  A full linear stability of a straight scroll wave in an excitable medium is
presented. The five eigenmode branches which correspond to deformation in the
third dimension of the five main modes of two-dimensional (2D) spiral dynamics
are found to play a dominant role. Modulations in the third dimension have
stabilizing or destabilizing effects on the different modes depending on the
parameter regimes. For untwisted scroll waves, our numerical results confirm
the relation between the long-wavelength behavior of the translation branches
and 2D spiral drift in an external field but show no similar direct relation
for the meander branches. The influence of twist on the different branches is
investigated. In particular, the sproing instability is seen to arise from the
twist induced deformation of the translation branches above a threshold twist.
