There is educational value in being wrong and being corrected. I am glad my stupid remarks about the etymology of "head" stirred up some scholarly response. It has been so long (47 years) since I took a PhD in English philology at Harvard that I have grown careless. Or I haven't grown, I have shrunk.
But I shall not shrink from admitting I failed to describe the Indo-European linguistic tree and its many branches and twigs. I also thoughtlessly made it appear that an English word, such as "head," came from a Latin word by passage through a sequence of Germanic languages and dialects.Professor Geist is correct in saying. "Common ancestry and direct borrowing are different linguistic phenomena."
Also I thank Thomas L. Robertson for reminding me of Grimm's Law, more important to students of linguistics than Murphy's Law or the Peter Principle. However it was the editor, in a brief note, who derived "to know" from the Latin "tongere," not I. In this connection, though, let me point out that one of the meanings of "tongue" is "language."
Professor Geist mentions "the availability of information about the Indo-European family of languages in many dictionaries." To be specific, I suggest two excellent articles in the front part of "The American Heritage Dictionary." One is "A Brief History of the English Language," by Morton W. Bloomfield. The other is "The Indo-European Origin of English," by Calvert Watkins. Penitently, I have just reread both articles.
I should have my head (from the Middle English "hed," which is from the Anglo-Saxon or Old English "heafod") examined, but would regret having my head removed or decapitated (from the Latin "decapitatus," the past participle of "decapitare," which is from "de" plus "caput").
So I am back to "head" and " caput" where I started.