Keyword searches use natural language and are effective at discovering items that might not be retrieved by a subject search. Experiment with different types of searches or combinations of subjects and keywords.
Keyword searches:
Library of Congress Subject Headings:
Black's Medical Dictionary
The Language of Medicine by 
Stedman's Medical Dictionary for the Health Professions and Nursing
Medical Terminology Made Incredibly Easy! by 
Medical Terminology Simplified by 
A Thesaurus of Medical Word Roots by 
Using Medical Terminology by