According to the Mishna, “with regard to a kulit – the thigh-bone – of a human corpse or of a consecrated animal, he who touches it, whether it be sealed or perforated, becomes ritually impure.”
Although the word kulit refers specifically to the thigh-bone, in fact the same rule would apply to any marrow bone, since the marrow is deemed edible, which changes our view of the bone from “bone” to “meat” with regard to the laws of ritual defilement. Furthermore, while the Mishna groups the cases of a human corpse and that of a consecrated animal together, their rules are not exactly the same. In the case of a human bone, the bone itself becomes tameh on a biblical level based on the passage in Sefer Bamidbar (19:16). This stands in contrast to the case of a consecrated animal that only becomes tameh on a rabbinic level (see Pesaḥim daf 120b), and whose bones only impart impurity if they contain marrow.
The Mishna continues:”With regard to a kulit of an animal carcass or of a creeping animal, if it was sealed up he who touches it remains ritually pure, but if it was at all perforated it imparts impurity by contact.”
These cases stand in contrast to the cases of a human corpse or of a consecrated animal, in that the bones do not convey ritual defilement in-and-of themselves; it is the edible marrow that is the source of tum’ah. For that reason, if the bone is totally sealed ritual defilement will not be conveyed.
One question that is raised with regard to this issue is that the bone should serve as a shomer – as protection – to the marrow within, and we have learned that a shomer is considered part-and-parcel of the thing that it protects (see above, daf 118), so that touching it should be considered as if physical contact had been made with marrow itself. Rashi explains that we must distinguish between situations where the shomer protects the food, but the food itself can be touched, and cases where the food is totally sealed off and cannot be touched under any circumstances. In the latter case, touching the shomer will not be considered as if the food had been touched.