Unsolved Mysteries

Earlier this week, Lizzy and I were pondering the origin of the word "quadricep." How did the front thigh muscles get that name? And what about triceps and biceps? The prefixes mean four, three, and two, but that's not helpful. You have two biceps, but you don't have three triceps or four quadriceps. Are they being ranked in some way? And if so, which muscle is first? Is there such a muscle as a monocep? Someone who has taken anatomy, please enlighten me!