  In two-dimensional space-time, point particles can experience a geometric,
dimension-specific gravity force, which modifies the usual geodesic equation of
motion and provides a link between the cosmological constant and the vacuum
$\theta$-angle. The description of such forces fits naturally into a gauge
theory of gravity based on the extended Poincar\'e group, {\it i.e.\/}
``string-inspired'' dilaton gravity.
