as the external pressure of the intergalactic medium [@SilichTenorio-Tagle01] may also play an important role.

To summarize, the present results show that gravity (in particular, the self-gravity of the gas) should not be neglected in studies of the structure of the atomic and molecular layers of a photoionized region, especially when one is interested in their total extent.

Concerning the specific problem of the vertical structure of the Milky Way ISM, the models discussed here are able to reproduce the observed distribution of atomic and molecular Hydrogen with an accuracy of about a factor of two, but the amount of diffuse ionized gas is severely underestimated at all heights. Much more realistic results could be obtained by providing support for distributed radiation sources, as well as a treatment for the coexistence of different gas phases at the same point.

Acknowledgments {#acknowledgments .unnumbered}
===============

We would like to thank G. Ferland and the [Cloudy]{} team for developing, maintaining, and making the software publicly available. It is also a pleasure to acknowledge useful comments from G. Tenorio-Tagle and E. Pérez on a preliminary version of this manuscript. Implementation in the development version of [Cloudy]{} would not be possible without the help from 