Just how many scholarly articles are posted on the web? An article published in open access journal PLOS ONE (http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0093949) says that there's at least 114 million English language articles out there. The research shows that Google Scholar had less than 100 million articles at the time of writing and that only 27 million (24%) are freely available.
So if you want to ensure that your literature searching is as comprehensible as possible, the advice is to run your searches in as many databases as possible. Databases available to HYMS researchers are listed at http://libguides.hull.ac.uk/medicine/FindArticles.