Negaim, Chapter 6, Mishnah 8. Much of this chapter has been occupied with the measurements of a nega and with michyah, the patch of healthy flesh that appears inside a beheres and renders it tomei. Our Mishnah now teaches that there are certain places on the human body where michyah simply does not function as a siman tumah at all.
The Mishnah begins with a count: "Esrim ve'arba roshei evarim ba'adam she'einam mitam'in mishum michyah" - a human being has twenty-four limb-ends, and in none of them does michyah bring about tumah.
Why specifically michyah
The Tiferes Yisroel draws attention to the precise wording. Our Mishnah speaks of michyah, whereas the listing that follows speaks of beheres. Furthermore, the two lists are not presented together as one continuous unit; the second one begins with the word "eilu," separating it off as its own subject. From both of these points he concludes that our Mishnah is addressing one narrow case: a beheres that would become tomei because of a michyah standing within it. In these twenty-four places, that particular siman does not apply. Should the same spot develop pisyon (spreading) or se'ar lovon (two white hairs), however, our Mishnah has nothing to say; there these places would indeed be susceptible.
What makes michyah different? Apparently the healthy skin of the michyah sits slightly higher than the white area of the beheres surrounding it. The Tosfos Yom Tov elsewhere cites the peirush of the Ralbag on Chumash, that the white discoloration of a nega is a form of ipush, a kind of decay or rotting of the skin. Healthy skin remaining in its midst therefore stands raised above it.
Now, the Torah says "vera'ahu hakohen", and the emphasis is on "ra'ahu," that the kohen must see it. The Rav explains that we require the kohen to take in the nega together with the michyah in a single view, all at once. The tips of the limbs are rounded and sloping surfaces: part of the area faces one way and part faces the other, and with the michyah raised above the beheres, the kohen cannot capture both in one look. Hence these places are excluded from tumas michyah.
Counting the twenty-four
"Roshei etzba'os yadayim veraglayim": the ends of the fingers and the ends of the toes. Ten on the hands and ten on the feet bring us to twenty.
"Veroshei oznayim" - the tips of the ears. The Tosfos Yom Tov brings Rashi in Kiddushin (Perek Kama, daf chaf hey), who explains this as sfas ha'ozen, the cartilage rim running around the top and back of the ear. The Rav points out that although the digits were counted individually as ten and ten, both ears together are counted here as a single item. We are now at twenty-one.
"Verosh hachotem": the end of the nose, which brings the tally to twenty-two. "Verosh hagviyah": the end of the male member, twenty-three. "Verosh hadadim shebe'ishah": a woman's nipples, completing the twenty-four.
The dissenting opinions
"Rabbi Yehudah omer: af shel ish" - Rabbi Yehudah adds the nipples of a man as well. The meforshim explain that we do not follow him because a man's are small enough that the kohen can indeed take in the whole area, together with the michyah, in one glance.
"Rabbi Eliezer omer: af hayabolos vehadidulin einam mitam'in mishum michyah" - Rabbi Eliezer would add warts to the list, along with didulin, generally taken to mean dangling flaps of skin, what we would nowadays call skin tags. The Rav notes a variant reading of tiltulin: flesh that has swelled upward kemo tel gavoha, like a high mound (tel means a mound, familiar from the city name Tel Aviv), for instance the bump that remains where a boil or something similar had been.
Neither Rabbi Yehudah nor Rabbi Eliezer is accepted as the halachah. We are left with a total of twenty-four: twenty at the ends of the fingers and toes, one for both ears together, one at the nose, one at the male member, and one for a woman's nipples.