Ohalos, Chapter 2, Mishnah 1. The second perek begins with a catalogue: "Eilu hametam'in b'ohel," the following are the sources of tumas meis whose tumah spreads by way of an ohel. Before working through the items themselves, it is worth recalling the three channels through which a meis conveys tumah. "Maga" is physical contact with the meis. "Massa" is bearing its weight, even where no part of one's skin ever touches it. "Ohel" is finding oneself beneath a single covering, roof or tent together with the meis. Of the three, ohel has the widest reach, and therefore whenever the Mishnah rules that a certain item is metamei b'ohel, it follows automatically that the identical item transmits tumah by maga and by massa as well.
Hameis: even a nefel
Heading the list is "hameis," a corpse that is intact. That seems self evident, so what does the Mishnah come to teach? Its novelty lies in a case one might have doubted: a nefel, a fetus expelled at such an undeveloped stage that, in the words of the sugya in Nazir cited here by the Bartenura, "lo nikshu evarav b'giddim," its limbs were not yet fastened to one another by sinews. The body never fully formed, and it does not contain even a k'zayis of flesh. Even so, because what lies before us is an entire body, it is metamei b'ohel.
K'zayis basar and k'zayis netzel
The Mishnah then records "k'zayis basar min hameis," an olive's volume of a corpse's flesh, followed by "k'zayis netzel." What is netzel? When the flesh of a body breaks down it turns into a putrid liquid, which the Bartenura calls "leicha serucha." And why that name? The Tosafos Yom Tov traces it to the posuk "Vayatzeil Elokim es mikneh avichem," where the root conveys removal and separation. That is precisely the state of this matter: it has dissolved away and separated off, so that it no longer appears before us as a piece of a human being.
Melo tarvad rakov
The Mishnah continues with "umelo tarvad rakov," a full large spoon of rakov. Rakov is flesh of a meis that decomposed and dried out into a dust like substance, in contrast to netzel, which is still moist. How much is a melo tarvad, a full spoon? The measure is the amount that would fill two hands held together.
The Bartenura, again drawing on the sugya in Nazir, records two qualifications that are essential here. One: the substance must be unmixed "rekev shel meis," consisting of nothing besides the remains of the body itself. In practice that means burial in a marble aron. Had the body been laid to rest in its clothing, or in a coffin built of wood, the resulting dust would not be metamei, since a handful of it holds not only the body but particles of fabric and of wood as well. Two: rekev carries this tumah only when the body it came from was whole. Should a limb already have been missing at the time of burial, the dust that forms afterwards does not convey this tumah.
The spine and the skull
"Hashidrah v'hagulgoles," the spinal column and the skull. These are metamei b'ohel even when there is no flesh on them whatsoever. Why should bare bone be metamei b'ohel? Because in the shidrah and in the gulgoles one still sees the tzurah of a person. The Torah speaks of "etzem adam," the bone of a man, and these bones qualify as etzem adam with regard to tumas ohel.
A detached limb, from the dead and from the living
Two further entries follow: "eiver min hameis" and "eiver min hachai," a severed limb taken off a corpse, and a severed limb that came off a person who is alive, in either case "sheyeish aleihem basar karoi," where a suitable quantity of flesh remains attached. What defines basar karoi? It is enough flesh to have made recovery possible: for a limb of a meis, we ask whether that much flesh would have enabled healing had the limb been on a living body; for a limb of a living person, whether it would have healed had it never been cut off in the first place. Given that quantity, the limb is metamei b'ohel, and no full k'zayis of flesh upon it is required.
Rova atzamos, rov binyano and rov minyano
"Rova" means a quarter of a kav of bones from a meis. Such a quantity is metamei b'ohel "af al pi she'einam me'rov habinyan o me'rov haminyan," even though these bones amount to neither the majority of the structure of the body nor the majority of the count of the bones. In other words, as the Bartenura explains, a quarter kav of bones is metamei even when I do not have most of the person's body before me and I do not have most of the individual bones of the person.
The Mishnah also states the opposite situation: "rov binyano v'rov minyano." When what lies before me is most of the physical structure of the meis, or most of the total number of his bones, the tumah applies "af al pi she'ein bahem rova," although those bones fall short of a quarter kav. Every one of these amounts renders tamei, and does so even at a distance, through the covering of an ohel.
The Mishnah closes by defining the second measure: "Kamah hu rov minyano?" How many bones constitute the majority of the count of the body? "Me'ah v'esrim vachamishah," one hundred and twenty five. The full number of bones in a person is two hundred and forty eight; half of that is one hundred and twenty four; therefore one hundred and twenty five is already a majority.
Reviewing the list
Let us collect the entries. Metamei b'ohel are: a meis, even a minute nefel whose limbs were never joined by sinews; an olive's measure of a corpse's flesh; an olive's measure of netzel, the moist matter left behind by decay; a full tarvad of rakov, two handfuls of a corpse's dust, so long as it comes from the body alone; the shidrah and the gulgoles, where a person's form is still to be seen; a severed limb, whether taken off a corpse or off someone alive, when basar karoi clings to it, the measure of flesh with which healing would have been possible; a quarter kav of bones, though it be short of the majority of the frame and short of the majority of the bone count; and the majority of the frame or the majority of the bone count, though these fall short of a quarter kav. And the majority of the count stands at one hundred and twenty five bones.
Why death brings the greatest tumah
One beautiful thought is worth taking along from this Mishnah. In Parshas Chukas the Or Hachaim Hakadosh raises the question of why a human being, of all creatures, generates the harshest tumah at death. His answer comes by way of a mashal. Pour water onto the ground and hardly a creature will take notice. Do the same with honey and leave the mess untouched, and before long insects will be teeming over it. The greater the value of what was there, the more powerful is whatever rushes to fill the space it left. Where lofty kedusha withdraws, fierce tumah moves in.
Here is another way to sense it. At a wedding the photographer switches on an enormous, brilliant light. When he shuts it off, the hall suddenly feels dark. Had you walked in only at that moment, never having seen the great light, the room would not have struck you as dark at all. The darkness is measured against the brightness that was there.
So as we learn these halachos of tumas meis, with all their measures of flesh and bone and dust, we are really being shown something else: the chashivus and the kedusha that reside in a person while he is alive.