  Experimental nuclear moments of inertia at high spins along the yrast line
have been determined systematically and found to differ from the rigid-body
values. The difference is attributed to shell effects and these have been
calculated microscopically. The data and quantal calculations are interpreted
by means of the semiclassical Periodic Orbit Theory. From this new perspective,
features in the moments of inertia as a function of neutron number and spin, as
well as their relation to the shell energies can be understood. Gross shell
effects persist up to the highest angular momenta observed.
