Ohalos, Chapter 1, Mishnah 7. Up to this point the perek has occupied itself with sizes and with counting: how much of something is needed, how many items combine. Now the Mishnah sets down a principle that steps outside the world of quantities altogether: "Ha'eivarim ein lahem shi'ur", for limbs no minimum amount is fixed at all. An eiver of a meis, and equally an eiver of an animal or of a sheretz, transmits tumah however small a piece it may be.
The force of this becomes clear only against the background of the standard case. Detached flesh, whether it came from a meis, from the neveilah of a beheimah or of a chayah, or from a sheretz, must reach a stated bulk before it can pass tumah along. Those amounts are listed right here in our Mishnah. And im yirtzeh Hashem, the Mishnah that follows will make clear that the tumah spoken of here is the full tumah of a meis in all three of its channels: be'maga, through direct contact; be'masa, through carrying the object even when one never touches it; and be'ohel, through sharing a roof with it. An intact eiver, however, is exempt from any threshold of size.
What counts as an eiver
The Bartenura supplies the definition here, and it will stay with us throughout the mesechta: an eiver is a section of the body in which three things are found together, basar, atzamos and giddin, that is flesh, bone and sinew. And what exactly are giddin? They are what our tefillin are stitched and bound with, the gid drawn from an animal: a nerve, a blood vessel, a ligament. Look at one and you would take it for an ordinary piece of string. Nothing less than all three together qualifies.
Take care not to misread the term. Nobody is asking for a full arm or a full leg. The Mishnah's "limb" means that what I am holding in my hand is a segment of the body in which there is bone, and vessels of that sort, and flesh. The Mishnah ahead of us fills in further detail.
The language of the Mishnah
Having stated that eivarim carry no shi'ur, the Mishnah spells it out. "Afilu pachos mi'kezayis min ha'meis": even a fragment that falls short of a kezayis taken from a meis, though a kezayis is generally what one would have to have. "U'pachos mi'kezayis min ha'neveilah": and likewise a fragment under a kezayis taken from a neveilah, a kezayis being the bulk of an olive, which is the usual requirement for a piece of meat that simply came off a neveilah. "U'pachos mi'ka'adashah min ha'sheretz": and a piece smaller even than a lentil taken from a sheretz, the lentil being the measure assigned to sheratzim, so that sheretz flesh would ordinarily have to amount to an adashah to be metamei. Yet where what lies before you is a genuine eiver, the halachah is "metam'in tum'asan", they pass on their tumah, and the reason is exactly this: basar, giddin and atzamos are all in your hand at once.
The Bartenura mentions in passing that the same law applies to an eiver that came away from a person who is still alive, Rachmana litzlan. Whether it separated from a living body or from a body already dead, such a limb is metamei with the tumah of an entire meis, and no kezayis is called for. Eivarim have no shi'ur, but only on condition that giddin and atzamos are there along with some measure of basar.
A striking application: the kidney
Think of a kidney. It is a whole organ in its own right, and still there is not a bone in it. Consequently a kidney that came away from a meis does not convey tumah in the category of an eiver, even should it amount to a kezayis in volume, because one of the three required elements is simply absent.
The Mishnah also yields the law for animals: an eiver that separated from a beheimah or a chayah, no matter whether the creature was still living or had already died, transmits tumas neveilah with no kezayis needed, and here too only when basar, giddin and atzamos are found together. As for a sheretz, flesh that came off it requires the bulk of an adashah, a lentil's worth, before it is metamei, whereas a sheretz in its entirety, and just as much an eiver that separated from it, is metamei with no such amount.
A closing thought: bones that lived again
Let us finish on a brighter note. The Eibeshter led Yechezkel HaNavi to the valley known as Bikas Dura, the very site where Nevuchadnezzar had erected his colossal pesel. Scattered there were the bones of the shevet of Ephraim, who had walked out of Mitzrayim before Moshe Rabbeinu authorized their departure, and alongside them the bones of those who had refused to prostrate themselves before the king's image. Chananyah, Mishael va'Azaryah emerged from the furnace untouched, and Nevuchadnezzar poured out his rage upon the rest.
Hashem put a question to Yechezkel: can you believe that bones such as these will live once more? Then the Eibeshter sent down the dew of techiyas hameisim and unlocked the Otzar haNeshamos, and there upon those bones came basar, and giddim, and ohr, flesh and sinews and skin, and bone after bone knit itself to its fellow. Men rose to their feet and began to walk, and every one of them was in tears. Yechezkel wanted to know why they should weep at the moment they had been granted techiyas hameisim. They explained their fear: perhaps their share had now been spent, and they would have no part in the great techiyah still to come. Hashem instructed Yechezkel to calm them: just as they had come to life at this hour, so would they come to life then. Thereafter they lived out normal lives, took wives and raised families. The Tanna Rebbi Yehudah ben Beseira testified about himself that he stemmed from those who arose in that techiyah.
In that same hour Nevuchadnezzar happened to be raising to his lips a goblet carved out of bone, and it began to shudder in his hand. The king shouted, what is going on? He was told that this was the doing of the bones of Bnei Yisrael, and that techiyas hameisim was under way. The goblet then flew up and struck him across the face. So shaken was Nevuchadnezzar that he stood and broke into shirah before HaKadosh Baruch Hu, and that shirah of his came close to outshining the shirah of Dovid HaMelech, until a malach arrived and dealt him a blow on the cheek to silence him. The Kotzker Rebbe pointed out where the two men part ways: Dovid HaMelech sang even while he was being struck across the face, whereas Nevuchadnezzar needed to see the dead rise before a single note left his mouth.
So while we sit and study eivarim, and atzamos, and giddim, let us keep in mind that a day of techiyas hameisim lies ahead. Chazal teach that these things are metamei for one reason: life once resided in them. And if life once resided in them, life is bound to come back to them.