  The differential energy spectrum of cosmic rays exhibits a change of slope,
called "knee" of the spectrum, around the nominal energy of 3 x 10^15 eV, and
individual "knees" for single ions, at different energies. The present work
reports a detailed account of the characteristics and the origin of the knees
for Helium and Iron. Current observational data regarding the magnetic field,
the insterstellar matter density, the size of the Galaxy and the galactic wind,
are incorporated in appropriate algorithms which allow to simulate millions of
cosmic-ion trajectories in the disk. Bundles of ion trajectories define
galactic regions called basins utilized in the present analysis of the knees.
The fundamental role of the nuclear cross sections in the origin of the helium
and iron knees is demonstrated and highlighted.
  The results of the calculation are compared with the experimental data in the
energy interval 10^12 eV - 5 x 10^17 eV. There is a fair agreement between the
computed and measured energy spectra of Helium and Iron; rather surprisingly
their relative intensities are also in accord with those computed here. The
results suggest that acceleration mechanisms in the disk are extraneous to the
origin of the "knees".
