Regarding the features of this galaxy:
in "Starfog," Earth lies in the spiral arm behind the one containing the interstellar civilization served by the Commonalty, an organization covering an estimated ten million planets;
the human race has occupied two or three spiral arms in total;
the colonized planet Serieve is near the northern edge of the Commonalty's spiral arm;
thus, beyond Serieve is only the galactic halo containing thin gas with little dust and old, widely scattered, globular clusters.
In two different works by James Blish, it seems that random stellar movements have opened a galactic chasm or valley called "the Rift" which is so immense that it is difficult or impossible to traverse it even with the existing faster than light drive. But random movements do not work like that, do they? In conversation, Blish said that he probably had in mind something like the space between two of the spiral arms.
In The Quincunx Of Time (New York, 1983), Blish describes a color-coded photograph, taken from at least ten million light years distance, of the Milky Way and the Magellanic Clouds. A southern area of the disc and the Clouds are colored red and the north is green while gray wanders between them and spreads to the west. The colors are political and need not concern us here but I find it interesting to read of the points of the compass being applied to the galaxy. Earth, in this period called "High Earth," is "...far out on a spiral arm in the red area, near the Clouds..." (p. 95).
In Blish's Mission To The Heart Stars (London, 1980), Jack Loftus views "'...a galactic model or map...'" (pp. 31-32) that appears initially as a glowing sphere, each evenly distributed point of light within it representing a three-hundred-light-years-diametered globular cluster of about a hundred thousand Population One stars. Secondly, the map adds a line representing the galactic equator; thirdly, a central sphere, occupying a fifth of the total and containing individual Population One stars; fourthly, the Heart Stars (a political term), a spindle spreading out along the equator; fifthly, all the remaining Population Two stars, which transform the spindle into a lens stretching across the larger sphere along the equator; sixthly, an equatorial band representing remaining dust from which more Population Two stars may condense.
Rivers of dust run from the rim between the spiral arms towards the Heart Stars. In fact, according to this text, the dark areas between the arms are not empty but are places where the dust conceals the stars. Thus, when that dust has condensed into stars, the Milky Way will join the class of elliptical galaxies.
Poul Anderson's World Without Stars is set in a planetary system in intergalactic space. In Cities In Flight (London, 1981), Blish discloses that it was discovered in 1953 that "...tenuous bridges of stars...connected the galaxies like umbilical chords..." (p. 577). This is mentioned only because at that point in the narrative a planet is flying through intergalactic space. Blish hoped to find a way to incorporate the bridges of stars into an sf story.