  The local Lorentz group is introduced in flat space-time, where the resulting
Dirac and Yang-Mills equations are found, and then generalized to curved
space-time: if matter is neglected, the Lorentz connection is identified with
the contortion field, while, if matter is taken into account, both the Lorentz
connection and the spinor axial current are illustrated to contribute to the
torsion of space-time.
