Negaim, Chapter 11, Mishnah 1. Our subject here is karachas and gabachas, the fourth and last of the nig'ei adam. A single mishnah covers the entire topic, and there is a good reason for such brevity. Karachas and gabachas are not some newly introduced nega at all; they are plain tumas baheres, the tzora'as of or basar, appearing on portions of the head once those portions have lost their hair. A bald scalp counts as ordinary flesh, no different from skin anywhere else on a person, and therefore the tumah of or basar operates there.
An overview from the Rambam
Before we turn to the mishnah itself, the Rambam's summary is worth hearing. Karachas and gabachas convey tumah through two simanim rather than three, namely michyah and pisyon. Michyah applies because a baheres is what we are dealing with, and the siman is a patch of living flesh the size of an adashah showing up inside the baheres; pisyon applies should the baheres expand. Their hesger likewise runs for two weeks, since the pasuk describes them as "kemar'eh tzora'as or basar", that their appearance is the appearance of the regular tzora'as of the skin. And because such places carry no hair, sei'ar lavan does not function as a siman tumah in them. By definition we are speaking of a hairless region, so white hairs will not sprout there to begin with, and even if somehow they did, they would not render him tamei.
The mishnah's ruling
The mishnah rules: "Hakarachas vehagabachas", the hairless region at the rear of the scalp and the hairless region at the front, "mitam'os bishnei shavu'os", convey tumah over two weeks of hesger, "bishnei simanim, bemichyah uvefisyon", by way of two signs: michyah, a patch of live flesh, and pisyon, the expansion of the baheres.
Why the person became bald makes no difference. It may be min hashamayim, that his hair simply fell out on its own. It may be the result of radiation, or of some medication, Rachmana litzlan, or of a depilatory smeared on the scalp.
The derivation
The Rav brings the source for these two simanim from the pasuk "tzora'as porachas hi bekarachto". The word "tzora'as" teaches that it is mitam'ah bemichyah, like the other tzora'as of or basar, and "porachas", spreading, teaches that it is mitam'ah befisyon. The word "hi" the Rav learns as a gezeiras hakasuv, a mi'ut teaching that it is not mitam'ah through sei'ar lavan. The Rambam, as we saw, needs no special pasuk: these are bald areas, so white hair is not shayach there at all.
Defining the two places
The mishnah continues with the causes and then the boundaries. One who ate neshem, a substance whose ingestion prevents hair from growing, or who smeared neshem on the scalp as a depilatory, or who received a makah, a wound or condition, she'einah re'uyah legadel sei'ar, which leaves the skin incapable of growing hair. In all such cases we have a scalp that cannot grow hair, and the halachos of karachas and gabachas apply.
The mishnah asks and answers: "Eizohu karachas? Min hakodkod hashofei'a le'achorav", identifying the karachas as the rear section: it begins at the kodkod, the top of the head, and slopes down behind, "ad pikah shel tzavar", as far as the small protruding bone one can locate where the hairline of the neck begins.
"Eizohu gabachas? Min hakodkod hashofei'a lefanav", the gabachas is the front portion: from the crown of the head sloping forward, "keneged sei'ar milma'lan", up to where the hairline lies above the forehead. In short, gabachas is the front and karachas is the back.
The Tosfos Yom Tov asks what this last phrase comes to teach, and writes "nir'eh li la'afukei", that it comes to exclude the hair of the eyebrows. He adds that if it were possible for hair to grow on the forehead itself, as the Kesef Mishneh at the end of Hilchos Avodah Zarah seems to hold, this phrase would exclude that as well, though the Tosfos Yom Tov himself is quite skeptical of that possibility. Either way, the boundary of the gabachas is the hairline, or where the hairline had been.
The Tiferes Yisrael fills in the picture: the case is where all the hairs fell out from the midpoint of the skull forward, that is, the entire half of the skull toward the face, running down the sides of the head from ear to ear, as far as the hairs above the forehead, and including the temple areas on either side of the forehead. Kol shetzad hapanim, everything on the face side of the head, is called gabachas. There does not appear to be any dispute about this.
Two names, two nega'im
The mishnah teaches: "Hakarachas vehagabachas", these two, "einan mitztarfos zo im zo", do not join together, "ve'einan posos mizo lazo", nor does either one spread into the other. Though both sit on one and the same head, each stands by itself. Half a gris of baheres in the front region alongside half a gris in the rear region, even immediately adjacent across the crown, will not add up to a full gris. So too, a baheres in the front area that widens over the crown into the rear area has not thereby produced a pisyon, and the identical rule applies when the movement goes the other way.
This is precisely why the Torah gave them two different names. It is not merely a description of the contour of the skull: the two names establish two distinct shemos of nega, which therefore neither combine with nor spread into each other.
"Rebbi Yehudah omer", Rebbi Yehudah says, "im yeish sei'ar beineihem", if some hair still remains between the two regions, "einan mitztarfos", they do not join, "ve'im lav", but where there is none, "harei eilu mitztarfos", they do join. For Rebbi Yehudah the deciding factor is an actual divider of hair. The halachah, however, does not follow him. Even where the head is entirely bald and nothing whatsoever stands between the two regions, they still do not join.
Looking back over the nig'ei adam
With this mishnah the four nega'im that come upon a person are brought to a close. There is or basar, tzora'as on the skin, which is the primary case. There are michveh and shechin, a burn and a boil, injuries within which a baheres develops. There is the nesek, in which no baheres figures at all, but rather hair drops out in patches on the scalp or in the beard. And there are karachas and gabachas, on a hairless scalp, where a baheres does indeed form, its simanei tumah being an adashah-sized michyah within the baheres, or spreading of the baheres at the close of the first week, at the close of the second week, or once he has already been declared tahor.
Karei'ach hu, tahor hu
Having spent this mishnah on baldness, it is a fitting moment for a lovely vort from Rav Menachem Azariah miFano, the Rama miFano, one of the foremost Italian acharonim: a posek who left us she'eilos uteshuvos, the author of Alfasi Zuta and much besides, and above all else a mekubal. The Chasam Sofer, in a teshuvah dealing with depilatories at a period when Jews were being assaulted by anti-Semites who seized them by the beard, calls him rosh hamekubalim. Should someone wish to claim that Kabbalah requires a man to wear a beard, the answer is that the Rama miFano removed his own beard in this very manner, and the rabbanim of Italy generally did the same.
Among his sefarim is Asarah Ma'amaros, and toward the conclusion of the opening ma'amar, the lengthiest of them, he sets out to resolve a passage in the closing sugya of Bechoros in a way that clears Ben Azzai of having trampled upon derech eretz. The Gemara there relates that Ben Azzai declared all the chachamim to be, in his eyes, nothing more than klipas hashum, the thin papery covering of a garlic clove, adding one exception: "chutz min hakarei'ach hazeh", apart from this bald fellow, by whom he meant Rebbi Akiva. Taken at face value this smells of ga'avah, and worse, it is hardly the manner in which one speaks about Rebbi Akiva. In fact, certain rishonim identify Yehoshua ben Korchah as Rebbi Akiva's own son, reasoning from the fact that Rebbi Akiva had a bald head, and Tosfos discards that identification precisely because such a way of referring to Rebbi Akiva is unthinkable. Here too, then, the word "karei'ach" cannot have been meant as a slur.
The Rama miFano therefore reads the statement in an entirely different key. Using atbash, the cipher in which alef exchanges with tav, beis with shin, gimmel with reish and onward through the alphabet, the letters of shum yield befi, "in my mouth": garlic is a code for Torah shebe'al peh. Furthermore, anyone who has grown garlic knows how vulnerable the crop is to tiny worms that bore into it, and these he takes as standing for the Baytusim, the Tzedukim and the assorted minim and heretical currents of the era of Chazal, from which the Torah had to be safeguarded. And what does the husk of the garlic accomplish? It acts as a shomer, a natural covering that keeps the infestation away from what lies within.
Read this way, Ben Azzai's words are both modest and true. The rest of chachmei Yisrael functioned as klipas hashum: they stood guard over Torah shebe'al peh and shielded it against everything the heretics threw at it, "chutz min hakarei'ach hazeh". And the word karei'ach is itself a remez, for the pasuk the Rav cites in this very perek, speaking of a man whose nesek has covered his whole head, states "karei'ach hu, tahor hu". A bald head is a mark of taharah. Rebbi Akiva was that tahor hu, the purest of the Tannaim of his generation, and his role was not to protect Torah shebe'al peh but to be it. This is what stands behind the statement in Sanhedrin concerning the great Tannaitic works that emerged after him, the Sifra and Sifrei, the Mechilta and the Seder Olam, all of them produced by his talmidim: kula aliba d'Rebbi Akiva, every one of them follows the way that Rebbi Akiva laid down. He was the shum, the befi, Torah shebe'al peh embodied, while his colleagues were the husk that guarded it.
Ahead of us now lie the nig'ei begadim, then two chapters of nig'ei batim, and finally the chapter dealing with the taharah of the metzora and the korbanos he brings when he is ready to become tahor and re-enter society.