Sometimes massive digitization projects happen and it takes a while to notice. Virginia just tipped off Stacey to the existence of the Medical Heritage Library, http://www.medicalheritage.org/. Founded in 2010, the MHL has quietly grown on Archive.org to contain over 76,000 items, 3,000 of which are volumes out-of-copyright historical American medical journals published from 1797 to 1923.
Unlike the Hathi Trust documents that many of us encounter through OCLC or I-SHARE, the Medical Heritage Library is not currently included in WorldCat. The Medicine in the Americas subcollection is included in the National Library of Medicine Digital Collections, but in general, you need to go to the MHL collections on Archive.org to experience this material. And I strongly encourage you to do so! If you need something nifty for a Facebook post, a cool image for an exhibit, or a patron needs a great historical image for a presentation, there is tons of stuff here that will fit the bill. Everything from 18th century textbooks to videos the Marlboro Man is included.