  A sample of 22 spiral galaxy rotation curves, measured in the 21 cm line of
neutral hydrogen, is considered in the context of Milgrom's modified dynamics
(MOND). Combined with the previous highly selected sample of Begeman et al.
(1990), this comprises the current total sample of galaxies with published (or
available) extended rotation curves and photometric observations of the light
distribution. This is the observational basis of present quantitative
understanding of the discrepancy between the visible mass and classical
dynamical mass in galaxies. It is found that the gravitational force calculated
from the observed distribution of luminous material and gas using the simple
MOND formula can account for the overall shape and amplitude of these 22
rotation curves, and in some cases, the predicted curve agrees with the
observed rotation curve in detail. The fitted rotation curves have, in 13
cases, only one free parameter which is the mass-to-light ratio of the luminous
disk; in nine cases, there is an additional free parameter which is M/L of a
central bulge or light concentration. The values of the global M/L (bulge plus
disk) are reasonable and, when the gas mass is also included, show a scatter
which is consistent with that in the Tully-Fisher relation. The success of the
MOND prescription in predicting the rotation curves in this larger, less
stringently selected sample, lends further support to the idea that dynamics or
gravity is non-Newtonian in the limit of low accelerations and that it is
unnecessary to invoke the presence of large quantities of unseen matter.
