  The fusion hindrance in heavy ion collisions is studied in the framework of
the two-center liquid drop model. It appears that the neck and the radial
degrees of freedom might both be hampered by an inner potential barrier on
their path between the contact configuration to the compound nucleus. Heavy ion
reactions with and without the two kinds of fusion hindrance are classified
through systematic calculations. It is found that the number of reactions
without radial fusion hindrance is much smaller than that without neck fusion
hindrance, and for both kinds of fusion hindrance the number of reactions
without fusion hindrance at small mass-asymmetry parameter $\alpha$ is smaller
than that at large $\alpha$. In the formation of a given compound nucleus, if a
reaction with $\alpha_c$ is not hindered, then other reactions with $\alpha >
\alpha_c$ are also not hindered as it is well known experimentally.
