  We show that the equations of motion governing the dynamics of strings in a
compact internal space can be written as dispersion relations, with a local
speed that depends on the velocity and curvature of the string in the large
dimensions. From a $(3+1)$-dimensional perspective these can be viewed as
dispersion relations for waves propagating in the string interior and are
analogous to those for current-carrying topological defects. This allows us to
construct a unified framework with which to study and interpret the internal
structure of various field-theoretic and fundamental string species, in a
simple physically intuitive coordinate system, without the need for dimensional
reduction or approximate effective actions. This, in turn, allows us to
identify the precise conditions under which higher-dimensional strings and
current-carrying defects are observationally indistinguishable, for macroscopic
observers. Our approach naturally incorporates the description of so-called
`cosmic springs', whose dynamics are expressed in terms of an effective Finsler
geometry, for circular loops, or generalised Finsler geometry, for non-circular
configurations. This demonstrates the importance of these novel geometric
structures and their utility in modelling complex physical phenomena in
cosmology and astrophysics.
