What Does It Mean To Think in a Marrow Bone?

In one of my favorite poems, “A Prayer for Old Age,” W.B. Yeats writes:
God guard me from those thoughts men think
In the mind alone;
He that sings a lasting song
Thinks in a marrow-bone.
Here Yeats makes the provocative claim that thinking is not restricted to the mind, and that the wise person is the one who is [...]