From the National Library of Medicine:
Following the signing of a 2014 memorandum of understanding (MOU) to make thousands of complete back issues of historically-significant biomedical journals freely available through the National Institutes of Health life sciences repository PubMed Central (PMC), and through its European counterpart, Europe PMC, the US National Library of Medicine and the Wellcome Trust have released multiple titles—encompassing more than 45,000 individual articles and nearly a half million pages—with more to come.
The NLM/Wellcome collaboration to make these titles and others publicly available builds on the Medical Journal Backfiles Digitization Project of 2004-2010) and contributes to the current PMC archive of over 4 million articles from medical journals. The content from these titles is free to read, and also to download, text mine, and re-use via the PMC Text Mining Collections, which includes the PMC Open Access Subset and the Historical OCR Collection. License terms vary by title; see PMC Back Issue Digitization for more details.