Q: A burning question in publishing: Books may have appendices, but what about people? If a doctor removes more than one appendix, has he removed appendices? We suspect not.
A: The short answer is yes, way back when.
Over time, plural endings of foreign-derived words tend to become Anglicized. Many English dictionaries now list “gymnasiums” before “gymnasia,” “memorandums” before “memoranda,” “syllabuses” before “syllabi,” “cactuses” before “cacti,” and so on.
Similarly, 150 years ago, your dictionary would have recommended “appendices” for the plural of “appendix” (in both the literary and medical senses). These days “appendixes” is preferred for both. Many dictionaries, however, give the two plurals, as in this entry for “appendix” in the American Heritage Dictionary of the English Language.