MEDLINE® is the United States National Library of Medicine's (NLM®) premier bibliographic database providing information from the following fields: Medicine, Nursing, Dentistry, Veterinary medicine, Allied health, Pre-clinical sciences. as well as information from international literature on biomedicine, including the following topics as they relate to biomedicine and health care: Biology, Environmental science, Marine biology, Plant and animal science, Biophysics, Chemistry. The database contains bibliographic citations and author abstracts from approx. 5,000 biomedical journals published in the United States and in 70 other countries. MEDLINE contains well over 13 million citations dating back to 1946. NLM uses a controlled vocabulary of biomedical terms to index articles, to catalog books and other holdings. MEDLINE’s controlled-vocabulary thesaurus contains Medical Subject Headings (MeSH®) to describe the subject of each journal article in the database. MeSH terms provide a consistent way of retrieving information that uses different terminology for the same concept. Within MEDLINE’s thesaurus, MeSH terms display hierarchically by category, with more specific terms arranged beneath broader terms.