  The transition from the microscopic Heisenberg model to the macroscopic
elastic theory is carried out for the chiral magnetics of MnSi-type with the
B20 crystal structure. Both exchange and Dzyaloshinskii-Moriya (DM)
interactions are taken into account for the first, second, and third magnetic
neighbors. The particular components of the DM vectors of bonds are found,
which are responsible for (i) the global magnetic twist and (ii) the canting
between four different spin sublattices. A possible mechanism for effective
reinforcement of the global magnetic twist is suggested: it is demonstrated
that the components of the DM vectors normal to corresponding interatomic bonds
become very important for the twisting power. The Ruderman-Kittel-Kasuya-Yosida
(RKKY) theory is used for model calculation of exchange parameters. It is found
that just the interplay between the exchange parameters of several magnetic
shells rather than the signs of DM vectors can be responsible for the
concentration-induced reverse of the magnetic chirality recently observed in
the Mn_(1-x)Fe_xGe crystals.
