Casey's inner arm tattoo reads "J'entends ta voix dans tous les bruits du monde" (I hear your voice in all the sounds of the world).
It is a verse from Paul Éluard's "Ta bouche aux lèvres d'or" (Your mouth with lips of gold),
found in "Au défaut du silence" (In the absence of silence) which was published in 1925,
and most notably in the poetry collection "Capitale de la douleur" (Capital of pain), published the following year.