The rapidity distributions of relativistic charged secondaries produced in inelastic Si-28 and C-12 ions at 4.5A GeV/c with nuclear emulsion nuclei have been studied for various effective target as well as beam thicknesses. The variation of particle number density in rapidity space with the number of participating nucleons is also given. With the increase in the participating volume of colliding nuclei the particle density is observed to decrease in the most forward region of the lab rapidity, whereas its value increases in the central and target fragmentation regions. It has been found that the nucleus-nucleus collision may be interpreted as a superposition of nucleon-nucleon collisions.