PubMed provides access to: older references from the print version of Index Medicus, back to 1951 and earlier, references to some journals before they were indexed in Index Medicus and MEDLINE, for instance, Science, BMJ, and Annals of Surgery, very recent entries to records for an article before it is indexed with Medical Subject Headings (MeSH) and added to MEDLINE, a collection of books available full-text and other subsets of NLM records, PMC citations, NCBI Bookshelf, Many PubMed records contain links to full-text articles, some of which are freely available, often in PubMed Central and local mirrors such as UK PubMed Central. Information about the journals indexed in MEDLINE, and available through PubMed, is found in the NLM Catalog.