Chullin, Chapter 9, Mishnah 5. Our chapter has been working through the laws of what serves as a yad (a handle) and what serves as a shomer (a protective covering) for transmitting tum'ah. This mishnah applies those principles to a particular case: the kulit, the marrow bone, whose hard shell encloses soft marrow within it. The question is when the tum'ah passes through to the person who touches the bone.
The first clause reads: "Kulit hamet vekulit hamukdashim" - a kulit that came from a corpse, and likewise a kulit of consecrated meat that became pigul or notar. Concerning both, the ruling is "hanogea bahen", one who makes contact with them, "bein setumim", whether they are sealed shut, "bein nekuvim", or whether they have been perforated, "tamei" - he contracts tum'ah either way.
Why is the condition of the bone irrelevant in these two instances? Because the tum'ah originates in the bone itself and not in the marrow lodged inside it. The second half of the mishnah will teach that marrow shut up within a bone cannot render a person tamei when he handles the outer shell, since he never comes into contact with that marrow at all. Where the bone is itself the source, though, there is nothing concealed that he needs to reach. Scripture states outright that a human bone left behind by the dead conveys tum'ah, and with regard to pigul and notar the chachamim enacted that whoever handles such meat contracts tum'ah.
The purpose of that decree is worth pausing on. The chachamim wanted the kohanim to be vigilant in the avodah: careful that no korban be disqualified through an improper intention (pigul), and careful that the meat be eaten in its proper time rather than left over past the deadline (notar). By attaching tum'ah to such meat, they gave the kohanim a strong incentive to guard the korbanot properly. And since the bones are part of the korban, the decree was extended to them as well, so that one who touches them becomes tamei.
Now the mishnah presents the opposite situation: "Kulit neveilah vekulit hasheretz" - a kulit taken from a neveilah, and a kulit taken from one of the eight sheratzim enumerated in the Torah. Regarding these, "hanogea bahen setumim, tahor": as long as they remain sealed, whoever handles them stays tahor. In these cases the bone carries no tum'ah on its own; the tum'ah belongs to the flesh alone. Hence, when the shell is completely closed, without puncture or opening of any kind, contact with the flesh or marrow within is impossible, and the person remains tahor.
"Nekuvim kol shehu, mitam'in bemaga." If the bones were pierced, even by the smallest hole, they convey tum'ah by touch. The measure of the hole is such that a person could insert even a single hair of his and reach the flesh or marrow within. Once the tamei substance inside has become accessible in this way, the surrounding bone takes on the status of a shomer, a protective covering for the flesh, and a shomer transmits the tum'ah of what it guards. So touching the outside of the bone is enough.
From Touching to Carrying
The mishnah's ruling that a sealed marrow bone conveys no tum'ah rests on a broader principle: whatever cannot convey tum'ah through contact also cannot convey tum'ah through being carried. The mishnah now supplies the source for that principle.
"Minayin she'af bemassa?" What is the scriptural basis for saying that an item incapable of imparting tum'ah when touched is equally incapable of imparting it when carried? "Talmud lomar" - the verse dealing with neveilah mentions in a single breath "hanogea vehanosei," the one who touches alongside the one who carries. Since the Torah pairs the two acts, the derivation follows: "et sheba likhlal maga", an item that belongs to the category of transmitting tum'ah by contact, "ba likhlal massa", belongs to the category of transmitting it by carrying as well; while "lo ba likhlal maga", an item excluded from transmitting tum'ah by contact, such as marrow shut inside its bone, "lo ba likhlal massa", is excluded from transmitting it by carrying too.