In the beginning of LL XXI, we have the phrase
"... et sanguis de naso eius fluit".
Ok, that makes sense. But later, we see the question:
"Sed cur sanguis de naso fluit Marco?"
I was surprised to see "Marco", not "Marci", genitive, as he's the owner of the nose in question. Also, the word order seems a little funny -- why did "Marco" slip past the verb?
The next sentence goes
"Sanguis ei de naso fluit",
so it seems that Marco is dative, like ei, so maybe this is something like dative of possession, but I thought that only worked with forms of esse?