In Medicine and in Surgery, the convention is similar to that of the Physical Sciences with the most significant contributor being first or last, or with the *owner* of the lab equipment or funding getting senior author position as the last author.

However, there is a curve ball in Medical and Surgical Journals in that the **first three authors** are the ones who gain the most credit.  The reason for this is that back in the pre-WWW-historic era, when I wrote papers that went into Surgical journals and when I went through medical school and surgical residency, the medical journal articles were all indexed in the **Index Medicus**.

The *Index Medicus* was a hard-copy index prepared at the end of each year and found in every medical library with three sets of listings sorted by Medical E-something Subject Headings (MeSH), title of the journal article, and the *last name of the* **first three authors**.  This paper index was how people found journal articles of interest and how the authors gained "publication cred."  I ended up as third author on many papers giving me a lot of cred even above some grad students and post-docs who helped with experiments but had not supervised or designed (or originally proposed some of) the experiments in these papers.