Most searches will show a Mind Map, which can be used to view related topics. Example: Middle Ages
Credo Reference

Topic Finder gives you a visual of related, broader, and narrower topics. (Choose Advanced Search the click Topic Finder.)
Key in a topic. Click a topic in the wheel or tile to narrow the results and view a list of documents containing that topic or term.
Gale Ebooks 

Sample Article:

Amundsen, D. W. (2004). Medical Ethics, History of Europe: I. Ancient and Medieval. B. Early Christianity. In S. G. Post (Ed.), Encyclopedia of Bioethics (3rd ed., Vol. 3, pp. 1562-1568). Macmillan Reference USA.

Selected Reference Titles