I think it's just the top layer of skin or cartilage-type layer right underneath that. The exposed part underneath is a silvery color and doesn't seem to have much blood supply. I would think if it was the muscle it would be pinker and have some blood/bleeding, but I'm not certain.
Edited to Add: It's after 1 am now, so I'm going to try to sleep a bit. I'll report back tomorrow when we return from the vet.