  We put forward a hydrodynamic theory of nematic liquid crystals that includes
both anisotropic elasticity and dynamic relaxation. Liquid remodeling is
encompassed through a continuous update of the shear-stress free configuration.
The low-frequency limit of the dynamical theory reproduces the classical
Ericksen-Leslie theory, but it predicts two independent identities between the
six Leslie viscosity coefficients. One replicates Parodi's relation, while the
other-which involves five Leslie viscosities in a nonlinear way-is new. We
discuss its significance, and we test its validity against evidence from
physical experiments, independent theoretical predictions, and
molecular-dynamics simulations.
