Mikvaos, Chapter 9, Mishnah 4. This chapter deals with chatzitza, the substances that come between the mikvah water and the body of the person immersing. Since the water must reach the whole body, anything clinging to the skin can invalidate the tevilah, and the deciding factor is hakpada: whether a person is particular to remove it. Our mishnah lists a series of things that people simply tolerate on their bodies, and which therefore do not form a barrier.
"Liflif sheba'ayin" - the secretion that gathers in the eye. So long as it is still in the eye itself, it is not a chatzitza, since people do not necessarily trouble themselves to wipe it away.
"V'gelad she'al hamaka" - the scab that forms over a wound, lying directly upon it. "V'saraf lach" - tree sap that has gotten onto a person and has not yet hardened; while it is still moist it does not interpose. "V'lichluchei tzo'ah she'al besaro" - moist grime and soil on the skin, the ordinary residue of sweat and dirt, which again is not something a person is particular to remove.
"V'tzo'ah shetachas hatziporen" - dirt lodged under the fingernail. Here too, as long as it is still soft, it is not something a person insists on cleaning out, and it does not interpose. This stands in contrast to the case taught earlier in the chapter of dough that gets caught beneath the nail, which is a chatzitza.
"V'tziporen hameduldeles" - a nail that has partly torn away and is hanging loose. "Kashash shel katan" - the hairs of a child that are destined to fall out later on in the course of his growth. Since in certain places that hair is going to be shed on its own, it is not treated as a barrier.
The mishnah then closes with a related point about tumah. These items are lo tamei v'lo metamei: they do not receive tumah, and they do not transmit it. If something touches such hair, the hair does not become tamei; and if the person himself is tamei and something touches those hairs, the tumah is not conveyed through them, because they are not truly part of his body.
Against this the mishnah sets "krum she'al hamaka", the thin membrane of skin that forms over a wound. That membrane becomes part of the flesh itself, and so it is tamei u'metamei: it is susceptible to tumah, and it transmits tumah. Being regarded as the person's own skin, if he is tamei and something touches that covering, the tumah passes through it, and likewise in the reverse direction.