  I show how transition systems can be applied to the naturally concurrent
behaviour of excitable media. I consider structured excitable media, in which
excitations are constrained to propagate only in defined narrow channels, and
cannot propagate elsewhere. I define a type of transition system that can be
used to describe the complete set of behaviours exhibited by simple structures.
The composition rules that result from this definition can be used to
automatically deduce the behaviour of more complex structures composed from
simpler structures. Several examples illustrate the method, and a software
implementation is provided.
