Ohalos, Chapter 3, Mishnah 3. Our chapter is devoted to forms of tumah whose shiur is reached only by adding separate portions together, and among its chief subjects is a revi'is of blood that came from a meis, which is metamei with the full stringency of the meis itself. The preceding mishnah discussed such a revi'is located inside a house that then seeped away, whether into the floor or into a piece of cloth. Here the Mishnah turns to a revi'is lying in an uncovered place, and the whole halachah, as we will see, hinges on the contour of the surface underneath it.
"Nishpach ba'avir": blood spilled in the open
The Mishnah opens: "nishpach ba'avir," the blood was spilled into the air. What does "into the air" mean here? That it is not lying beneath any roof. Picture an open courtyard with nothing overhead, no ohel of any kind spread above the blood.
Now the Mishnah distinguishes: "im hayah mekomo katafras," if the spot where the blood came to rest was a katafras. This word is worth learning well, because a whole family of halachos rests on it, and it will return with special force when we reach Maseches Mikvaos, be'ezras Hashem Yisborach. A katafras is a sloping surface, a place where liquid does not stand still but keeps running downhill. And the rule we have is: katafras eino chibur, a slope does not join liquids together. Think of water poured down a slide: we do not regard what is at the top and what is at the bottom as one connected body of liquid. Every drop is judged as an individual drop, standing on its own.
The practical result: a revi'is of blood that spread out over a sloping surface is not a single revi'is at all. So if a person bent over the blood and formed an ohel above part of it, he stays tohor. The part he overshadowed is not linked to the rest, and a mere portion of a revi'is does not carry the tumah of a meis. He never had a full revi'is beneath him.
"Eshboren" and blood that congealed
The Mishnah continues: "hayah eshboren, tamei." An eshboren is a place where liquid collects and stays put, such as a depression or a small hollow in the ground. There the blood is not running off in separate drops; it sits together as one pool, and therefore the whole of it counts as one connected revi'is. Consequently, if a person leans over even a part of that pool, he is tomei, since overshadowing part of a single connected revi'is is like overshadowing all of it.
The same applies in a second case the Mishnah gives, "o shekarash," or the blood congealed. Once it has thickened into a solid mass it is no longer a flowing liquid at all, but a single lump. Here even a slope makes no difference: congealed blood on a sloping surface is still one unit, and someone who overshadows part of it is tomei.
"Al ha'iskufah": blood on the threshold platform
The Mishnah then presents a second scenario: "nishpach al ha'iskufah." An iskufah is the raised threshold in front of a house, a small step or platform on which you stand before entering the doorway. The blood spilled there, "vehayah katafras," and that surface was sloped, "bein mifnim bein mibachutz," whether the slope tilted inward, so that the blood would run toward the inside of the house, or outward, so that it would run off in front of the platform. The Mishnah tells us that this detail changes nothing.
But there is a complicating factor: "vehabayis mayil alav." When you stand on that platform and look up, there is a roof above your head, because the roof of the house projects out past the wall and covers the threshold. Very convenient in practice: you can stand there closing your umbrella without getting soaked. For our purposes, though, that overhang raises a real question of ohel, since a roof spread over blood and over the interior of the house could serve to bring the tumah inside.
The halachah, nevertheless, is "tahor." Nobody in the house, and nothing in it, becomes tomei. The reason is precisely the principle set out above: blood resting on a sloped threshold is never reckoned as one quantity. Each drop is judged separately, so there is no revi'is at all for that overhang to convey inside.
Once again the Mishnah supplies the opposite ruling: "hayah eshboren o shekarash, tamei." Should the blood have pooled in a spot that holds it in one place, or should it have set into a solid clot, whoever is in the house, along with the keilim there, contracts tumah. That is so even though the step is on an incline, and even though the projection above shades no more than a portion of the blood. Since the blood forms one continuous quantity (either because it is held together in a single place or because it has stiffened into a lump instead of running off), overshadowing a part of it is reckoned as overshadowing the entirety, and the din is tumah.
"Kol shebameis tomei": which parts of a meis convey tumah
The Mishnah ends by teaching a new rule: "kol shebameis tomei." Whatever comes from a meis is metamei, even once it has separated from the body, "chutz min hashinayim vehasei'ar vehatzipornayim", teeth, hair and nails being the exceptions. Those three are left out only after they have come off. And the Mishnah adds, "bish'as chiburan hakol tomei": for as long as they remain fastened to the meis they share in its tumah, so that touching the hair of a meis while it is still in place, or its teeth or nails, makes a person tomei. Just how such a situation comes about is what the next mishnah will explain.
Why do these three drop out once they are detached? The Torah's source for this tumah speaks of "be'etzem," a bone. A bone comes into being along with the person and stays as it is: if, chas vesholom, someone should lose a bone, nothing grows to take its place, not until techiyas hameisim. Hence a piece of the body is metamei on its own only if it shares those features of a bone: formed together with the person, and not subject to being replaced. On both counts teeth, hair and nails do not qualify. An infant comes into the world with no teeth at all, while hair and nails renew themselves endlessly: one has a haircut and pares one's nails lichvod Shabbos, they grow out again, and again they are trimmed lichvod Shabbos. Such a thing is termed "gizo machlif," it grows back after being cut, and for that reason these parts, once they have come away, are not metamei.
Reviewing the Mishnah
A revi'is of blood that came from a meis and spilled in an uncovered place, with nothing spread overhead. Should it have come to rest on an incline, the halachah is taharah, since a slope never unites liquids: each drop is reckoned on its own, and one drop falls short of the shiur that conveys tumah, so a person stooping over the blood never has a complete revi'is beneath him. If instead the blood gathered in a place that holds it, say a small dip in the floor, or if it set and hardened, the halachah is tumah: bending over it and forming an ohel above any portion of it renders a person tomei, because the full quantity is treated as one body.
Blood that landed on the raised step outside a doorway: a sloping step yields the same halachah whether the tilt runs toward the interior of the house or away from it, and the fact that the overhanging roof spreads above that step is likewise of no consequence. Nothing indoors picks up tumah, since drops resting on an incline are never reckoned together and a revi'is is therefore lacking; each drop is judged by itself. Should the blood pool in an eshboren, a spot that holds it in place, or should it stiffen into a solid clot, the halachah is tumah even on a slope, and even where the projection shades no more than a fraction of it, because the whole quantity counts as a single body.
The Mishnah then introduces the subject of the next mishnah: every part of a meis conveys tumah even after separating from the body, apart from teeth, hair and nails, which are excluded because they are not created with the person and because they regenerate, and therefore do not resemble the bone from which the Torah teaches this tumah.