The Holy Floor
In order to properly perform the sacred service in the Beit Hamikdash a kohen had to have his feet on the floor that had been sanctified to serve as the platform for this service. There could not be anything between his feet and the floor, not even the foot of another kohen.
Who sanctified the floor of the Beit Hamikdash?
Our natural inclination would be to suggest that the same King Shlomo who built the Beit Hamikdash did this. Our gemara, however, clearly identifies his father David as the one who invested this sanctity in the Beit Hamikdash floor.
Although G-d did not allow him to build the Beit Hamikdash, we do find that David was intensely involved in preparing its construction by his son. It was David, together with the Prophet Shmuel, who determined the exact location where the Beit Hamikdash was to be built (Zevachim 54b). It was he who purchased that site from Aravnah the Jebusite in order to establish there an altar and offer sacrifices, which brought an end to the plague which had afflicted the land (Shmuel II 24:18-25), and it was he who dug the ducts that carried the libations to the very foundations of the universe.
The Midrash (Pesikta Rabba 43) relates that when David went to establish this altar he found the altar upon which Adam and Noach had offered sacrifices and upon which Avraham had prepared to sacrifice Yitzchak. Upon finding it he began to measure distances from it, determining exactly where the Azarah Courtyard would be, where the sacrificial services would be performed and where the Holy and Holy of Holies sections would be.
It may be that it was during this measuring that David sanctified the floor of the Beit Hamikdash that his son Shlomo would build. According to Rashi, this was achieved with two thanksgiving sacrifices, and according to Tosefot with the remnants of the flour offering.
All or Nothing at All
How much of the kohen must be inside the Beit Hamikdash courtyard when he performs the slaughtering of the animal as a sacrifice or receives its blood?
This question was posed to Rabbi Zeira by Rabbi Yirmiyah who asked if it mattered that only the hair of the kohen was not within this sacred area designated for all of the sacrificial services.
Rabbi Zeiras response was a reference to the passage (Shmot 28:43) which speaks of the need for kohanim to be dressed in their priestly garments "upon entering" the Sanctuary. This term is interpreted as indicating a need for a "total entry", including even the hair of the kohen.
The commentaries draw a parallel between this interpretation of the term "upon entering" with a similar phrase used by the Torah in regard to the mitzvah of separating challa from dough and giving it to a kohen. This obligation by Torah Law was limited to the grain of Eretz Yisrael and it was incumbent on Jews "upon your entering into the land" (Bamidbar 15:18). Although this meant that the obligation began even before they finished conquering and dividing the land, it also indicated that it depended on the entry of the entire people, not just a few advance scouts (Mesechta Ketubot 25a). Once again we see the term "upon entering" understood as total with no reservation.
While the sole ramification for sacrifices of this insistence on totality was the need for the kohen to be completely inside the Sanctuary, the ramifications of the phrase for challa is relevant even today. Since the Torah made the obligation conditional on all Jews being in Eretz Yisrael, this is not the case today, nor was it even when Ezra led the return to the land from Babylonian exile. Our obligation to separate challa ever since our ancestors went into exile is only of rabbinic nature. It is interesting to note that in regard to challa there is also a rabbinic requirement to perform the mitzvah outside of Eretz Yisrael. Our Sages saw a need to institute this decree so that the mitzvah of challa would not be forgotten, since every Jewish household is involved, in contrast with the need for tithing terumot and maasrot which involved only agriculturists.