  Differential equations on metric graphs model disparate phenomena, including
electron localisation in semiconductors, low-energy states of organic
molecules, random laser networks, pollution diffusion in cities, dense neuronal
networks and vasculature. This article describes the continuum limit of the
edgewise Laplace operator on metric graphs, where vertices fill a given space
densely, and the edge lengths shrink to zero (e.g. a spider web filling in a
unit disc). We derive a new, coarse-grained partial differential operator which
depends on the embedding space and local graph structure and has interesting
similarities and differences with the Riemannian Laplace-Beltrami operator. We
highlight various subtleties of dense metric graph systems with several semi-
and fully analytic examples.
