Negaim, Chapter 9, Mishnah 2. This perek deals with negaim that appear on the site of a wound or a burn rather than on ordinary skin. The previous Mishnah taught the halachah: such a nega, a baheres of gris size that has formed upon the thin membrane (as thin as the peel of a clove of garlic) that covers the wound in the first stage of its healing, is judged in one week of hezger only, and its simanim of tumah are two white hairs and spreading. Our Mishnah now defines the two names: which sort of injury is called a shechin, and which is called a michvah.
The two names
The Tiferes Yisroel, in his Mar'eh Kohen, based on the Rambam, describes them as follows. A shechin is skin that became damaged either by an internal toxin in the body that attacked the flesh at that place, or by being struck there with a piece of wood or a stone or the like, so that the skin was injured by the blow and then "nischamem betiv'o," grew hot and inflamed by its own nature (an expression Rashi uses constantly in Chumash). A michvah is skin that was burned by fire, by glowing coals, by red hot iron, by hot ash, by scalding water, or by anything else heated by a fire that a person kindled, as opposed, for instance, to the hot springs of Chamei Teveriah.
Eizehu hashechin
"Lokoh be'eitz o be'even o begefes o beChamei Teveriah." He was struck with wood, or with a stone, or injured by gefes, the refuse of pressed olives, whose chaff apparently generates heat on its own, or he was burned in the hot springs of Chamei Teveriah, whose heat comes from volcanic activity beneath the earth and is not a product of a fire kindled by human beings.
"Kol she'eino machmas ho'eish, zehu shechin." That is the governing rule: any injury to the skin whose cause is not a man-made fire is called a shechin.
Eizohu hamichvah
"Nichvah begochalos o beremetz." A person was scorched by burning embers, or by smoldering ashes. Then comes the defining principle: "Kol shehu machmas ho'eish, zu hi michvah." Wherever fire lies behind the injury, whether the flame directly or a toldah, some object that fire had heated, the resulting wound bears the name michvah.
Only the origin of the injury separates the one category from the other. Halachically, though, both behave in precisely the same manner: the nega under discussion is a baheres that arises upon that gossamer covering, no thicker than a garlic peel, which forms over the shechin or the michvah as it first begins to close up.