It sends forth its light by way of the hairs of the head... The hairs of the head are the first radiant splendor: this is comparable to the beard, which is the first radiant splendor of the face, as discussed in the previous Opening. It is from the hairs of the head that the light of AV goes out as a second radiant splendor, comparable to the second radiant splendor discussed in the case of the face....through which the brain reveals its contents...
The secret of the hairs, in every place they are found, is that they are connected with what is arranged within the brain inside the head. This is revealed outside, on the head, in the category of “hairs”. (The letters of the Hebrew word sei’ar, a “hair”, are the same as those of shaar, a “gateway” and the root sha’er, “form a mental estimate”.) Everything bound up with the hairs and all of their laws are in accordance with the nature of the brain as it is inside.
This will be explained further in the appropriate place (Opening 105)....while itself remaining concealed. The light of AV that emerges is concealed, since it corresponds to Chochmah, whose nature is to be concealed.