In The Illuminated Blake, Erdman goes through the illuminated works of William Blake plate by plate describing what he sees. This is not a commentary on the text, but on the images. But the text illuminates the pictures, just as the pictures illuminate the text.
Let's have a look at Milton, Plate 19, (E 110). Click on image and page down for enlarged view.
Without Erdman's explanation this image at the bottom of the page would be difficult to understand. Says Erdman, "Los shoots his limbs forth 'like the roots of trees' - and we see that he is almost headless...Urizen, nothing but head, peers from the ground and beholds 'the immortal Man'...Milton's task is to annihilate their separation...Looking at Plate 18 we can see that Milton is replacing Urizen's head."
Erdman considers this to be a representation of events in the poem, but also commentary on the politics of England. Milton was a supporter of the Civil War of 1649 which separated England's 'head and body'; "But the naked Milton now confronts the problem of making whole what is already asunder, of resurrecting the Spiritual Body."
So this is another expression of the old task of reconciling the division between Los and Urizen, imagination and reason, spirit and body, as well as the republicans and the monarchists.
Read more about it in Chapter 24 of Erdman's Prophet Against Empire including, "The moral is that Satan must be forgiven or vengeful slaughter will never end."