משנה על כלים 17:12

The following defile by contact and carriage but not by overshadowing: A bone of barleycorn size, Earth from a foreign country, A bet peras, A limb of a corpse, or a limb [severed] from a living person which has no longer its appropriate flesh, A spine or a skull which is deficient. How much is [considered] a deficiency in the spine? Bet Shammai say: two vertebrae, But Bet Hillel say: even one vertebra. And in the skull? Bet Shammai say: [the size of a] hole [made] by a drill, But Bet Hillel say: as much as would be taken from a living person and he would die. Of what drill did they speak? Of the small one [used] by physicians, the words of Rabbi Meir. But the sages say: of the large one in the Temple-chamber.

[An object] one handbreadth square and one handbreadth high conveys uncleanness and blocks uncleanness. How does it [block uncleanness]? In the case of a covered drain beneath a house, if it has a space a handbreadth wide and its outlet was a handbreadth wide, and there is uncleanness inside it, the house remains clean; And when there is uncleanness in the house, that which is within [the drain] remains clean, for the manner of the uncleanness is to go out and not to go in. If it had a space one handbreadth wide but its outlet was not one handbreadth wide, when there is uncleanness in it, the house becomes unclean; But when there is uncleanness in the house, that which is within it remains clean, for the manner of the uncleanness is to go out and not to go in. If it did not have a space one handbreadth wide and its outlet was not one handbreadth wide, when there is uncleanness within it, the house becomes unclean; And when there is uncleanness in the house, it [also] becomes unclean. It makes no difference if the cavity was carved out by water or by a sheretz or if it had been eaten out by salt. And similarly [if it is in] a row of stones or a pile of beams. Rabbi Judah says: any "tent" not made by a person is not considered a tent’. But he agrees that crevices and crags [can be considered as ‘tents’].