Kesubos, chapter seven, mishnah ten. This mishnah opens with a list of physical defects and occupations on account of which we force a husband to give his wife a get, if she wishes it.
"V'eilu shekofin oso l'hotzi":
"Mukeh shechin" - one who is afflicted with a type of skin disease.
"U'ba'al polipus" - one who has a growth near the nose that makes breathing difficult, and the Gemara explains that a foul odor issues from his nose.
"V'hamekametz" - one whose profession is collecting dog excrement, which is used in the process of tanning hides, a rather foul-smelling trade.
"V'hametzaref nechoshes" - those who mine copper and smelt it, who are likewise filthy and foul-smelling.
"V'haborsi" - the tanner, who processes hides.
What they all have in common: occupations with an exceedingly harsh odor.
The mishnah adds that there is no difference between one who had these defects or worked at these trades before they married - and the case is where she did not know about it - and one who developed them after the marriage, such as where he took up this trade when he was already married. Either way, if the wife says that she cannot bear it, we force him to divorce her.
The dispute between Rabbi Meir and the Sages:
"V'al kulan amar Rabbi Meir: af al pi shehisnah imah - yecholah hi shetomar: sevurah hayisi she'ani yecholah lekabel, v'achshav eini yecholah lekabel" - regarding all of these defects Rabbi Meir says that even if he made a condition with her that she would tolerate his defects and she agreed, she is able to say, just as we learned in the previous mishnah: I thought that I would be able to bear it, and now I have discovered that I cannot. On the strength of this claim she leaves him, and she is given a get and her kesubah.
"Va'chachamim omrim: mekabalto al karchah, chutz mimukeh shechin" - the Sages disagree with Rabbi Meir: if he had these defects before she married him and she knew about them, and certainly where he made a condition with her that she accept them, we do not force him to give her a get. This applies to all the defects the mishnah listed, except for one afflicted with skin disease, where we do force him to divorce her.
And the reason for this: "mipnei shememakaso" - marital relations are extremely harmful to this skin affliction and ultimately bring him to danger. Therefore, even if she is willing to remain married to him, we do not allow them to continue their marriage.
The incident in Tzaidan:
The mishnah brings an incident that occurred in a place called Tzaidan: a certain tanner died childless, and he left a brother who was also a tanner, and his widow came before that brother for yibum. She is able to say to the yavam: "Your brother and the odor of his trade I was able to accept, but you I cannot accept. I am unable to enter into yibum with you; rather, you should perform chalitzah for me and give me my kesubah."
In summary: In this mishnah we learned the list of defects and foul-smelling occupations - one afflicted with skin disease, one with a nasal growth, the collector of dog excrement, the copper smelter and the tanner - on account of which we force the husband to divorce his wife, whether he had them before the marriage or developed them afterward. We examined the dispute between Rabbi Meir and the Sages in a case where he made a condition with her: according to Rabbi Meir she can say, "I thought that I would be able to bear it, and now I cannot bear it," while according to the Sages she must accept him against her will, except for one afflicted with skin disease, which the relations aggravate, so that even with her consent we do not allow them to remain married. And finally we learned from the incident in Tzaidan that even a yevamah is able to say that the brother she could accept but the yavam she cannot accept, and she demands chalitzah and her kesubah.