Sefer HaBahir 85

1"[The students asked,] ""And what is Nun [in the word ozen]?""[Rabbi Rachumai responded,] ""To teach you that the brain is the essence of spinal cord, and from there, he always draws [שואב].* If it were not for the spinal cord, the brain would not exist, and without the brain the body could not exist. All of the body is for the sake of the brain, and if the whole body does not exist, the brain cannot exist. Therefore, the spinal cord pours out to the whole body from the brain. And this is the bent Nun. But this Nun[, the one in ozen,] is straight [i.e., the final Nun]. The straight Nun is always in the completion of a word, to teach you that a straight Nun contains the bent and the straight. But the bent Yesod, to teach you that the straight Nun includes from masculine and feminine.""