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מסכת חולין פרק ט משנה א
העור The hide of an animal (to which less than an olive's volume of meat is attached, and so it serves to protect the meat - this gives it the status of a shomer and thus allows it to contract tumah even though it is not considered food by itself),
והרטב and the gravy (which is ordinarily not eaten, but in this case has congealed and is therefore commonly eaten with meat),
והקיפה and the spices (which are not ordinarily eaten by themselves),
והאלל and the shreds (According to Reish Lokish, these are shreds of meat that remain stuck to the hide when a carcass is skinned, and which are not ordinarily eaten; R' Yochanan defines this as something else which some rishonim define as sinews of the neck and spine, others as shriveled flesh),
והעצמות and the bones (which contain marrow, and so the bone serves as a shomer for the marrow, which is food),
והגידים and sinews (this refers to major blood vessels, tendons, membranes, connective tissue, nerve bundles, which, though not normally eaten, are permitted),
והקרנים והטלפים

and the [roots of] horns and hooves (this part is soft and will bleed if cut, but is not normally eaten),

מצטרפין לטמא טמאת אכלים combine [with less than an egg's volume of food] to impart the tumah of foods [when the aggregate is at least an egg's volume]
אבל לא טמאת נבלות but not the tumah of neveilas.
כיוצא בו Similarly,
השוחט בהמה טמאה one who slaughters a non-kosher animal
לנכרי for a gentile
ומפרכסת and it is convulsing (and so its meat is prohibited to a gentile as being from a living animal, or אבר מן החי)
מטמאה טמאת אכלים imparts the tumah of food (as the slaughterer's intent to create food for the gentile gives it the status of food, even though it cannot yet be eaten)
אבל לא טמאת נבלות but not the tumah of neveilas
עד שתמות until it dies
או עד שיתיז את ראשה or until he chops off its head.
רבה לטמא טמאת אכלים [The Torah] has included that which impart the tumah of food
ממה שרבה more than that which it included
לטמא טמאת נבלות to impart the tumah of neveilas (i.e. these different rules result from Scriptural sources).
רבי יהודה אומר Rabbi Yehuda says
האלל המכנס the shreds that were gathered (showing that the gatherer gives these shreds some value and significance apart from the hide)
אם יש כזית במקום אחד if there is an olive's volume in one spot
חיב עליו he is liable for it [for tumah of neveilas].

פרק ט משנה ב

אלו שעורותיהן כבשרן The hide of these is as their flesh [for the purpose of imparting tumah]:
עור האדם human skin (as one is born with it, and it does not regenerate, it is as significant as bone; however there is an opinion in the gemara that since it does re-grow to some extent, this rule is only rabbinical),
ועור חזיר של ישוב and the skin of a domestic pig (which is soft and edible),
רבי יוסי אומר Rabbi Yosi says
אף עור חזיר הבר also the skin of a wild pig,
ועור חטוטרת של גמל הרכה and the skin of the hump of a tender camel (which is soft and edible),
ועור הראש של עגל הרך and the skin of the head of a tender calf [that is unweaned],
ועור הפרסות and the skin of the hooves (in the gemara, Rav takes this literally, while R' Chanina says it refers to the skin of the animals lower limb, at the cannon bone),
ועור בית הבשת and the skin of the pudenda,
ועור השליל and the skin of a fetus,
ועור שתחת האליה and the skin that is under the fat-tail (where there is no hair and it is tender),
ועור האנקה and the skin of the hedgehog (Prof. Jastrow translates this is as a species of lizard)
והכח and the chameleon,
והלטאה and the lizard
והחמט and the snail (the last four species are of the eight species listed in the Torah that impart the tumah of a sheretz).
רבי יהודה אומר Rabbi Yehuda says
הלטאה כחלדה the lizard is like the weasel (in that its skin is not considered soft enough to have the staus of flesh which can convey tumah).
וכולן שעבדן And all of them, when tanned
או שהלך בהן כדי עבודה or trodden on in order to work them (which constitutes partial tanning)
טהורין are tahor
חוץ מעור האדם except for human skin.
רבי יוחנן בן נורי אומר Rabbi Yochanan ben Nuri says
שמונה שרצים The eight sheratzim (listed in the Torah as imparting the tumah of a sheretz)
יש להן עורות have skins (i.e. none of their skins impart tumah like their flesh).