The techincal term is "pizzle", but I always say penis because I so rarely get to say penis in polite company. Penis.bully sticks are....um...meat? Muscle? Tendon? (a bull's boy part )
You're right about them being digestible, but they are still a choking hazard; I take them away from my dog just as they approach a size they can be swallowed.
Moderators, please don't ban me. And penis.