tricyclesinskirts: Henry Gray’s Anatomy of the Human Body: Posterior surface of the forearm, superficial muscles. Note how there aren’t any muscles in the fingers. Your fingers are controlled by tendons that connect to muscles in the forearm.
For better grip, flip the bobby pin over so that the wavy side is against your head. If you have a hard time keeping your bobby pins in place, try flipping the pin over. The straight side is slicker and won't grip as well as the wavy, more textured side.