Middos, Chapter 4, Mishnah 2. This chapter is describing the Heichal building, and the previous mishnah dealt with the great gate that served as its entrance. Our mishnah continues with that entrance: the two small doorways that flanked it, and the procedure by which the kohanim unlocked the Heichal each morning.
Two small wickets
Alongside that entrance, the mishnah tells us, stood "shnei pishpashim," a pair of miniature doorways, and they belonged "l'sha'ar hagadol," to the great gate itself. One of them was placed "echad batzafon," on the northern side, and the other "v'echad badarom," on the southern side.
The two were not put to the same use. Concerning the southern one the mishnah records that "lo nichnas bo adam," no human being ever entered through it, "me'olam," not once in all the years the Beit Hamikdash stood. It was simply never unlocked.
The verse in Yechezkel
Why construct a doorway only to seal it permanently? The mishnah answers that the matter is laid out in prophecy: "v'alav hu meforash," concerning this door it is stated explicitly, "al yedei Yechezkel," through Yechezkel. The pasuk records: "Vayomer eilai Hashem: hasha'ar hazeh sagur yihiyeh," this gate shall remain shut, "lo yipatei'ach," it is not to be opened, "v'ish lo yavo vo," and no person shall pass through it, "ki Hashem Elokei Yisrael ba vo, v'haya sagur." The reason for the sealing is the honor of the One who enters there: this is the passage through which Hashem enters His House, and for that very reason it stays locked to everyone else.
Opening the Heichal: the first opinion
The mishnah now turns to the northern wicket, which was very much in use, since it was the route by which the Heichal doors were opened each day.
"V'natal et hamaftei'ach": the kohen charged with opening the building each morning first took up the key, "u'patach et hapishpash," and used it on the northern wicket, which was situated inside the Ulam. "V'nichnas mehatatah": having gone through, he continued westward into one of the cells constructed against the northern flank of the Heichal, "u'mehatatah laheichal," and from that cell a further opening on his left brought him into the Heichal proper. Standing now within, he reached the western, inner pair of doors and swung them open, and afterwards the eastern, outer pair, leaving the main entryway of the Heichal wide open. Such is the account of the Tanna Kamma.
Rabbi Yehudah's route
"Rabbi Yehudah omer, betoch ovyo shel kotel haya mehalech." Rabbi Yehudah pictures it otherwise. Once the kohen had unlocked the northern wicket and stepped into the cell, no such circuit was necessary: a narrow corridor had been built inside the thickness of the wall that divided the Ulam from the Heichal, and along it he walked "ad sheyimtza omed," until he came to a stop "bein shnei she'arim," between the two sets of doors described above, the inner and the outer.
The difference between the two opinions is not merely architectural. Rabbi Yehudah maintains that one may not enter the Heichal without a purpose, and his passageway solves the problem neatly: it runs within the wall, in front of the Heichal, and delivers the kohen directly into the space between the doors without his ever setting foot inside. According to the Tanna Kamma, the kohen walks into the Heichal, crosses it, and comes back out to the doors in order to open them, which means he is treating the Heichal as a corridor and passing through it for no purpose of its own. That is exactly what Rabbi Yehudah is unwilling to accept, and so he presents this alternative route as the way it was actually done.
Which doors first
"U'patach et hachitzoniyot": from that position he opened the outermost pair, "mi bifnim," handling them from where he stood on their inner side, "v'et hapnimiyot mi bachutz," while the inner pair he swung open from the side facing out. With that, the Heichal building was open for the day.