Arnold Schwarzenegger -- Suffers Head Wound on Movie Set

Blood was trickling down Arnold Schwarzenegger's forehead yesterday -- after the 64-year-old actor got "banged up" on the set of his new movie ... and of course, he tweeted about it.

Arnold -- who's shooting "The Last Stand" in New Mexico -- explained, "Got a little banged up on set today. Thanks to the medical staff who got me back in action an hour later!"

So far, no word on exactly how Arnold split his melon.