  We have probed the Landau levels of nodal quasi-particles by tunneling along
a nodal direction of (110) oriented YBa$_2$Cu$_3$O$_{7-x}$ thin films with a
magnetic field applied perpendicular to the $CuO_2$ planes, and parallel to the
film's surface. In optimally doped films and at low temperature, finite energy
nodal states are clearly observed in films thinner than the London penetration
depth. Above a well defined temperature the order parameter reverts to a pure
\emph{d}-wave symmetry.
