  BF theory is a topological theory that can be seen as a natural
generalization of 3-dimensional gravity to arbitrary dimensions. Here we show
that the coupling to point particles that is natural in three dimensions
generalizes in a direct way to BF theory in d dimensions coupled to
(d-3)-branes. In the resulting model, the connection is flat except along the
membrane world-sheet, where it has a conical singularity whose strength is
proportional to the membrane tension. As a step towards canonically quantizing
these models, we show that a basis of kinematical states is given by `membrane
spin networks', which are spin networks equipped with extra data where their
edges end on a brane.
