National Library of Medicine (2009). "Chapter 5 - Semantic Networks". UMLS Reference Manual. Bethesda, MD: U.S. National Library of Medicine, National Institutes of Health.
Browne AC, McCray AT, Srinivasan S (June 2000). The Specialist Lexicon(PDF). Bethesda, MD: Lister Hill National Center for Biomedical Communications, National Library of Medicine. p. 1.