74a -Sugya starts from Mufneh
The gemara is wondering if a vessel made from hair from the tail of horse/cow is tamay. It is trying to suggest that the tumah can be contracted by the chamur case of 7 day tumah (maga b'meis) from the kal case of one day tumah (maga b'sheretz) - See last Rashi on 73b
Through a series of hekeshim the gemara proves that the tumah is in fact contacted through tamai meis.
Look very carefully at the two rashi's "Sh'hee Merubah" and the following one "Nirbah Anu". Rashi clearly distinguishes that touching a dead body is 7 day teumah and touching SOMEONE who touched a dead body receives 1 day teumah.
MY QUESTION: Why does the gemara seem to make a hekesh from sheretz and shichvat zera to MAGA B'TEUMAS MEIS (Va'yikra 22:4) - when our goal is to prove MAGA B'MEIS. (What's even stranger is that Rashi to Va'Yikra 22:5 explains that "Oh B'Adam" there is refering davka to MAGA B'MEIS - WHY DOESN'T THE GEMARA QUOTE THIS PASUK AS THE HE'KAISH TO SHERETZ WHEN THIS IS WHAT WE NEED
I hope I articulated the question well. It seems to be a blantantly obvious problem that slips through Rashi, Tosfot, Schottenstein, Steinzaltz, etc.)
Betzalel Gersten, Mevaseret, Israel
The Gemara (64a) concludes that a vessel made from the hair of a horse/cow is Tamei because of the Gezeirah Shaveh of "Beged v'Or," which is said both by Maga b'Meis and Sheretz. The Gemara goes on to explain that these Pesukim are Mufnah, meaning that they are open to be used for a Gezeirah Shaveh. The reason that the Pasuk of "Beged v'Or" regarding Maga b'Meis is Mufnah (which it only has to be according to the opinion of "Lemeidin u'Meishivin") is because it could have been learned from Shichvas Zera, as Maga b'Meis is compared to Shichvas Zera. This prompted your question that the Pasuk brought is in fact from Maga b'Tumas Meis, and not Maga b'Meis. The Rishonim all take for granted that Maga Meis is in fact compared to Shichvas Zera.
If you take a look at the Pesukim (Vayikra 22:4-5), you will see that these topics are all one "run-on sentence." It is very possible that though the Gemara quotes the Pasuk you mentioned (22:4), it is in fact comparing Shichvas Zera (22:4) to Maga b'Meis (22:5). The Gemara often will cite a Pasuk in order to best illustrate a Hekesh, even though these might not be the exact words which the Hekesh is learned from. Especially in this case, where the commentaries explain that Maga b'Meis is really the first topic which would be discussed if not for the fact that the Torah organized Tumas Maga only (22:4) before things which could also have Tumas Masa (22:5), Tumas Meis is actually inferred through the mere mention of Maga b'Tumas Meis.
Regarding your second question, it is clear from Tosfos' question (DH "Shma Minah") that the Gemara was not looking for a Hekesh, as it could have cited a clear Hekesh from Shichvas Zera to teach us that this hair is also Tamei b'Maga Meis (similar to your question). The RAMBAN and RASHBA answer that the Torah will often teach us something even though we could have learned it from a Hekesh (See SFAS EMES for a more complicated answer).