"MeSH is the National Library of Medicine's controlled vocabulary thesaurus. It consists of sets of terms naming descriptors in a hierarchical structure that permits searching at various levels of specificity. MeSH descriptors are arranged in both an alphabetic and a hierarchical structure. At the most general level of the hierarchical structure are very broad headings such as 'Anatomy' or 'Mental Disorders'. More specific headings are found at more narrow levels of the twelve-level hierarchy, such as 'Ankle' and 'Conduct Disorder'. There are 26,142 descriptors in 2011 MeSH. There are also over 177,000 entry terms that assist in finding the most appropriate MeSH Heading, for example, 'Vitamin C' is an entry term to 'Ascorbic Acid'. In addition to these headings, there are more than 199,000 headings called Supplementary Concept Records (formerly Supplementary Chemical Records) within a separate thesaurus. The MeSH thesaurus is used by NLM for indexing articles from 5,400 of the world's leading biomedical journals for the MEDLINE®/PubMED® database. It is also used for the NLM-produced database that includes cataloging of books, documents, and audiovisuals acquired by the Library. Each bibliographic reference is associated with a set of MeSH terms that describe the content of the item. Similarly, search queries use MeSH vocabulary to find items on a desired topic."