Tosefta su Levitico 7:33

A Kohen who takes upon himself all the ritual obligations of the priesthood, except for one thing, we do not accept him. A Levite who takes upon himself all the Levitical obligations except for one thing, we do not accept him, as it is written (Lev. 7:33), "he who offers the blood of the peace-offering, and the fat [of the peace-offering] from among the sons of Aaron shall get the right thigh as his portion." [With reference to Kohanim (following Steinsaltz to Hul. 132b:15 to Hul. 133a:2)], I have only derived [that a Kohen does not share in the priestly gifts] only [if he does not believe in the validity of] sprinkling blood and burning of fats [on the altar]. How do we know [that they are also obligated in the rites of] the pouring [of oil onto the minchah-offering], the mixing [the ingredients together], the waving and bringing [the meal offering to the corner of the altar], the removing the handful and burning [on the altar], cutting [the neck of bird-sacrifices], receiving and sprinkling [the blood], the Sotah ceremony, the breaking the red heifer's [neck], and purifying the metzora [= leper], and the lifting the hands [to bless the people] inside and outside [the Temple]? As the Torah teaches (Lev. 7:33) "[among the] sons of Aaron," that is [to say] "all the rites" that are incumbent upon the sons of Aaron. Said Rabbi Shimon, you might think [lit. "it can be"] we don't prohibit them except from partaking in the priestly gifts within the Temple; how do we know [we also prohibit them partaking in priestly gifts given] in the countryside (see Y. Chal. IV.4.17)? As the Torah teaches (Deut. 18:4), "You shall also give him the first fruits of your new grain [and wine and oil, and the first shearing of your sheep]." Because [as it says, Deut. 18:5], "For the LORD your God has chosen him [etc.]"; thus, everyone who takes upon himself the service, he receives the gifts. All those who do not take upon themselves the service, he does not receive the gifts. During the time that the Kohanim do the will of [their Father in heaven, following the GR"A], what it says about them (Lev. 6:10), "I have given it as their portion from My offerings by fire." What is theirs, they take, and they do not take what is mine. And during the time that they do not do His will, what is said about them (Mal. 1:10), "If only you would lock My doors[, and not kindle fire on My altar to no purpose! I take no pleasure in you—said the LORD of Hosts—and I will accept no offering from you.]"

What is the shankbone? It is taken from the socket and all the socket with it. The "shankbone" of the Temple is the "shankbone" of the countryside. What is the "cheek"? It is taken from the place of the slaughter and the entire place of the slaughter with it. The "maw" is as it implies. The fats that are on the maw are given to the Kohen. Said Rabbi Yehuda, the Kohanim used to behave generously, and they would give it [back] to the owners. The "shankbone" is the right shankbone. You may say, is it the right shankbone, or perhaps it is only the left shankbone? [But] behold, you may deduce, since the [breast and] thigh (Lev. 7:33-34) is a gift to the Kohen, and the shankbone is a gift to the Kohen, since the [gift of the breast and] thigh are only applicable to the right, the [gift of the] shankbone also is only applicable to the right. The thigh is from the joint and downward, the words of Rabbi Yehuda, and the Sages say, from the joint and upwards. What is the "breast"? Whatever faces the ground and upwards, until it reaches the neck, [and] downward until it reaches the belly. Two ribs should be cut off and given [to the Kohen] along with it (i.e., the breast). And these are the sacrificial parts [of the animal]: The two kidneys and the lobe of the liver and that which is attached to it. And these are the [sacrificial] fats: "The fat that covers the innards and all the fat that surrounds the innards" (Lev. 3:3, Fox tr.), those are the fats. [The fats] surrounding the rumen and all the fats that are on the small intestine, the words of Rabbi Yishmael. Rabbi Yishmael says, the fats that are on the maw, and the Sages did not agree with him. [With regard to the fat] that is on the loins between the sinews, one scrapes around it in every place that it is found and takes it away, the words of Rabbi Yosei HaGalili. Rabbi Yishmael says, the fat that is on the maw, which is spread out [and possesses] a membrane, and is [easily] peeled off, is forbidden. Rabbi Yehuda says, the membrane that is on the kidney is forbidden, and one is liable [for eating] it. The sinews that are in the kidneys and in [other] offerings are forbidden, but [bedi'eved] one is not liable for them.