  The probability content of a convex polyhedron with a multivariate normal
distribution can be regarded as a real analytic function. We give a system of
linear partial differential equations with polynomial coefficients for the
function and show that the system induces a holonomic module. The rank of the
holonomic module is equal to the number of nonempty faces of the convex
polyhedron, and we provide an explicit Pfaffian equation (an integrable
connection) that is associated with the holonomic module. These are
generalizations of results for the Schl\"afli function that were given by
Aomoto.
