Q: I want my students to read this article. Can I post a link to it? It's freely available on PubMed Central.
Steps to take to figure this out:
1) Click on the link for copyright information that PMC offers.
2) In this case, the authors are permitting "...unrestricted use, distribution and reproduction in any medium," provided you give appropriate credit... (a Creative Commons Attribution 4.0 license)
A: Yes you can!
Q: Can I use this very old illustration as a visual aid?
Steps to find out: This is from the NLM's "History of Medicine" online exhibit, in the Rare Books and Journals Collection. Even though the NLM is a government agency, that does not mean everything it publishes is in the public domain, especially in cases such as this where the image is part of a compilation of works not created by NLM employees. Copyright information for this illustration reads:
"NLM Copyright Information
"Government information at NLM Web sites is in the public domain. Public domain information may be freely distributed and copied,... When using NLM Web sites, you may encounter documents, illustrations, photographs, or other information resources contributed or licensed by private individuals, companies, or organizations that may be protected by U.S. and foreign copyright laws. Transmission or reproduction of protected items beyond that allowed by fair use as defined in the copyright laws requires the written permission of the copyright owners. Specific NLM Web sites containing protected information provide additional notification of conditions associated with its use."
A: As there are no obvious "additional notification of conditions associated with its use" and the website is from the NLM, giving credit should suffice, under fair use guidelines.