  We report calculations of the stellar and gaseous response to a Milky Way
mass distribution model including a spiral pattern with a locus as traced by
K-band observations, over imposed on the axisymmetric components in the plane
of the disk. The stellar study extends calculations from previous work
concerning the self-consistency of the pattern. The stellar response to the
imposed spiral mass is studied via computations of the central family of
periodic and nearby orbits as a function of the pattern rotation speed,
$\Omega_p$, among other parameters. A fine grid of values of $\Omega_p$ was
explored ranging from 12 to 25 $km s^{-1} kpc^{-1}$. Dynamical self-consistency
is highly sensitive to $\Omega_p$, with the best fit appearing at 20 $km s^{-1}
kpc^{-1}$. We give an account of recent independent pieces of theoretical and
observational work that are dependent on the value of $\Omega_p$, all of which
are consistent with the value found here; the recent star formation history of
the Milky Way, local inferences of cosmic ray flux variations and Galactic
abundance patterns. The gaseous response, which is also a function of
$\Omega_p$, was calculated via 2D hydrodynamic simulations with the ZEUS code.
For $\Omega_p = 20 km s^{-1} kpc^{-1}$, the response to a two-armed pattern is
a structured pattern of 4 arms, with bifurcations along the arms and interarm
features. The pattern resembles qualitatively the optical arms observed in our
Galaxy and other galaxies. The complex gaseous pattern appears to be linked to
resonances in stellar orbits. Among these, the 4:1 resonance plays an important
role, as it also determines the extent over which the imposed spiral pattern is
dynamically self-consistent.
