Our galaxy is an extremely complex object, and it is not only changing through time, but evolving as well. There are changing chemical gradients, acoustic and shock waves in the interstellar medium, gravitationally coupled orbits driving density waves to create spiral arms which precipitate collapse of molecular clouds and provoke stellar genesis, non-Keplerian rotation, different evolving stellar populations, a complex porous foamy structure to the interstellar medium caused by supernovae detonations and stellar winds, fountains of gas erupting out of the disc, galactic magnetic and radiation fields, and a complex family of globular and open clusters with fascinating histories of their own. And at the center of it all, a million solar mass black hole. And we are just now learning about dark matter and it's effects. If the universe consisted of the Milky Way alone, it would still be an extravagantly outrageous place.

I highly recommend the standard textbook in galactic structure, Binney and Merrifield's "Galactic Astronomy", (1998). It's probably about due for a 2nd edition.