Gemara

People would take dirt from Rav's grave to cure a fever on the first day.

Shmuel: It is permitted. The ground does not become forbidden. "Va'Yashlach Es Afarah (of the Asheirah) Al Kever Benei ha'Am" equates a grave to idolatry. Idolatry attached to the ground is permitted. The Torah forbids "Eloheihem Al (on) he'Harim", but the mountains are not their gods. Likewise, attached dirt of a grave is permitted.

Avodah Zarah 29b: One may not benefit from Korbanos to idolatry. "...Va'Yochelu Zivchei Mesim" equates them to a Mes, which we learn from a Gezerah Shavah "Sham-Sham" from Eglah Arufah (a calf beheaded when a murdered corpse is found). One may not benefit from Eglah Arufah, for it is called 'Kaparah', just like Kodshim.

Erchin 7b (Mishnah): If a woman was executed (we may benefit from her hair. An animal that was executed is (totally) forbidden.)

Question: Why is this permitted? One may not benefit from a Mes!

Answer #1 (Rav): It is not her hair. It is a wig.

Objection (and Answer #2 - Rav Nachman bar Yitzchak): Presumably, the case of her hair is like (the other case in the Mishnah,) an animal's hair, i.e. its own hair! Rather, one may benefit from her until she dies (and death does not affect her hair, so it does not forbid it). An animal is different, for it is (totally) forbidden once it is sentenced to die.

Support (for Rav - Beraisa): If a woman was being taken to be executed and she said "give my hair to my daughter," we give it. If she died (without saying this), we do not give it, for one may not benefit from a Mes.

This refers to Noyei (adornments of a) Mes (a wig).

Another Beraisa supports Rav Nachman (bar Yitzchak).

Nidah 55a (Ula): Mid'Oraisa, human skin is Tahor. Chachamim decreed that is Tamei, lest people make mats from their parents' skin.

Shabbos 93b (Mishnah): If one takes a k'Zayis of a Mes to Reshus ha'Rabim, he is liable;

R. Shimon exempts.

Rishonim

Rambam (Hilchos Evel 14:21): One may not benefit from any part of a Mes, except for the hair, which is permitted because it is not the body.

Kesef Mishneh: What is his source to permit hair? Erchin 7a forbids!

Lechem Mishneh: The Rambam rules like Rav Nachman.

Mishneh l'Melech (b'Sof, DH Asher): A Mishnah connotes that people do not guard less than a k'Zayis of a Mes. If one may benefit from the skin, perhaps this refers to the skin (one need not bury it). The Yerushalmi says that it refers to a Mes Nochri, from which one may benefit.

Rashba (365): Ula explains that Chachamim decreed that skin of a Mes is Tamei, lest one make rugs from his parents. Even though one may not benefit from the skin, he does not get physical benefit from the rugs. We forbid benefit from Yayin Nesech, for it is equated to a Korban, which is equated to a Mes. Perhaps it is equated to a dead Nochri! Rather, we must say that one may benefit even from dead Nochrim. In any case, one may benefit from skin of a Mes, for we learn the Isur from the Hekesh to Kodshim, and one may benefit from skin of Kodshim. This is why Chachamim needed to decree that it is Tamei. Alternatively, the Torah forbids skin of a Mes. Chachamim decreed due to fools who are more concerned for Tum'ah (lest others separate from them) than for Isur. I favor this, for the skin is no better than the hair, which is forbidden (Erchin 7b). The Rambam permits, but I disagree.

Tosfos (Nidah 55a DH Shema): The Torah forbids benefit from a Mes, but people are more concerned for Tum'ah than for Isur. We say that people are more concerned for Tum'ah of a Keli than for murder (Yoma 23a). Actually, it seems that one may benefit from skin of a Mes. We learn the Isur from the Hekesh to Kodshim, and one may benefit from skin of Kodshim after Zerikah (when there is Kaparah). This is why we need "Lo Yochal" to forbid skin of an executed animal. We could not learn it from the Isur of the meat.

Pri Megadim (YD 79:3 Sifsei Da'as): Perhaps the skin is forbidden only mid'Rabanan.

Rashba (330): One may not benefit from hair of a Mes. Rav Nachman discusses hair detached between sentencing and execution. Rav holds that the Mishnah connotes that we discuss after death. Rav found no Heter for her own hair after death, so he needed to say that it discusses a wig. All forbid hair after death. The Ramban says so, and the Rambam in Perush ha'Mishnayos, but the Rambam retracted in the Mishnah Torah. The Stam Gemara asked 'one may not benefit from a Mes!' Rav Nachman does not argue. If he did, why did he explain 'an animal is different, for it is forbidden once it is sentenced.' The primary question was not 'why are animals different?'! The Beraisa explicitly forbids benefit from her hair, for one may not benefit from a Mes. It is like her body.

Chasam Sofer (Avodah Zarah 29b DH Kaparah): Tosfos (Sanhedrin 48a DH Meshamshin) says that skin is no less (forbidden) than shrouds! We must say that the clothes he was wearing are forbidden only if he will be buried in them. Likewise, skin is forbidden if he will be buried in it, but not if one flays the Mes and makes rugs from the skin. However, Rav and Rava forbid even hair (detached) from a Mes! Rav Nachman permits, for death does not affect hair. Even if he says the same about skin, it is still difficult for Rav and Rava. I answer that we hold like R. Yanai, who forbids Eglah Arufah while it is alive, for it is equated to Kodshim. Therefore, the skin is permitted. Rava forbids Eglah Arufah only after death. He learns from "there it will be buried", so the skin and hair are just like the rest. Ula, who decreed Tum'ah on skin lest people make rugs from it, holds like R. Yanai, who permits the skin. The Rambam permits hair of a Mes, like R. Yanai. Even though he connotes that the Torah forbids the skin, perhaps he expounds "beheaded" - when it is whole.

Poskim

Shulchan Aruch (YD 349:2): One may not benefit from hair of a Mes.

Beis Yosef (DH Kosav): The Rambam and Tosfos permit benefit from hair of a Mes. I do not know their source. The Rashba challenged them. Also the Ramban forbids.

Nekudas ha'Kesef: The Sugya favors the Rambam.

Beis Hillel: The opinion that forbids benefit from hair must hold like the answer in Tosfos (Nidah 55a) that the Torah forbids the skin.

Ginas Veradim (YD 1:4, cited in R. Akiva Eiger): One may not benefit from a Mes even in an abnormal way. We say that the Torah forbids only normal benefit only when it says "do not eat." Regarding meat and milk, Kilai ha'Kerem, and a Mes, which is learned from Eglah Arufah, the Torah forbids Stam. This includes even abnormal Hana'ah. Shmuel permitted dirt from a grave for a cure only because the ground does not become forbidden. Whether they made mud to annoint the Choleh, or whether he ate the dirt, this is abnormal Hana'ah, and it would be forbidden if not for the Heter of what is attached. The Radvaz (547) permits to eat from Egyptian mummies or sell them, for this is abnormal Hana'ah, and they are of Nochrim. I proved that even abnormal Hana'ah is forbidden. Most Poskim forbid Hana'ah even from dead Nochrim. In any case one must be stringent about a Safek mid'Oraisa.

Kol Mevaser (1:70): It is clear from Shulchan Aruch 526:4 and Nazir 51 that one need not bury hair of a Mes with the Mes. Surely this is like the opinion (Erchin 7) that one may benefit from it. Rav forbids benefit, but surely he agrees with the Beraisa in Mo'ed Katan that says that we cut the hair of a Mes. We must say that even though hair is forbidden, for it is not worse than shrouds, it is not like the body, and "you will bury it" does not apply to it. The Torah commands "bury, and do not be Metamei the land." Since hair, teeth and nails are not Metamei (Nazir 51), there is no need to bury them.