This TED talk looks at the different metaphors that we use to visualise knowledge.
According to Lima, we used to map knowledge as trees with trunks and branches, but increasingly we are moving towards using the network as the metaphor with many-to-many connections.
Sometimes things are obvious when they are pointed out, and this is one of them. Whilst I love mind-maps, which tend to follow a tree structure, they are so often inadequate for visualising the multitude of connections that exist:
Graham Chastney 