shechin or a michvah, he places the person under hezger, but for one week only and not more. If at the end of that single week the baheres has not spread, the person is tohor. There is no second week of hezger here.
"Bese'ar lovon ufisoyon." Only two simanim of tumah operate in this nega: se'ar lovon, two hairs standing inside the baheres that have turned white, and pisoyon, the patch expanding beyond the borders it had when the cohen first saw it. Should such expansion occur once that week has ended, the person is declared tomei muchlos.
Notice which siman is missing: michyah. Michyah does not apply to shechin and michvah at all, because michyah requires a patch of whole, healthy skin within the nega, and here we are dealing with nothing more than the thin membrane that has formed over the wound. There is no healthy skin present for a michyah to be found in.
To summarize: the nega of shechin and michvah exists only during the first stage of healing of a wound or a burn, when a baheres of gris size appears upon the thin membrane covering it. Such a nega is judged in a single week of hezger, and its only simanim of tumah are se'ar lovon and pisoyon.