About the NLM Archives
The NLM archives contain older Web pages from the National Library of Medicine’s main Website. All pages in the archives are considered to be of permanent value. They are moved from the current site to the archives when their contents are no longer current. NLM has made a commitment to keep them available permanently.
To see more recent versions of a document, click on “Replaced By”.
To see earlier versions of a document, click on “Previous Version".