FYI Index Medicus began in the 19th century and continued through to the early 21st century. During the 1960s the National Library of Medicine began to use a mainframe computer to assist with the production of the Index Medicus -- a print-only publication. I believe that it was an annual cumulative volume with monthly updates. Soon, someone realized that the computer could also be used for an electronic database. This was known as MEDLARS later this became MEDLINE. Access to MEDLARS MEDLINE was only available through subscription and search fees were quite costly. There was a connection fee and fees per character in the search string, per item returned from the query, and (if you wanted more than one or two years of data) tape change charges. Results were available only after several hours or even after a day or two. Only the most skilled librarians could construct a query that returned everything the information seeker wanted and with minimal irrelevant items. Throughout the 1960s to the early 21st century both the electronic and print versions were available. Index Medicus should not be confused with Excerpta Medica, a similar index published by a corporation in Europe (Elsevier, I think). When PubMed became available in 1997, the (now free) database included all of the electronic MEDLARS MEDLINE records. PubMed also began including items from journals that were not included in MEDLINE. Current day PubMed indexes many publications that aren't included in MEDLINE. Knowing this is important because only the MEDLINE journals receive MeSH index terms.
That said, there _were_ journals in the early 20th century that requested manuscripts include references formatted the way that they appeared in the print Index Medicus. Whether that could be called a citation style could be debated.
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Index Medicus is not a citation style. It's an index of journals (which is no longer in use and replaced by the NLM catalog, I believe).
That said, there _were_ journals in the early 20th century that requested manuscripts include references formatted the way that they appeared in the print Index Medicus. Whether that could be called a citation style could be debated.