출애굽기 26:28 해설: Chizkuni, Rabbeinu Bahya & Rav Hirsch

בתוך הקרשים, within the planks. Seeing that the other bolts were fastened to the golden outside of the planks and therefore were visible to the eye, the centre bolt which was fastened to the wood underneath the golden overlay, and therefore invisible to the naked eye, was described as “inside the planks.”

והבריח התיכון בתוך הקרשים מן הקצה אל הקצה, “and the middle bar inside the beams shall extend from end to end.” Rashi explains on this verse that five of the bars were on the northern wall, five on the southern wall, and five on the western wall. These “five” were in reality only three, as the upper and lower bars respectively consisted of two bars each (on the same level), whereas the middle one was a single bar joining all the beams on its respective side. It was thirty cubits long. This is the meaning of the words: “from one end to the other end.” The western wall’s middle bar then was 12 cubits long, from east to west. [Our version of Rashi differs from that quoted by the author. Ed.]
According to Shabbat 98 the middle bar stood miraculously. Tosafot on that folio say that the middle bar extended through all three directions, i.e. one bar which made two 90 degree turns and must have been 70 cubits long. The two corner beams which overlapped detracted from what otherwise would have been 72 cubits.

It is well to remember that this middle bar which joined every one of the beams to one another in the manner of a belt symbolized the קו האמצעי, the “center line” we have often mentioned as part of the diagram of the ten emanations. In this instance it stabilized and supported all the extremities of the Tabernacle providing input from earth into the celestial Sanctuary by symbolizing the spiritual force generated on earth by Yaakov and his 70 family members (descendants) who emigrated to Egypt. This is why it was Yaakov who was chosen (by history under G’d’s guidance) to be the one to bring all these people down to Egypt. In fact we have a Midrash Tanchuma (Terumah 9) according to which the בריח התיכון this “middle bar” descended to Egypt with Yaakov at the time. He told his sons that when the time would come to leave Egypt they should make sure to take it with them.