The Morphology of Hydrogen and of Other Tracers in the Galaxy

It has recently become clear that, among the constituents of the interstellar medium, atomic hydrogen is unique in its distribution. In particular, the galactic disk as defined by atomic hydrogen has a diameter fully twice as large as that defined by the ionized and molecular states of hydrogen, as well as by other molecules, supernova remnants, pulsars, ..gamma..-radiation, and synchrotron radiation. The focus of this review is on the relative overall morphological characteristics of those constituents of the disk which can be observed along transgalactic paths. 13 figs., 153 refs. (GHT)