A limb that was cut off from a living person is considered as an entire corpse and imparts impurity when it is touched, carried, or one is under the same structure. This applies even to a limb of a newborn infant, for there is no minimum measure that applies with regard to complete limbs. This is derived from Numbers 19:16 which states: "Anyone who touches a corpse slain by the sword... on the open field." It is a known matter that the laws applying to one slain by the sword are the same as one slain by a stone or through other means. According to the Oral Tradition, we learned that the verse comes only to deem a limb severed by a sword as impure.
When does the above apply? When the limb was intact as it was when it came into being with flesh, sinews, and bones, as the abovementioned verse states: "Or the bone of a man." Implied is that the bone must be like a man, i.e., a human corpse. Just as a human corpse has flesh, sinews, and bones, so too, a limb from a living person must be intact as it was when it came into being and have flesh, sinews, and bones. In contrast, a kidney and a tongue, and the like, even though they are considered as complete organs, since they do not contain bones, they are considered as the remainder of a person's flesh.
If even the slightest amount of bone was missing from a limb severed from a living person, the entire limb is pure. The following rules apply if some of its flesh was missing: If there remained enough flesh on it that, were the person to be alive, his flesh would regenerate, be healed, and return to a state of wholeness, the limb imparts impurity when it is touched, carried, or when one is under the same structure. If not, it imparts impurity when it is touched, carried, but not when one is under the same structure.
Flesh that is separated from a living person is ritually pure. Similarly, a bone without flesh that is separated from a living person is ritually pure.
א. בב2 לית.
ב. בד׳ לית. אך בגמ׳
שבת עח. כבפנים.
ג. ב2: הכוליה. ב3: הכוליא. וכך ד. אך בפיהמ״ש
חולין ג, ב בכ״י רבנו כבפנים.
ד. ב3: עצמן. וכך ד (גם פ).
ה. בב2 נוסף: ששעורו בכזית.