15 “Between the shoulders,” however. This is indeed a part of the body, it is a part about the region of the heart, at the hinder parts however, that is, at the back: which part of that dove silvered he says is “in the greenness of gold,” that is, in the vigour of wisdom, which vigour I think cannot be better understood than by love. But why on the back, and not on the breast? Although I wonder in what sense this word is put in another Psalm, where there is said, “Between His shoulders He shall overshadow you, and under His wings you shall hope:” forasmuch as under wings there cannot be overshadowed anything but what shall be under the breast.
And in Latin, indeed, “between the shoulders,” perchance in some degree of both parts may be understood, both before and behind, that we may take shoulders to be the parts which have the head between them; and in Hebrew perchance the word is ambiguous, which may in this manner also be understood: but the word that is in the Greek, μετ·φρενα, signifies not anything but at the back, which is “between the shoulders.” Is there for this reason there the greenness of gold, that is, wisdom and love, because in that place there are in a manner the roots of the wings?
Or because in that place is carried that light burden? For what are even the wings themselves, but the two commandments of love, whereon hangs the whole Law and the Prophets? what is that same light burden, but that same love which in these two commandments is fulfilled? For whatever thing is difficult in a commandment, is a light thing to a lover. Nor on any other account is rightly understood the saying, “My burden is light,” but because He gives the Holy Spirit, whereby love is shed abroad in our hearts, in order that in love we may do freely that which he that does in fear does slavishly; nor is he a lover of what is right, when he would prefer, if so be it were possible, that what is right should not be commanded.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)