It's obviously thin in direction perpendicular the galactic plane, otherwise it would cover the whole sky, not just a strip. We can count objects and map them on the sky.
More distant arms are hard to trace.
But we note that the arms are the site of star-formation, because hot O and B stars and HII regions are concentrated in the arms. Contains the globular clusters. It is likely that this is a supermassive black hole . The brightest stars in the halo are the red giants in the globular clusters. These are old objects and there isn't much star formation taking place nowadays there.