An observational distances model has been determined based on the observations at the Instituto Argentino de Radioastronomia. The results are shown in diagrams that relate the sun relative radial velocity with the heliocentric distance in the range of galactic longitudes between 297° and 342° at constant intervals of 3° ( b = 0 ). Typical examples of these are given in Figure 1. From the comparison of this model curves and the ones of the homogeneous mass distribution, it is possible to determine the number of more important structures in the fourth quadrant (v < 0 ) and its tangential points. Those galactic longitudes has been compared with those from the continuum. There is a good agreement between them ( Table I). The galactic structure in the fourth quadrant was studied applying this model. There were found 26 concentrations of neutral hydrogen that are shown in (l, b ) diagrams in Figures 4. Since, for negative velocities it is not possible to give a single value of the distance for each concentration, a comparison was made with optical objects, Weaver’s predictions for the occidental galactic hemisphere and Lin’s model. The conclusions are shown in Figures 7 and 8.Asociación Argentina de Astronomí
