Niddah 49 - One but Not the Other - Vessels, Limbs, Purity
We just finished talking about upper and lower signs of maturity , where the first cannot be without the second, but the second can be without the first. By analogy, if a vessel lets water in, it surely lets water out, but there can be a vessel that lets water out but not in. What's the difference? If it lets water out, it is still a vessel, albeit a leaky one, and can be used to hold purification water of the ashes of red heifer, to remove the impurity of the dead body. However, if it lets water in, it is not a vessel anymore, and will not purify.
An extra finger, if it has a fingernail, surely has a bone, but there can be an extra finger with a bone but without a fingernail. To what law does this refer? If it has a fingernail, then even if it is very small, it is a full-fledged limb, and transmits the impurity of the dead body when under the same roof. However, if it does not have a fingernail, then it it only transmits impurity because of the bone in it, which is the impurity of touching and carrying it, but not of the roof.
Any vessel receptive of impurity from a man who had an emission ( zav ) sitting on it, is also receptive to the impurity of the dead body. However, the other way around is not true: if a vessel is so large that it is not designated for sitting, it does not become impure when a zav sits on it, since they can tell him "stand up, and let us do our work."
Art: David The Younger Ryckaert - An old woman holding a stoneware vessel