  We use dynamical models that include bulk rotation, velocity dispersion
anisotropy and both stars and dark matter to explore the conditions that give
rise to the early-type galaxy scaling relations referred to as the Fundamental
Plane (FP) and Manifold (FM). The modelled scaling relations generally match
the observed relations and are remarkably robust to all changes allowed within
these models. The empirical relationships can fail beyond the parameter ranges
where they were calibrated and we discuss the nature of those failures. Because
the location of individual models relative to the FP and FM is sensitive to the
adopted physical scaling of the models, unconstrained rescaling produces a much
larger scatter about the scaling relations than that observed. We conclude that
only certain combinations of scaling values, which define the physical radial
and kinematic scale of the model, produce low scatter versions of the FP and
FM. These combinations further result in reproducing a condition observed
previously for galaxies, $r_c \rho_0 = $ constant, where $r_c$ is the scaling
radius and $\rho_0$ is the central density. As such, we conclude that this
empirical finding and global galaxy scaling relations are not independent and
that finding the physical cause of one should lead to the solution to the
other. Although our models are strictly for pressure supported galaxies, these
results may well hold generally because the central density constraint was
first identified in dwarf spheroidals but later extended to rotating giant
galaxies and the FM applies to galaxies of any morphological type and
luminosity class.
