Machshirin, Chapter 6, Mishnah 6. This chapter is occupied with the liquids that render food susceptible to tumah, the Shiva Mashkin and their derivatives. Our mishnah takes up a further group: fluids that are themselves sources of tumah, and which, upon contact with food, accomplish two things at once.
Tamei and Machshir Together
The mishnah opens: "eilu mitam'in umachshirin," these liquids convey tumah and also effect hechsher. In other words, they are tamei in their own right, and when they land on food they simultaneously make that food susceptible to tumah and render it tamei on the spot.
Heading the list is "zovo shel zav," the emission that defines a man as a zav. Such a man contracts tumah through a specific type of flow, and the Torah gives him the status of an av hatumah: his very body is tamei, and so too the liquids that come out of him. Should any of them fall upon food, that food is at once made fit to receive tumah and rendered tamei by the very same contact.
The mishnah continues with the rest of his fluids: "rukko," his saliva, "v'shichvas zaro," his semen, "umei raglav," his urine. Each carries the identical double status.
Then the mishnah leaves the zav and adds two further items. "Urevi'is min hameis," a revi'is, a quarter log, of blood from a corpse; and "v'dam hanidah," the blood of a nidah. Both are tamei in themselves, and both, on contact, make food susceptible while conveying tumah to it at once.
Three Dissenting Views
"Rabbi Eliezer omer, shichvas zaro eino machsheres." Rabbi Eliezer removes semen from the list of liquids that effect hechsher. His reasoning is that it has taken on an identity of its own and is no longer to be classified with water. Since it has departed from the category of water, it cannot serve as a hechsher. It remains tamei; it simply does not render food susceptible.
"Rabbi Elazar ben Azaryah omer, dam hanidah eino machshir." Rabbi Elazar ben Azaryah makes the parallel claim about the blood of a nidah. He compares it to blood that issues from hemorrhoids or from a wound, blood which all agree does not effect hechsher. Nidah blood, in his view, belongs to that same category of blood produced by a bodily affliction rather than blood in the ordinary sense. It is tamei, but it does not make food susceptible.
Rabbi Shimon applies the same limitation to blood taken from a corpse, teaching "dam hameis eino machshir." Blood earns its place among the seven liquids only while it is flowing in a living human being. After death, it can no longer be regarded as a drink the way the blood of the living is regarded, and with that it forfeits its capacity to bring about hechsher.
The halachah does not follow any of these three. We rule like the Tanna Kamma: all the fluids on his list effect hechsher l'kabel tumah.
The Gourd
The mishnah closes with the practical consequence of Rabbi Shimon's position: "v'im nafal al hadela'as, yigod v'hi tehorah." Since, in his view, the blood of a corpse does not effect hechsher, if such blood falls onto a gourd, one scrapes off the blood and the gourd remains tahor.
The gourd may be eaten with no difficulty whatsoever. The blood has to be removed only on account of the prohibition against consuming blood, and not because of any question of tumah, for according to Rabbi Shimon no tumah has been transmitted to the gourd at all.