MEDLINE, produced by the National Library of Medicine, is an index to journal articles in medicine, nursing, dentistry, veterinary medicine, the health care system, and the preclinical sciences, including both basic biomedical sciences and clinical practice. It indexes over 4,000 journals. The database contains citations from 1950 to the present with some older citations. An important feature of MEDLINE is the use of Medical Subject Headings (MeSH), a powerful tool for searching specific topics or for comprehensive general searches.