MEDLINE is the U.S. National Library of Medicine's premier bibliographic database, containing approximately 12 million references to journal articles in life sciences with a concentration on biomedicine. Articles cover topics in medicine, nursing, dentistry, veterinary medicine, the health care system, pre-clinical sciences, and much more. Medline uses MeSH (Medical Subject Headings) indexing with tree, tree hierarchy, subheadings and explosion capabilities to search citations from over 4,800 current biomedical journals.