Menachos, Chapter 4, Mishnayos 4-5. These mishnayos deal with the chavitei kohen gadol - the minchas ha'chavitin that the Kohen Gadol brings every day.
What is the minchas ha'chavitin:
How it is made - it is prepared essentially on a griddle (machavas), and that is where its name comes from.
Its measure - a standard issaron of fine flour, except that it is richer: three lugin of olive oil, and frankincense in addition.
Its form - it is baked into twelve loaves.
Its time - half is offered in the morning and half in the afternoon.
Who pays for it - the Kohen Gadol pays for it out of his own pocket.
How the minchah is divided into two halves - a dispute among the Rishonim:
According to most of the commentators (the Riva, the Rash and others) - twelve loaves are baked, six of which are offered in the morning and six in the afternoon.
According to the Rambam - twelve loaves are baked in the morning, and each one of them is divided in two; the twelve halves are offered in the morning, and the remaining twelve halves in the afternoon.
From here on the mishnah discusses the details that arise from this arrangement.
The mishnah opens: "Chavitei kohen gadol, einan ba'os chatza'im" - the Kohen Gadol may not bring half of the recipe in the morning and half of it in the afternoon. According to the straightforward understanding, he cannot bake six loaves in the morning and bake another six later on.
"Ela meivi isaron shalem" - he brings a full issaron, which is a measure of 43.2 eggs, roughly a gallon of flour. "V'chotzehu" - after preparing it he divides it in two. "U'makriv mechetzah b'shacharis" - half of the recipe, which is six loaves according to most opinions, or twelve halves according to the Rambam, "u'mechetzah bein ha'arbayim" - and the second half is offered in the afternoon. That is how it is supposed to be done.
A Kohen Gadol who died and another was appointed in his place:
What is the halachah if the Kohen Gadol brought the full issaron, prepared it as chavitin, six of the loaves were offered in the morning - and then he died? If a new Kohen Gadol is appointed that same day, what must he bring: does he complete the half that remains from the first one, or does he bring a new minchah - a full one or a half one?
The mishnah says: "V'kohen shehikriv mechetzah b'shacharis u'meis, u'minu kohen acher tachtav" - the Kohen Gadol died, half of his chavitin had not yet been offered, and they managed to appoint a new Kohen Gadol that same day. "Lo yavi chatzi isaron mibeiso" - the new kohen does not bring half an issaron and prepare half a minchah from it, which would be six loaves; and neither does he bring "v'chatzi isaron shel rishon" - he does not offer the remaining half of the first kohen's loaves, since the first one is no longer the Kohen Gadol, for he has passed away, and now a new Kohen Gadol must be brought.
"Ela meivi isaron shalem v'chotzehu u'makriv mechetzah" - the second Kohen Gadol brings a full issaron, makes complete chavitin from it, divides them in half and offers the one half. "U'mechetzah aveid" - the second half goes to waste, since there is no way to offer it: it becomes disqualified through linah (being left overnight), and the next day it is burned in the beis ha'sereifah.
The Mishnah then summarizes the whole picture: "Nimtze'u shnei chatza'im kereivim u'shnei chatza'im ovdin" - two halves were offered: the first half in the morning from the first Kohen Gadol, and the second half in the afternoon from the second Kohen Gadol. And two halves were lost: the remaining half of the first and the remaining half of the second, which become invalid by being left overnight and are burned in the place designated for burning.
No Other Kohen Gadol Was Appointed - From Whose Funds Is It Brought?
Until now we have been dealing with a case in which an additional Kohen Gadol was appointed in time for the afternoon offering. What is the law when the Kohen Gadol died after the morning chavitin were offered, before the afternoon chavitin, and no new Kohen Gadol was appointed? The verses prove that the chavitin must still be offered in the afternoon, but there is no Kohen Gadol from whom they can be brought, and they are not brought from the Kohen who died. The question, then, is who pays for them and where the money for purchasing them comes from.
"Mishel mi hayesah kereivah?" - from whose pocket does the money come to purchase these afternoon chavitin? The Tannaim disagree about this:
"Rabbi Shimon omer: mishel tzibur" - and this is derived from a verse teaching that the minchah must come from the community. Every year each member of Israel gives a half shekel; the contributions are collected from all over the Jewish world, from Eretz Yisrael and from the diaspora, brought to Jerusalem and stored in those shofaros - large chests - and used to purchase the various communal offerings. From these public funds the minchas chavitin is purchased on behalf of all of Israel at a time when there is no Kohen Gadol.
"Rabbi Yehudah omer: mishel yorshin" - the chavitin come from the money of the heirs of the Kohen Gadol who died, and it is they who must pay for preparing new chavitin. This opinion too is based on the exposition of verses, and there are two such expositions.
The Bartenura rules like Rabbi Yehudah, that the chavitin come from the funds of the heirs of the Kohen Gadol who died; and some say otherwise.
In any case, "u'sheleimah hayesah kereivah" - it is offered as a complete minchah. That is, as long as there is no Kohen Gadol to bring the chavitin, whether they come from public funds or from the funds of the heirs, complete chavitin are brought - twelve loaves from a full issaron of fine flour - twice a day. In the afternoon they bring twelve complete loaves from a full issaron, and likewise the next morning and afternoon, and so on each day until a new Kohen Gadol is appointed: a full issaron in the morning, and another full issaron, twelve loaves, in the afternoon.