What is the relationship between the realist and the fantasy sections of Alasdair Gray’s Lanark?

What is the relationship between the realist and the fantasy sections of Alasdair Gray’s Lanark?
Subject is Scottish Literature. Essay must be written by someone with knowledge of Glaswegian author Alasdair Gray, the city of Glasgow, has a familiarity with the Scots language and the essay must include several (approx 5/6) references.