Chullin, Chapter 10, Mishnah 4. This chapter is devoted to the matanot, the three gifts that an owner must hand over to a kohen when he slaughters an animal for ordinary, non-sacred use: the zero'a (foreleg), the lechayayim (jaw) and the keivah (stomach). Our mishnah begins with a question of timing: what about a person who became Jewish while already owning livestock? Is the animal he brought with him into Judaism subject to the gifts?
The convert and his cow
"Ger shenitgayer vehayeta lo parah": someone from the nations entered the covenant, and a cow already belonged to him when that happened. Does such an animal carry the obligation of the three gifts?
Everything turns on when the shechitah took place. If the knife was drawn while he was still a gentile, "nishchetah ad shelo nitgayer", the ruling is "patur", and no gift is owed. If the slaughtering came only after he entered the covenant, "mishenitgayer", he is "chayav", and the matanot must be handed over.
The principle behind both rulings is one and the same. The obligation of matanot is generated at the instant of slaughter, not at the moment of purchase or ownership. Therefore the only question worth asking is what his status was when the knife did its work. Jewish at that moment: obligated. Gentile at that moment: exempt.
When the timing is in doubt
"Safek, patur." If we cannot establish whether the animal was slaughtered before or after the conversion took effect, the convert is exempt.
The reason is a general rule of monetary law: "shehamotzi mechavero alav hare'ayah," one who wishes to extract property from his fellow bears the burden of proof. In any machloket over money, possession is decisive until the claimant proves his case. Here the gifts are still in the convert's hands, and it is the kohen who seeks to take them from him. As long as the kohen cannot demonstrate that the slaughter took place after the conversion, the convert keeps what he holds.
Which part of the leg is the zero'a?
Among the three gifts is the zero'a, the foreleg, and it is the right one that goes to the kohen. But a foreleg is not one continuous piece of bone. It is built of three segments: a lower one adjoining the hoof, a middle one above it, and a third that runs up to meet the shoulder blade. The mishnah now marks off precisely how much of that limb the kohen takes.
"Eizehu hazero'a?" Which portion earns the name zero'a? The answer is given by two landmarks: "min haperek shel arkubah", starting at the joint that divides the lowest segment from the one above it, "ad kaf shel yad", ending where the topmost segment meets the shoulder blade. What the kohen receives, then, is the middle segment together with the uppermost one; the lowest segment remains with the owner.
The zero'a of the nazir and the shok of the shelamim
"Vehu shel nazir." This same section, middle bone plus upper bone, is the zero'a that the kohanim receive from the shelamim offering brought by a nazir.
"Uk'negdo baregel, shok." The matching portion of the right hind leg, once again the middle and upper bones, bears the name shok, the thigh, and the kohanim receive it from every shelamim offering.
"Rabbi Yehudah omer": he draws the boundaries of the shok elsewhere. It stretches, in his words, "min haperek shel arkubah", from the joint above the lowest bone, and reaches only "ad sovech shel regel", the knee. By his reckoning, what the kohanim take from an ordinary shelamim is the middle bone alone.
Which part is the lechi?
The mishnah closes by marking off the jaw that belongs among the matanot. "Eizehu lechi?" Its limits are two: "min haperek shel lechi", the joint where the jawbone meets the temple, and from there downward "ad pika shel gargeret", the cartilage at the head of the trachea. Picture a line stretched between those two points: whatever lies in front of it, toward the animal's snout, is the kohen's portion.