  A general continuum theory for the distribution of hairs in a bundle is
developed, treating individual fibers as elastic filaments with random
intrinsic curvatures. Applying this formalism to the iconic problem of the
ponytail, the combined effects of bending elasticity, gravity, and
orientational disorder are recast as a differential equation for the envelope
of the bundle, in which the compressibility enters through an 'equation of
state'. From this, we identify the balance of forces in various regions of the
ponytail, extract a remarkably simple equation of state from laboratory
measurements of human ponytails, and relate the pressure to the measured random
curvatures of individual hairs.
