Biological metaphors for the city abound in daily use. You may live close to an “arterial” road or in the “heart” of a metropolis. You may work in one of the city’s “nerve centres” or exercise in a park described as the city’s “lungs”.

Ever noticed how few rivers you can see in most city centres? It’s easy enough to spot the big, usually tamed, main river such as the Thames in London, the Seine in Paris, the Aire in Leeds or the Don in Sheffield. But you will be hard pressed to find any of their tributaries.