1) "EVEN MESAMA"
OPINIONS: The Gemara explains that one might have thought that just as a Nidah is Metamei through Even Mesama, her blood should be Metamei through Even Mesama. The Gemara quotes Rav Ashi who derives from a verse that blood is not Metamei with Tum'as Mishkav u'Moshav, and it also is not Metamei with Tum'as Even Mesama. It is Metamei, however, through carrying (Metamei b'Masa).
What is the difference between being Metamei with Even Mesama, and being Metamei with Maga and Masa? It seems that Even Mesama and Masa are the same type of transmission of Tum'ah.
(a) According to RASHI (DH Even Mesama), Even Mesama is an extension of Tum'as Masa (and applies even to objects which are not suited for sitting upon). It differs from the normal Tum'as Masa, since generally only something which is fit to be moved by the Nidah becomes Tamei through Masa, while something which lies on a very heavy stone which is above the Nidah cannot be moved by her (since the stone is too heavy to carry).
(b) According to RASHI in Shabbos (82b, DH b'Even Mesama), Even Mesama refers to a large stone suspended on poles, on which a Nidah or Zav sits, while utensils are underneath the stone. The utensils are Tamei even though no pressure is put on them by the stone on which the Nidah or Zav sits. This seems to apply even to objects that are not made for sitting upon (see Tosfos), and it is a completely new type of Tum'ah.
(c) According to RABEINU TAM (cited by Tosfos here and in Eruvin 27a, DH Kol), an Even Mesama is such a heavy stone that the weight of the Nidah above it in no way adds any noticeable pressure to any of the objects beneath it. Objects beneath the stone become Tamei with Rishon l'Tum'ah. It is not the same as Tum'as Masa, since Masa requires moving the object and not just holding it (as Rashi explains in Chulin 21b).
(d) TOSFOS (ibid.) cites Rabeinu Tam's definition of the Even Mesama, but explains that any objects fit for sitting upon that lie below the stone will become Tamei. (That is, Even Mesama is an extension of Mishkav u'Moshav). (See also Insights to Shabbos 82:3.)
2) HALACHAH: CORNEAL TRANSPLANTS
QUESTION: The Mishnah states that the skin of a deceased person is Metamei like his flesh. Ula explains that, mid'Oraisa, the skin of a person is not Metamei, but the Rabanan decreed that it is Metamei in order to prevent people from making bedspreads out of human skin.
Why does Ula say that the Rabanan decreed that human skin is Tamei in order to prevent people from making human skin into bedspreads? No one would use human skin for such a purpose, because it is already prohibited to derive benefit from any part of a dead body (Avodah Zarah 29b)! (TOSFOS DH Oros)
(a) TOSFOS (DH Shema) and the RASHBA answer that people tend to be more scrupulous in keeping themselves Tahor than in observing other Mitzvos of the Torah (the Rashba in Teshuvos 1:365 calls such people "fools"), and therefore the Rabanan saw a need to decree that human skin is Metamei.
(b) Tosfos suggests further that the skin of a person is not considered part of his flesh, and it is not included in the prohibition against deriving benefit from the body of a corpse. This also seems to be the view of RASHI in Chulin (DH Devar Torah) and other Rishonim. (The Rashba (ibid.), however, argues and maintains that skin is included in the prohibition.)
The MISHNEH L'MELECH (end of Hilchos Avelus) cites an interesting proof to permit the skin of a corpse. The Navi relates that David ha'Melech gave to Michal for her betrothal (Kidushin) one hundred Philistine foreskins, removed from Philistine warriors that he killed in battle (Shmuel II 3:14). The Mishneh l'Melech asserts that this proves that one is permitted to derive benefit from the skin of the dead.
There is a practical difference between these two answers, one which is relevant to a modern medical procedure. It is now common to replace a damaged cornea with the healthy cornea of a deceased person. The cornea is the transparent membrane (made of epithelial tissue) that is the outermost cover of the pupil and iris of the eye, which protects the eye from harmful matter and serves as the eye's outer lens. Due to its composition and function, the cornea can be classified as skin according to Halachah. Since its replacement is not always necessary to prevent blindness (in which case its replacement might be classified as Piku'ach Nefesh and would override other Isurim), it is important to determine whether one may use the cornea taken from a deceased person. (This question is relevant even with regard to a cornea taken from the corpse of a Nochri, because the Halachah prohibits deriving benefit from any human corpse, whether that of a Jew or of a Nochri. See SHULCHAN ARUCH YD 349:1.)
RAV YECHIEL YAKOV WEINBERG in SERIDEI ESH (2:120) and RAV YEKUSIEL GREENWALD in KOL BO AL AVEILUS permit corneal transplants based on the words of Tosfos here who permits deriving benefit from the skin of a dead person. Others, however, disagree and assert that a cornea is considered flesh and not skin. Others permit the procedure on the grounds that using a cornea for a transplant is not considered a normal form of benefit and is "she'Lo k'Derech Hana'asan" (see YABI'A OMER 3:20).
(c) The RADVAZ (Teshuvos 1:262) explains that the skin of a human corpse was considered to have mystical powers of protection. A superstitious person would take the skin of a dead person, preferably a dead person who was a close relative such as a parent, and bring it with him to war as a talisman for protection. As the Gemara in Chulin (123a) states, no Roman battalion went to war without taking along the skin of the head of a dead person (see Rashi there, DH Karkaflin). Accordingly, some considered this to be permitted, reasoning that it did not involve deriving benefit from the corpse (but merely protection). The Rabanan therefore decreed that the skin is Tamei in order to prevent people from using the skin for such purposes.
3) THE "TUM'AH" OF FLUIDS OF THE NOSE
QUESTION: The Mishnah (54b) states that the Rok (saliva) of a Zav makes something else Tamei only when it is moist, but not when it is dry. The Gemara asks what the source is that the Rok of a Zav is Tamei, and it answers by quoting a Beraisa that says that the source is the verse, "When the Zav shall spit..." (Vayikra 15:8). The Gemara asks that although the verse teaches that his Rok is Tamei, how do we know that "Kicho," "Ni'o," and "Mei ha'Af" are Tamei? ("Kicho" and "Ni'o" are types of Rok, and "Mei ha'Af" is, literally, "fluids of the nose.") The Gemara answers that these are derived from the extra "Vav" in the words, "v'Chi Yarok."
The Gemara later asks what exactly is "Mei ha'Af." Rav explains that "Mei ha'Af" refers to fluids of the nose that were expelled through the mouth (via the nasal passages). It is not possible for fluids of the nose that were expelled in this manner not to come into contact with "Tzichtzuchei Rok," drops of saliva.
According to Rav, the reason why Mei ha'Af is Tamei is that Rok is mixed with it. If this is the reason why Mei ha'Af is Tamei, then why is it necessary for the Gemara to derive that Mei ha'Af is Tamei from the extra "Vav" in the verse? According to Rav, Mei ha'Af is Tamei because of the Rok mixed with it, which is Tamei as explicitly stated in the verse itself, "When the Zav shall spit..."!
(a) The RASHBA, in his first answer, writes that the Tum'ah of Mei ha'Af indeed is identical to the Tum'ah of Rok. When the Gemara says that "Kicho," "Ni'o," and "Mei ha'Af" are derived from the extra "Vav" in the verse, it really means that only "Kicho" and "Ni'o" are derived from the extra "Vav." It mentions Mei ha'Af only incidentally. Rav is teaching that Mei ha'Af is Tamei only because of the Rok mixed with it.
However, the Rashba rejects this answer for two reasons. First, the Gemara later asks that according to Rav, why does the Beraisa earlier not mention also that the tears from the eyes of the Zav are Tamei? This question does not make sense according to this answer of the Rashba, because even Mei ha'Af is mentioned in the Beraisa only incidentally, and thus it is clear that the Beraisa is not particular about what it mentions.
Second, the Gemara later teaches that there are nine Mashkin (fluids) of a Zav. The Gemara says that it is understandable, according to Rav, why Mei ha'Af is not included in the list: since Mei ha'Af is Tamei only if it exits through the mouth, it is not mentioned. According to this answer of the Rashba, the Gemara could have given a more basic reason for why Mei ha'Af is not mentioned: it is included in Rok, which is mentioned.
(b) Therefore, the Rashba gives a different answer for why the extra "Vav" is necessary to teach that Mei ha'Af is Tamei. If the verse would have mentioned only Rok, one would have thought that Rok is Tamei only when it is not mixed with anything else. However, Tzichtzuchei Rok, or the Rok mixed with the fluids of the nose, might be Tahor. Therefore, the extra "Vav" teaches that even Rok mixed with Mei ha'Af is Tamei. (See also RITVA.)
The Rashba cites as a source for this answer the Gemara earlier (22a) which says that there is a difference between the fluid itself and its Tzichtzuchei. The Gemara there says that when a Zav is counting his seven clean days, if he sees Zov he loses all seven days. However, if he sees Tzichtzuchei Zivah, which is a mixture of Zov and Shichvas Zera, he loses only one day. Since there is a difference between a fluid and its Tzichtzuchei, an extra Derashah is necessary to teach that Tzichtzuchei Rok of a Zav is Tamei just as his Rok is Tamei.
The RITVA writes that according to this answer, although the Gemara says that Rebbi Yochanan maintains that even Mei ha'Af that exited through the nose is Tamei because Mei ha'Af is considered a "Ma'ayan" of a Zav which is Tamei, this does not mean that Rav maintains that Mei ha'Af is not a Ma'ayan. Rather, the Gemara means that according to Rebbi Yochanan, it is an entirely independent Ma'ayan (that comes from the nose), while according to Rav it is the Ma'ayan of Rok. (D. BLOOM)