The poem "An Elementary School Classroom in a Slum" by Stephen Spender is given below
Far far from gusty waves these children's faces.
Like rootless weeds, the hair torn round their pallor:
The tall girl with her weighed-down head. The paper-
seeming boy, with rat's eyes. The stunted, unlucky heir
Of twisted bones, reciting a father's gnarled disease,
His lesson, from his desk. At back of the dim class
One unnoted, sweet and young. His eyes live in a dream
Of squirrel's game, in tree room, other than this.
On sour cream walls, donations. Shakespeare's head,
Cloudless at dawn, civilized dome riding all cities.
Belled, flowery, Tyrolese valley. Open-handed map
Awarding the world its world. And yet, for these
Children, these windows, not this map, their world,
Where all their future's painted with a fog,
A narrow street sealed in with a lead sky
Far far from rivers, capes, and stars of words.
Surely, Shakespeare is wicked, the map a bad example.
With ships and sun and love tempting them to steal —
For lives that slyly turn in their cramped holes
From fog to endless night? On their slag heap, these children
Wear skins peeped through by bones and spectacles of steel
With mended glass, like bottle bits on stones.
All of their time and space are foggy slum.
So blot their maps with slums as big as doom.
Unless, governor, inspector, visitor,
This map becomes their window and these windows
That shut upon their lives like catacombs,
Break O break open till they break the town
And show the children to green fields, and make their world
Run azure on gold sands, and let their tongues
Run naked into books the white and green leaves open
History theirs whose language is the sun.
I do not understand what the following lines mean
"whose language is the sun"
"civilized dome riding all cities."
"Break O break open till they break the town"
Please help me understand the meanings of the above three lines.