Menachot 34: The Mezuza, the Tefillin, and the Holiness of the Head

We already learned that like the mezuza's holding of Torah, the tefillin hold representations of the four parshiyot.  Today's Gemara examines the differences between the tefillin worn on the head and those worn on the arm.  The head holds a box with four pieces of parchment.  Each is placed in a separate compartment that together form the box.  Rashi explains that the soft raw hide is placed over a mold with four protrusions that stretch the leather.  The four compartments can be see from the outside of the container because they are pressed together.  A baraita tells us that all four parshiyot are written down on one sheet of parchment in a single compartment.

Can one piece of parchment be exchanged for the other?  Do the tefillin placed on the head stay on the head because of their greater holiness?  Rashi teaches that two of the three letters of the holy name of G-d, shin/dalet/yud are on the head - the shin is seen on the box, the dale is on the knot at the back and the yud is made from a knot at the back of the tefillin. The yud is the only letter made as a knot on the tefillin placed on the arm.  Other rabbis argue that the head is the centre of our intellect and this is holier than the hand.