Trapezius
Shoulder & Arm
Named because the left and right together form a trapezoid shape — it's the muscle you shrug with.
Function
Elevates, retracts, depresses, and rotates scapula
Origin
External occipital protuberance, nuchal ligament, spinous processes C7-T12
Insertion
Lateral clavicle, acromion, spine of scapula
Innervation
Spinal accessory nerve (CN XI) and C3-C4