Sentence:  [H]e [the newly elected Pope] is placed on the altar, where all the cardinals, according to their rank, pay their reverence to him, and kiss his feet, hands, and mouth. I should observe, <WSD>"en passant</WSD>", that cardinals only have the right to kiss the Pope's hand. 
Meaning of the word marked with <WSD> tags:  In passing, by the way, incidentally. (from early 17th c.)