  If $C$ is a spherical fusion category, the string-net construction associates
to each closed oriented surface $\Sigma$ the vector space $Z_\text{SN}(\Sigma)$
of linear combinations of $C$-labelled graphs on $\Sigma$ modulo local
relations, in a way which is functorial with respect to orientation-preserving
diffeomorphisms of surfaces. We show how to extend this assignment to a
3-dimensional topological quantum field theory (TQFT), by defining how the
surgery generators in Juh\'{a}sz' presentation of the oriented 3-dimensional
bordism category act on the string-net vector spaces. We show that the
resulting TQFT, which is formulated completely in the two-dimensional graphical
language of string-nets, is an alternative description of the Turaev-Viro state
sum model.
